diff --git a/clang/include/clang/AST/ASTContext.h b/clang/include/clang/AST/ASTContext.h
--- a/clang/include/clang/AST/ASTContext.h
+++ b/clang/include/clang/AST/ASTContext.h
@@ -139,6 +139,12 @@
 
 } // namespace comments
 
+namespace vm {
+
+class Context;
+
+} // namespace vm
+
 struct TypeInfo {
   uint64_t Width = 0;
   unsigned Align = 0;
@@ -564,6 +570,7 @@
   const TargetInfo *Target = nullptr;
   const TargetInfo *AuxTarget = nullptr;
   clang::PrintingPolicy PrintingPolicy;
+  std::unique_ptr<vm::Context> ConstexprCtx;
 
 public:
   IdentifierTable &Idents;
@@ -573,6 +580,9 @@
   IntrusiveRefCntPtr<ExternalASTSource> ExternalSource;
   ASTMutationListener *Listener = nullptr;
 
+  /// Returns the constexpr VM context.
+  vm::Context &getConstexprCtx();
+
   /// Container for either a single DynTypedNode or for an ArrayRef to
   /// DynTypedNode. For use with ParentMap.
   class DynTypedNodeList {
diff --git a/clang/include/clang/Basic/DiagnosticASTKinds.td b/clang/include/clang/Basic/DiagnosticASTKinds.td
--- a/clang/include/clang/Basic/DiagnosticASTKinds.td
+++ b/clang/include/clang/Basic/DiagnosticASTKinds.td
@@ -228,6 +228,10 @@
 def note_constexpr_bit_cast_indet_dest : Note<
   "indeterminate value can only initialize an object of type 'unsigned char'"
   "%select{, 'char',|}1 or 'std::byte'; %0 is invalid">;
+def err_constexpr_compilation_failed : Error<
+  "constexpr cannot be compiled by the experimental interpreter">;
+def err_constexpr_interp_failed : Error<
+  "constexpr cannot be evaluated by the experimental interpreter">;
 
 def warn_integer_constant_overflow : Warning<
   "overflow in expression; result is %0 with type %1">,
diff --git a/clang/include/clang/Basic/LangOptions.def b/clang/include/clang/Basic/LangOptions.def
--- a/clang/include/clang/Basic/LangOptions.def
+++ b/clang/include/clang/Basic/LangOptions.def
@@ -286,6 +286,10 @@
                "maximum constexpr call depth")
 BENIGN_LANGOPT(ConstexprStepLimit, 32, 1048576,
                "maximum constexpr evaluation steps")
+BENIGN_LANGOPT(ConstexprInterpreter, 1, 0,
+               "enable the experimental constexpr interpreter")
+BENIGN_LANGOPT(ConstexprForceInterpreter, 1, 0,
+               "force the use of the experimental constexpr interpreter")
 BENIGN_LANGOPT(BracketDepth, 32, 256,
                "maximum bracket nesting depth")
 BENIGN_LANGOPT(NumLargeByValueCopy, 32, 0,
diff --git a/clang/include/clang/Basic/OptionalDiagnostic.h b/clang/include/clang/Basic/OptionalDiagnostic.h
new file mode 100644
--- /dev/null
+++ b/clang/include/clang/Basic/OptionalDiagnostic.h
@@ -0,0 +1,78 @@
+//===- OptionalDiagnostic.h - An optional diagnostic ------------*- C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+//
+/// \file
+/// Implements a partial diagnostic which may not be emitted.
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_CLANG_BASIC_OPTIONALDIAGNOSTIC_H
+#define LLVM_CLANG_BASIC_OPTIONALDIAGNOSTIC_H
+
+#include "clang/AST/APValue.h"
+#include "clang/Basic/PartialDiagnostic.h"
+#include "llvm/ADT/APFloat.h"
+#include "llvm/ADT/APSInt.h"
+#include "llvm/ADT/SmallVector.h"
+#include "llvm/ADT/StringRef.h"
+
+namespace clang {
+
+/// A partial diagnostic which we might know in advance that we are not going
+/// to emit.
+class OptionalDiagnostic {
+  PartialDiagnostic *Diag;
+
+public:
+  explicit OptionalDiagnostic(PartialDiagnostic *Diag = nullptr) : Diag(Diag) {}
+
+  template <typename T> OptionalDiagnostic &operator<<(const T &v) {
+    if (Diag)
+      *Diag << v;
+    return *this;
+  }
+
+  OptionalDiagnostic &operator<<(const llvm::APSInt &I) {
+    if (Diag) {
+      SmallVector<char, 32> Buffer;
+      I.toString(Buffer);
+      *Diag << StringRef(Buffer.data(), Buffer.size());
+    }
+    return *this;
+  }
+
+  OptionalDiagnostic &operator<<(const llvm::APFloat &F) {
+    if (Diag) {
+      // FIXME: Force the precision of the source value down so we don't
+      // print digits which are usually useless (we don't really care here if
+      // we truncate a digit by accident in edge cases).  Ideally,
+      // APFloat::toString would automatically print the shortest
+      // representation which rounds to the correct value, but it's a bit
+      // tricky to implement. Could use std::to_chars.
+      unsigned precision = llvm::APFloat::semanticsPrecision(F.getSemantics());
+      precision = (precision * 59 + 195) / 196;
+      SmallVector<char, 32> Buffer;
+      F.toString(Buffer, precision);
+      *Diag << StringRef(Buffer.data(), Buffer.size());
+    }
+    return *this;
+  }
+
+  OptionalDiagnostic &operator<<(const APFixedPoint &FX) {
+    if (Diag) {
+      SmallVector<char, 32> Buffer;
+      FX.toString(Buffer);
+      *Diag << StringRef(Buffer.data(), Buffer.size());
+    }
+    return *this;
+  }
+};
+
+} // namespace clang
+
+#endif
diff --git a/clang/include/clang/Driver/Options.td b/clang/include/clang/Driver/Options.td
--- a/clang/include/clang/Driver/Options.td
+++ b/clang/include/clang/Driver/Options.td
@@ -833,6 +833,10 @@
 def fconstant_string_class_EQ : Joined<["-"], "fconstant-string-class=">, Group<f_Group>;
 def fconstexpr_depth_EQ : Joined<["-"], "fconstexpr-depth=">, Group<f_Group>;
 def fconstexpr_steps_EQ : Joined<["-"], "fconstexpr-steps=">, Group<f_Group>;
+def fexperimental_constexpr_interpreter : Flag<["-"], "fexperimental-constexpr-interpreter">, Group<f_Group>,
+  HelpText<"Enable the experimental constexpr interpreter">, Flags<[CC1Option]>;
+def fexperimental_constexpr_force_interpreter : Flag<["-"], "fexperimental-constexpr-force-interpreter">, Group<f_Group>,
+  HelpText<"Force the use of the experimental constexpr interpreter, failing on missing features">, Flags<[CC1Option]>;
 def fconstexpr_backtrace_limit_EQ : Joined<["-"], "fconstexpr-backtrace-limit=">,
                                     Group<f_Group>;
 def fno_crash_diagnostics : Flag<["-"], "fno-crash-diagnostics">, Group<f_clang_Group>, Flags<[NoArgumentUnused, CoreOption]>,
diff --git a/clang/lib/AST/ASTContext.cpp b/clang/lib/AST/ASTContext.cpp
--- a/clang/lib/AST/ASTContext.cpp
+++ b/clang/lib/AST/ASTContext.cpp
@@ -12,6 +12,7 @@
 
 #include "clang/AST/ASTContext.h"
 #include "CXXABI.h"
+#include "ExprVM/Context.h"
 #include "clang/AST/APValue.h"
 #include "clang/AST/ASTMutationListener.h"
 #include "clang/AST/ASTTypeTraits.h"
@@ -737,6 +738,13 @@
   llvm_unreachable("Invalid CXXABI type!");
 }
 
+vm::Context &ASTContext::getConstexprCtx() {
+  if (!ConstexprCtx) {
+    ConstexprCtx.reset(new vm::Context(*this));
+  }
+  return *ConstexprCtx.get();
+}
+
 static const LangASMap *getAddressSpaceMap(const TargetInfo &T,
                                            const LangOptions &LOpts) {
   if (LOpts.FakeAddressSpaceMap) {
diff --git a/clang/lib/AST/CMakeLists.txt b/clang/lib/AST/CMakeLists.txt
--- a/clang/lib/AST/CMakeLists.txt
+++ b/clang/lib/AST/CMakeLists.txt
@@ -4,6 +4,8 @@
   Support
   )
 
+add_subdirectory(ExprVM)
+
 add_clang_library(clangAST
   APValue.cpp
   ASTConsumer.cpp
@@ -80,6 +82,7 @@
   VTTBuilder.cpp
 
   LINK_LIBS
+  clangExpr
   clangBasic
   clangLex
   )
diff --git a/clang/lib/AST/ExprConstant.cpp b/clang/lib/AST/ExprConstant.cpp
--- a/clang/lib/AST/ExprConstant.cpp
+++ b/clang/lib/AST/ExprConstant.cpp
@@ -32,13 +32,17 @@
 //
 //===----------------------------------------------------------------------===//
 
+#include "ExprVM/Context.h"
+#include "ExprVM/Compiler.h"
+#include "ExprVM/Frame.h"
+#include "ExprVM/State.h"
 #include "clang/AST/APValue.h"
 #include "clang/AST/ASTContext.h"
 #include "clang/AST/ASTDiagnostic.h"
 #include "clang/AST/ASTLambda.h"
+#include "clang/AST/CXXInheritance.h"
 #include "clang/AST/CharUnits.h"
 #include "clang/AST/CurrentSourceLocExprScope.h"
-#include "clang/AST/CXXInheritance.h"
 #include "clang/AST/Expr.h"
 #include "clang/AST/OSLog.h"
 #include "clang/AST/RecordLayout.h"
@@ -49,6 +53,8 @@
 #include "clang/Basic/TargetInfo.h"
 #include "llvm/ADT/Optional.h"
 #include "llvm/ADT/SmallBitVector.h"
+#include "clang/Basic/OptionalDiagnostic.h"
+#include "clang/Basic/TargetInfo.h"
 #include "llvm/Support/SaveAndRestore.h"
 #include "llvm/Support/raw_ostream.h"
 #include <cstring>
@@ -66,8 +72,8 @@
 
 namespace {
   struct LValue;
-  struct CallStackFrame;
-  struct EvalInfo;
+  class CallStackFrame;
+  class EvalInfo;
 
   using SourceLocExprScopeGuard =
       CurrentSourceLocExprScope::SourceLocExprScopeGuard;
@@ -480,7 +486,8 @@
   };
 
   /// A stack frame in the constexpr call stack.
-  struct CallStackFrame {
+  class CallStackFrame : public vm::Frame {
+  public:
     EvalInfo &Info;
 
     /// Parent - The caller of this stack frame.
@@ -574,6 +581,12 @@
     }
 
     APValue &createTemporary(const void *Key, bool IsLifetimeExtended);
+
+    void describe(llvm::raw_ostream &OS) override;
+
+    Frame *getCaller() const override { return Caller; }
+    SourceLocation getLoc() const override { return CallLoc; }
+    const FunctionDecl *getCallee() const override { return Callee; }
   };
 
   /// Temporarily override 'this'.
@@ -592,59 +605,6 @@
     const LValue *OldThis;
   };
 
-  /// A partial diagnostic which we might know in advance that we are not going
-  /// to emit.
-  class OptionalDiagnostic {
-    PartialDiagnostic *Diag;
-
-  public:
-    explicit OptionalDiagnostic(PartialDiagnostic *Diag = nullptr)
-      : Diag(Diag) {}
-
-    template<typename T>
-    OptionalDiagnostic &operator<<(const T &v) {
-      if (Diag)
-        *Diag << v;
-      return *this;
-    }
-
-    OptionalDiagnostic &operator<<(const APSInt &I) {
-      if (Diag) {
-        SmallVector<char, 32> Buffer;
-        I.toString(Buffer);
-        *Diag << StringRef(Buffer.data(), Buffer.size());
-      }
-      return *this;
-    }
-
-    OptionalDiagnostic &operator<<(const APFloat &F) {
-      if (Diag) {
-        // FIXME: Force the precision of the source value down so we don't
-        // print digits which are usually useless (we don't really care here if
-        // we truncate a digit by accident in edge cases).  Ideally,
-        // APFloat::toString would automatically print the shortest
-        // representation which rounds to the correct value, but it's a bit
-        // tricky to implement.
-        unsigned precision =
-            llvm::APFloat::semanticsPrecision(F.getSemantics());
-        precision = (precision * 59 + 195) / 196;
-        SmallVector<char, 32> Buffer;
-        F.toString(Buffer, precision);
-        *Diag << StringRef(Buffer.data(), Buffer.size());
-      }
-      return *this;
-    }
-
-    OptionalDiagnostic &operator<<(const APFixedPoint &FX) {
-      if (Diag) {
-        SmallVector<char, 32> Buffer;
-        FX.toString(Buffer);
-        *Diag << StringRef(Buffer.data(), Buffer.size());
-      }
-      return *this;
-    }
-  };
-
   /// A cleanup, and a flag indicating whether it is lifetime-extended.
   class Cleanup {
     llvm::PointerIntPair<APValue*, 1, bool> Value;
@@ -707,7 +667,8 @@
   /// rules.  For example, the RHS of (0 && foo()) is not evaluated.  We can
   /// evaluate the expression regardless of what the RHS is, but C only allows
   /// certain things in certain situations.
-  struct EvalInfo {
+  class EvalInfo : public vm::State {
+  public:
     ASTContext &Ctx;
 
     /// EvalStatus - Contains information about the evaluation.
@@ -727,6 +688,13 @@
     /// we will evaluate.
     unsigned StepsLeft;
 
+    /// Force the use of the constexpr interpreter, bailing out with an error
+    /// if a feature is unsupported.
+    bool ForceInterp;
+
+    /// Enable the constexpr interpreter.
+    bool EnableInterp;
+
     /// BottomFrame - The frame in which evaluation started. This must be
     /// initialized after CurrentCall and CallStackDepth.
     CallStackFrame BottomFrame;
@@ -837,7 +805,7 @@
 
     /// Are we checking whether the expression is a potential constant
     /// expression?
-    bool checkingPotentialConstantExpression() const {
+    bool checkingPotentialConstantExpression() const override {
       return EvalMode == EM_PotentialConstantExpression ||
              EvalMode == EM_PotentialConstantExpressionUnevaluated;
     }
@@ -845,12 +813,16 @@
     /// Are we checking an expression for overflow?
     // FIXME: We should check for any kind of undefined or suspicious behavior
     // in such constructs, not just overflow.
-    bool checkingForOverflow() { return EvalMode == EM_EvaluateForOverflow; }
+    bool checkingForOverflow() const override {
+      return EvalMode == EM_EvaluateForOverflow;
+    }
 
     EvalInfo(const ASTContext &C, Expr::EvalStatus &S, EvaluationMode Mode)
       : Ctx(const_cast<ASTContext &>(C)), EvalStatus(S), CurrentCall(nullptr),
         CallStackDepth(0), NextCallIndex(1),
         StepsLeft(getLangOpts().ConstexprStepLimit),
+        ForceInterp(getLangOpts().ConstexprForceInterpreter),
+        EnableInterp(ForceInterp || getLangOpts().ConstexprInterpreter),
         BottomFrame(*this, SourceLocation(), nullptr, nullptr, nullptr),
         EvaluatingDecl((const ValueDecl *)nullptr),
         EvaluatingDeclValue(nullptr), HasActiveDiagnostic(false),
@@ -862,8 +834,6 @@
       EvaluatingDeclValue = &Value;
     }
 
-    const LangOptions &getLangOpts() const { return Ctx.getLangOpts(); }
-
     bool CheckCallLimit(SourceLocation Loc) {
       // Don't perform any constexpr calls (other than the call we're checking)
       // when checking a potential constant expression.
@@ -907,118 +877,52 @@
     }
 
   private:
-    /// Add a diagnostic to the diagnostics list.
-    PartialDiagnostic &addDiag(SourceLocation Loc, diag::kind DiagId) {
-      PartialDiagnostic PD(DiagId, Ctx.getDiagAllocator());
-      EvalStatus.Diag->push_back(std::make_pair(Loc, PD));
-      return EvalStatus.Diag->back().second;
-    }
-
-    /// Add notes containing a call stack to the current point of evaluation.
-    void addCallStack(unsigned Limit);
-
-  private:
-    OptionalDiagnostic Diag(SourceLocation Loc, diag::kind DiagId,
-                            unsigned ExtraNotes, bool IsCCEDiag) {
-
-      if (EvalStatus.Diag) {
-        // If we have a prior diagnostic, it will be noting that the expression
-        // isn't a constant expression. This diagnostic is more important,
-        // unless we require this evaluation to produce a constant expression.
-        //
-        // FIXME: We might want to show both diagnostics to the user in
-        // EM_ConstantFold mode.
-        if (!EvalStatus.Diag->empty()) {
-          switch (EvalMode) {
-          case EM_ConstantFold:
-          case EM_IgnoreSideEffects:
-          case EM_EvaluateForOverflow:
-            if (!HasFoldFailureDiagnostic)
-              break;
-            // We've already failed to fold something. Keep that diagnostic.
-            LLVM_FALLTHROUGH;
-          case EM_ConstantExpression:
-          case EM_PotentialConstantExpression:
-          case EM_ConstantExpressionUnevaluated:
-          case EM_PotentialConstantExpressionUnevaluated:
-            HasActiveDiagnostic = false;
-            return OptionalDiagnostic();
-          }
+    vm::Frame *getCurrentFrame() override { return CurrentCall; }
+    const vm::Frame *getBottomFrame() const override { return &BottomFrame; }
+
+    bool hasActiveDiagnostic() override { return HasActiveDiagnostic; }
+    void setActiveDiagnostic(bool Flag) override { HasActiveDiagnostic = Flag; }
+
+    void setFoldFailureDiagnostic(bool Flag) override {
+      HasFoldFailureDiagnostic = Flag;
+    }
+
+    Expr::EvalStatus &getEvalStatus() const override { return EvalStatus; }
+
+    ASTContext &getCtx() const override { return Ctx; }
+
+    // If we have a prior diagnostic, it will be noting that the expression
+    // isn't a constant expression. This diagnostic is more important,
+    // unless we require this evaluation to produce a constant expression.
+    //
+    // FIXME: We might want to show both diagnostics to the user in
+    // EM_ConstantFold mode.
+    bool hasPriorDiagnostic() override {
+      if (!EvalStatus.Diag->empty()) {
+        switch (EvalMode) {
+        case EM_ConstantFold:
+        case EM_IgnoreSideEffects:
+        case EM_EvaluateForOverflow:
+          if (!HasFoldFailureDiagnostic)
+            break;
+          // We've already failed to fold something. Keep that diagnostic.
+          LLVM_FALLTHROUGH;
+        case EM_ConstantExpression:
+        case EM_PotentialConstantExpression:
+        case EM_ConstantExpressionUnevaluated:
+        case EM_PotentialConstantExpressionUnevaluated:
+          setActiveDiagnostic(false);
+          return true;
         }
-
-        unsigned CallStackNotes = CallStackDepth - 1;
-        unsigned Limit = Ctx.getDiagnostics().getConstexprBacktraceLimit();
-        if (Limit)
-          CallStackNotes = std::min(CallStackNotes, Limit + 1);
-        if (checkingPotentialConstantExpression())
-          CallStackNotes = 0;
-
-        HasActiveDiagnostic = true;
-        HasFoldFailureDiagnostic = !IsCCEDiag;
-        EvalStatus.Diag->clear();
-        EvalStatus.Diag->reserve(1 + ExtraNotes + CallStackNotes);
-        addDiag(Loc, DiagId);
-        if (!checkingPotentialConstantExpression())
-          addCallStack(Limit);
-        return OptionalDiagnostic(&(*EvalStatus.Diag)[0].second);
-      }
-      HasActiveDiagnostic = false;
-      return OptionalDiagnostic();
-    }
-  public:
-    // Diagnose that the evaluation could not be folded (FF => FoldFailure)
-    OptionalDiagnostic
-    FFDiag(SourceLocation Loc,
-          diag::kind DiagId = diag::note_invalid_subexpr_in_const_expr,
-          unsigned ExtraNotes = 0) {
-      return Diag(Loc, DiagId, ExtraNotes, false);
-    }
-
-    OptionalDiagnostic FFDiag(const Expr *E, diag::kind DiagId
-                              = diag::note_invalid_subexpr_in_const_expr,
-                            unsigned ExtraNotes = 0) {
-      if (EvalStatus.Diag)
-        return Diag(E->getExprLoc(), DiagId, ExtraNotes, /*IsCCEDiag*/false);
-      HasActiveDiagnostic = false;
-      return OptionalDiagnostic();
-    }
-
-    /// Diagnose that the evaluation does not produce a C++11 core constant
-    /// expression.
-    ///
-    /// FIXME: Stop evaluating if we're in EM_ConstantExpression or
-    /// EM_PotentialConstantExpression mode and we produce one of these.
-    OptionalDiagnostic CCEDiag(SourceLocation Loc, diag::kind DiagId
-                                 = diag::note_invalid_subexpr_in_const_expr,
-                               unsigned ExtraNotes = 0) {
-      // Don't override a previous diagnostic. Don't bother collecting
-      // diagnostics if we're evaluating for overflow.
-      if (!EvalStatus.Diag || !EvalStatus.Diag->empty()) {
-        HasActiveDiagnostic = false;
-        return OptionalDiagnostic();
       }
-      return Diag(Loc, DiagId, ExtraNotes, true);
-    }
-    OptionalDiagnostic CCEDiag(const Expr *E, diag::kind DiagId
-                                 = diag::note_invalid_subexpr_in_const_expr,
-                               unsigned ExtraNotes = 0) {
-      return CCEDiag(E->getExprLoc(), DiagId, ExtraNotes);
-    }
-    /// Add a note to a prior diagnostic.
-    OptionalDiagnostic Note(SourceLocation Loc, diag::kind DiagId) {
-      if (!HasActiveDiagnostic)
-        return OptionalDiagnostic();
-      return OptionalDiagnostic(&addDiag(Loc, DiagId));
+      return false;
     }
 
-    /// Add a stack of notes to a prior diagnostic.
-    void addNotes(ArrayRef<PartialDiagnosticAt> Diags) {
-      if (HasActiveDiagnostic) {
-        EvalStatus.Diag->insert(EvalStatus.Diag->end(),
-                                Diags.begin(), Diags.end());
-      }
+    unsigned getCallStackDepth() override {
+      return CallStackDepth;
     }
 
+  public:
     /// Should we continue evaluation after encountering a side-effect that we
     /// couldn't model?
     bool keepEvaluatingAfterSideEffect() {
@@ -1064,7 +968,7 @@
     /// Note that we hit something that was technically undefined behavior, but
     /// that we can evaluate past it (such as signed overflow or floating-point
     /// division by zero.)
-    bool noteUndefinedBehavior() {
+    bool noteUndefinedBehavior() override {
       EvalStatus.HasUndefinedBehavior = true;
       return keepEvaluatingAfterUndefinedBehavior();
     }
@@ -1321,62 +1225,6 @@
   return Result;
 }
 
-static void describeCall(CallStackFrame *Frame, raw_ostream &Out);
-
-void EvalInfo::addCallStack(unsigned Limit) {
-  // Determine which calls to skip, if any.
-  unsigned ActiveCalls = CallStackDepth - 1;
-  unsigned SkipStart = ActiveCalls, SkipEnd = SkipStart;
-  if (Limit && Limit < ActiveCalls) {
-    SkipStart = Limit / 2 + Limit % 2;
-    SkipEnd = ActiveCalls - Limit / 2;
-  }
-
-  // Walk the call stack and add the diagnostics.
-  unsigned CallIdx = 0;
-  for (CallStackFrame *Frame = CurrentCall; Frame != &BottomFrame;
-       Frame = Frame->Caller, ++CallIdx) {
-    // Skip this call?
-    if (CallIdx >= SkipStart && CallIdx < SkipEnd) {
-      if (CallIdx == SkipStart) {
-        // Note that we're skipping calls.
-        addDiag(Frame->CallLoc, diag::note_constexpr_calls_suppressed)
-          << unsigned(ActiveCalls - Limit);
-      }
-      continue;
-    }
-
-    // Use a different note for an inheriting constructor, because from the
-    // user's perspective it's not really a function at all.
-    if (auto *CD = dyn_cast_or_null<CXXConstructorDecl>(Frame->Callee)) {
-      if (CD->isInheritingConstructor()) {
-        addDiag(Frame->CallLoc, diag::note_constexpr_inherited_ctor_call_here)
-          << CD->getParent();
-        continue;
-      }
-    }
-
-    SmallVector<char, 128> Buffer;
-    llvm::raw_svector_ostream Out(Buffer);
-    describeCall(Frame, Out);
-    addDiag(Frame->CallLoc, diag::note_constexpr_call_here) << Out.str();
-  }
-}
-
-/// Kinds of access we can perform on an object, for diagnostics. Note that
-/// we consider a member function call to be a kind of access, even though
-/// it is not formally an access of the object, because it has (largely) the
-/// same set of semantic restrictions.
-enum AccessKinds {
-  AK_Read,
-  AK_Assign,
-  AK_Increment,
-  AK_Decrement,
-  AK_MemberCall,
-  AK_DynamicCast,
-  AK_TypeId,
-};
-
 static bool isModification(AccessKinds AK) {
   switch (AK) {
   case AK_Read:
@@ -1744,36 +1592,36 @@
 }
 
 /// Produce a string describing the given constexpr call.
-static void describeCall(CallStackFrame *Frame, raw_ostream &Out) {
+void CallStackFrame::describe(raw_ostream &Out) {
   unsigned ArgIndex = 0;
-  bool IsMemberCall = isa<CXXMethodDecl>(Frame->Callee) &&
-                      !isa<CXXConstructorDecl>(Frame->Callee) &&
-                      cast<CXXMethodDecl>(Frame->Callee)->isInstance();
+  bool IsMemberCall = isa<CXXMethodDecl>(Callee) &&
+                      !isa<CXXConstructorDecl>(Callee) &&
+                      cast<CXXMethodDecl>(Callee)->isInstance();
 
   if (!IsMemberCall)
-    Out << *Frame->Callee << '(';
+    Out << *Callee << '(';
 
-  if (Frame->This && IsMemberCall) {
+  if (This && IsMemberCall) {
     APValue Val;
-    Frame->This->moveInto(Val);
-    Val.printPretty(Out, Frame->Info.Ctx,
-                    Frame->This->Designator.MostDerivedType);
+    This->moveInto(Val);
+    Val.printPretty(Out, Info.Ctx,
+                    This->Designator.MostDerivedType);
     // FIXME: Add parens around Val if needed.
-    Out << "->" << *Frame->Callee << '(';
+    Out << "->" << *Callee << '(';
     IsMemberCall = false;
   }
 
-  for (FunctionDecl::param_const_iterator I = Frame->Callee->param_begin(),
-       E = Frame->Callee->param_end(); I != E; ++I, ++ArgIndex) {
+  for (FunctionDecl::param_const_iterator I = Callee->param_begin(),
+       E = Callee->param_end(); I != E; ++I, ++ArgIndex) {
     if (ArgIndex > (unsigned)IsMemberCall)
       Out << ", ";
 
     const ParmVarDecl *Param = *I;
-    const APValue &Arg = Frame->Arguments[ArgIndex];
-    Arg.printPretty(Out, Frame->Info.Ctx, Param->getType());
+    const APValue &Arg = Arguments[ArgIndex];
+    Arg.printPretty(Out, Info.Ctx, Param->getType());
 
     if (ArgIndex == 0 && IsMemberCall)
-      Out << "->" << *Frame->Callee << '(';
+      Out << "->" << *Callee << '(';
   }
 
   Out << ')';
@@ -5185,6 +5033,42 @@
                                         Frame.LambdaThisCaptureField);
   }
 
+  // Try to evaluate the function call in the bytecode VM.
+  if (Info.EnableInterp) {
+    auto &Ctx = Info.Ctx.getConstexprCtx();
+    if (auto Func = Ctx.compileFunction(Callee)) {
+      switch (Ctx.executeFunction(Info, *Func, ArgValues, Result)) {
+      case vm::InterpResult::SUCCESS: {
+        // Interpreter computed a value and converted it to the format
+        // used by the AST walker - continue interpretation here.
+        return true;
+      }
+      case vm::InterpResult::FAIL: {
+        // Diagnostics already emitted through the VM.
+        return false;
+      }
+      case vm::InterpResult::BAIL: {
+        // Interpreter bailed out due to unsupported features.
+        // Evaluation continues with the AST walked, unless force is set.
+        if (Info.ForceInterp) {
+          Info.FFDiag(Callee->getBeginLoc(), diag::err_constexpr_interp_failed);
+          return false;
+        }
+        break;
+      }
+      }
+    } else {
+      // Code cannot be compiled to bytecode - unless force flag is set,
+      // evaluation fall back to the AST walker.
+      if (Info.ForceInterp) {
+        handleAllErrors(Func.takeError(), [&Info](vm::CompilerError &Err) {
+          Info.FFDiag(Err.getLoc(), diag::err_constexpr_compilation_failed);
+        });
+        return false;
+      }
+    }
+  }
+
   StmtResult Ret = {Result, ResultSlot};
   EvalStmtResult ESR = EvaluateStmt(Ret, Info, Body);
   if (ESR == ESR_Succeeded) {
diff --git a/clang/lib/AST/ExprVM/CMakeLists.txt b/clang/lib/AST/ExprVM/CMakeLists.txt
new file mode 100644
--- /dev/null
+++ b/clang/lib/AST/ExprVM/CMakeLists.txt
@@ -0,0 +1,17 @@
+set(LLVM_LINK_COMPONENTS
+  )
+
+add_clang_library(clangExpr
+  Compiler.cpp
+  Context.cpp
+  Disasm.cpp
+  Frame.cpp
+  Function.cpp
+  Interp.cpp
+  InterpFrame.cpp
+  InterpStack.cpp
+  Program.cpp
+  Pointer.cpp
+  State.cpp
+  Type.cpp
+  )
diff --git a/clang/lib/AST/ExprVM/Compiler.h b/clang/lib/AST/ExprVM/Compiler.h
new file mode 100644
--- /dev/null
+++ b/clang/lib/AST/ExprVM/Compiler.h
@@ -0,0 +1,178 @@
+//===--- Compiler.h - Code Generator for the constexpr VM -------*- C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+//
+// Defines the constexpr bytecode compiler.
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_CLANG_AST_EXPRVM_COMPILER_H
+#define LLVM_CLANG_AST_EXPRVM_COMPILER_H
+
+#include "Pointer.h"
+#include "Type.h"
+#include "clang/AST/Decl.h"
+#include "clang/AST/StmtVisitor.h"
+#include "llvm/ADT/Optional.h"
+
+namespace clang {
+class QualType;
+
+namespace vm {
+class Compiler;
+class Context;
+class Function;
+class LocalScope;
+class Program;
+class SourceInfo;
+enum Opcode : uint32_t;
+
+/// Error thrown by the compiler.
+struct CompilerError : public llvm::ErrorInfo<CompilerError> {
+public:
+  CompilerError(SourceLocation Loc) : Loc(Loc) {}
+
+  void log(raw_ostream &OS) const override { OS << "unimplemented feature"; }
+
+  const SourceLocation &getLoc() const { return Loc; }
+
+  static char ID;
+
+private:
+  // Start of the item where the error occurred.
+  SourceLocation Loc;
+
+  // Users are not expected to use error_code.
+  std::error_code convertToErrorCode() const override {
+    return llvm::inconvertibleErrorCode();
+  }
+};
+
+/// Compilation context for a constexpr function.
+class Compiler : public ConstStmtVisitor<Compiler, llvm::Error> {
+private:
+  using LabelTy = int;
+  using EmitFn = void (Compiler::*)(PrimType, const SourceInfo &);
+
+  /// Current compilation context.
+  Context &Ctx;
+  /// Program to link to.
+  Program &P;
+  /// Function declaration being compiled.
+  const FunctionDecl *F;
+
+public:
+  /// Creates a compiler for a funciton.
+  Compiler(Context &Ctx, Program &P, const FunctionDecl *F);
+
+  /// Compiles the function into the module.
+  llvm::Expected<Function *> compile();
+
+public:
+  // Statement visitors.
+  llvm::Error compile(const Stmt *S);
+  llvm::Error compileCompoundStmt(const CompoundStmt *S);
+  llvm::Error compileDeclStmt(const DeclStmt *DS);
+  llvm::Error compileForStmt(const ForStmt *FS);
+  llvm::Error compileWhileStmt(const WhileStmt *DS);
+  llvm::Error compileReturnStmt(const ReturnStmt *RS);
+  llvm::Error compileIfStmt(const IfStmt *IS);
+
+  // Expression visitors - result returned on stack.
+  llvm::Error VisitCastExpr(const CastExpr *CE);
+  llvm::Error VisitIntegerLiteral(const IntegerLiteral *E);
+  llvm::Error VisitParenExpr(const ParenExpr *PE);
+  llvm::Error VisitBinaryOperator(const BinaryOperator *BO);
+
+  // If an unsupported construct is encountered, no code is generated.
+  llvm::Error VisitExpr(const Expr *E) { return bail(E); }
+  llvm::Error VisitStmt(const Stmt *S) { llvm_unreachable("not compiled"); }
+
+private:
+  /// Evaluates an expr for side effects and discards the result.
+  llvm::Error discard(const Expr *E);
+  llvm::Error discardBinaryOperator(const BinaryOperator *BO);
+
+  /// Bails out if a given node cannot be compiled.
+  llvm::Error bail(const Stmt *S) {
+    return llvm::make_error<CompilerError>(S->getBeginLoc());
+  }
+  llvm::Error bail(const Decl *D) {
+    return llvm::make_error<CompilerError>(D->getBeginLoc());
+  }
+
+private:
+  friend class LocalScope;
+
+  /// Parameter indices.
+  llvm::DenseMap<const ParmVarDecl *, unsigned> Params;
+
+  /// Parameter descriptors.
+  llvm::DenseMap<unsigned, std::unique_ptr<Descriptor>> ParamDescriptors;
+  /// Perform an action if a parameter is found.
+  llvm::Error withParam(const ParmVarDecl *P,
+                        std::function<llvm::Error(unsigned)> &&f);
+
+  /// Information about a local variable's storage.
+  struct Local {
+    /// Offset of the local in frame.
+    unsigned Location;
+    /// Size of the local, in bytes.
+    unsigned Size;
+  };
+  /// Variable to storage mapping.
+  llvm::DenseMap<const VarDecl *, Local> Locals;
+  /// Local descriptors.
+  llvm::SmallVector<SmallVector<Descriptor, 8>, 2> Descriptors;
+  /// Next offset for a local.
+  unsigned NextLocalOffset = 0;
+  /// Creates a local.
+  const Local &createLocal(const VarDecl *V, unsigned Size, unsigned ElemSize,
+                           PrimType ElemTy);
+  /// Current scope.
+  LocalScope *Top = nullptr;
+
+  /// Perform an action if a local var is found.
+  llvm::Error withLocal(const VarDecl *V,
+                        std::function<llvm::Error(const Local &)> &&f);
+
+  /// Index of the next available label.
+  LabelTy NextLabel = 0;
+
+  /// Create a label.
+  LabelTy getLabel() { return ++NextLabel; }
+
+  /// Define a label.
+  void emitLabel(LabelTy Label);
+
+  /// Helpers to emit instructions.
+  void emitJT(LabelTy Label);
+  void emitJF(LabelTy Label);
+  void emitJMP(LabelTy Label);
+
+#define GET_EMITTER_PROTO
+#include "Opcodes.inc"
+#undef GET_EMITTER_PROTO
+
+  /// Emits an opcode.
+  template <PrimType... Tys>
+  void emitOp(Opcode Op, const typename PrimConv<Tys>::T &... Args,
+              const SourceInfo &L);
+
+  /// Label information for linker.
+  llvm::DenseMap<LabelTy, unsigned> LabelOffsets;
+  /// Location of label relocations.
+  llvm::DenseMap<LabelTy, llvm::SmallVector<unsigned, 5>> LabelRelocs;
+
+  /// Emits a label or a lazy relocation for a forward pointer.
+  void emitReloc(LabelTy Label);
+};
+
+} // namespace vm
+} // namespace clang
+
+#endif
diff --git a/clang/lib/AST/ExprVM/Compiler.cpp b/clang/lib/AST/ExprVM/Compiler.cpp
new file mode 100644
--- /dev/null
+++ b/clang/lib/AST/ExprVM/Compiler.cpp
@@ -0,0 +1,653 @@
+//===--- Compiler.cpp - Code Generator for the constexpr VM -*- C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include "Compiler.h"
+#include "Context.h"
+#include "Function.h"
+#include "Opcode.h"
+#include "Program.h"
+#include "Type.h"
+
+using namespace clang;
+using namespace clang::vm;
+
+using Error = llvm::Error;
+
+char CompilerError::ID;
+
+namespace clang {
+namespace vm {
+
+class LocalScope {
+public:
+  LocalScope(Compiler *Ctx) : Ctx(Ctx), Parent(Ctx->Top) { Ctx->Top = this; }
+
+  virtual ~LocalScope() {
+    emitDestructor();
+    Ctx->Top = Parent;
+  }
+
+  virtual void addDescriptor(Descriptor &&Desc) {
+    if (!Idx.hasValue()) {
+      Idx = Ctx->Descriptors.size();
+      Ctx->Descriptors.emplace_back();
+    }
+
+    Ctx->Descriptors[*Idx].emplace_back(std::move(Desc));
+  }
+
+  LocalScope *getParent() { return Parent; }
+
+  void emitDestructor() {
+    // TODO: emit an opcode to destroy the frame.
+  }
+
+private:
+  /// Compiler instance.
+  Compiler *Ctx;
+  /// Link to the parent scope.
+  LocalScope *Parent;
+  /// Index of the scope in the chain.
+  llvm::Optional<unsigned> Idx;
+};
+
+class BlockScope final : public LocalScope {
+public:
+  BlockScope(Compiler *Ctx) : LocalScope(Ctx) {}
+};
+
+class ExprScope final : public LocalScope {
+public:
+  ExprScope(Compiler *Ctx) : LocalScope(Ctx) {}
+};
+
+} // namespace vm
+} // namespace clang
+
+Compiler::Compiler(Context &Ctx, Program &P, const FunctionDecl *F)
+    : Ctx(Ctx), P(P), F(F) {}
+
+Expected<Function *> Compiler::compile() {
+  // Set up argument indices.
+  unsigned ParamOffset = 0;
+  SmallVector<PrimType, 8> Args;
+  for (auto *ParamDecl : F->parameters()) {
+    if (auto T = Ctx.classify(ParamDecl->getType())) {
+      auto Size = primSize(*T);
+      auto Desc = llvm::make_unique<Descriptor>(ParamDecl, Size, Size, *T,
+                                                /* isMutable */ true,
+                                                /* isArray */ false,
+                                                /* isGlobal */ false);
+      ParamDescriptors.insert({ParamOffset, std::move(Desc)});
+      Params.insert({ParamDecl, ParamOffset});
+      ParamOffset += align(Size);
+      Args.push_back(*T);
+    } else {
+      return bail(F->getBody());
+    }
+  }
+
+  // Compile the function body.
+  unsigned Start, End;
+  {
+    Start = P.getOffset();
+    if (auto E = compile(F->getBody()))
+      return std::move(E);
+
+    // Emit a guard return to protect against a code path missing one.
+    if (F->getReturnType()->isVoidType())
+      emitOp(EO_RET_VOID, {});
+    else
+      emitOp(EO_NORET_VOID, {});
+
+    End = P.getOffset();
+  }
+
+  // Create scopes from descriptors.
+  llvm::SmallVector<Scope, 2> Scopes;
+  for (auto &DS : Descriptors) {
+    Scopes.emplace_back(std::move(DS));
+  }
+
+  // Create a handle over the emitted code.
+  return P.createFunction(F, Start, End, NextLocalOffset, ParamOffset,
+                          std::move(Scopes), std::move(Args),
+                          std::move(ParamDescriptors));
+}
+
+Error Compiler::compile(const Stmt *S) {
+  switch (S->getStmtClass()) {
+  case Stmt::CompoundStmtClass:
+    return compileCompoundStmt(static_cast<const CompoundStmt *>(S));
+  case Stmt::DeclStmtClass:
+    return compileDeclStmt(static_cast<const DeclStmt *>(S));
+  case Stmt::ForStmtClass:
+    return compileForStmt(static_cast<const ForStmt *>(S));
+  case Stmt::WhileStmtClass:
+    return compileWhileStmt(static_cast<const WhileStmt *>(S));
+  case Stmt::ReturnStmtClass:
+    return compileReturnStmt(static_cast<const ReturnStmt *>(S));
+  case Stmt::IfStmtClass:
+    return compileIfStmt(static_cast<const IfStmt *>(S));
+  case Stmt::NullStmtClass:
+    return Error::success();
+  default: {
+    if (auto *E = dyn_cast<Expr>(S)) {
+      if (E->getType()->isVoidType()) {
+        return Visit(E);
+      } else {
+        return discard(static_cast<const Expr *>(S));
+      }
+    } else {
+      return bail(S);
+    }
+  }
+  }
+}
+
+Error Compiler::compileCompoundStmt(const CompoundStmt *S) {
+  // Visitor for a block of code - generates bytecode.
+  BlockScope Scope(this);
+  for (auto *Stmt : S->body())
+    if (auto E = compile(Stmt))
+      return E;
+  return Error::success();
+}
+
+Error Compiler::compileDeclStmt(const DeclStmt *DS) {
+  for (auto *D : DS->decls()) {
+    // Variable declarator.
+    if (auto *VD = dyn_cast<VarDecl>(D)) {
+      auto DT = VD->getType();
+      if (!VD->hasLocalStorage()) {
+        // TODO: implement non-local variables.
+        return bail(DS);
+      }
+
+      // Integers, pointers, primitives.
+      if (DT->isFundamentalType() || DT->hasPointerRepresentation()) {
+        if (auto T = Ctx.classify(VD->getType())) {
+          const auto Size = primSize(*T);
+          const auto &L = createLocal(VD, Size, Size, *T);
+          // Compile the initialiser in its own scope.
+          {
+            ExprScope Scope(this);
+            if (auto E = Visit(VD->getInit()))
+              return E;
+          }
+          emitSET_LOCAL(*T, L.Location, VD);
+          continue;
+        }
+      }
+
+      return bail(DS);
+    }
+
+    // TODO: implement other declarations.
+    return bail(DS);
+  }
+
+  return Error::success();
+}
+
+Error Compiler::compileForStmt(const ForStmt *FS) {
+  // Compile the initialisation statement in an outer scope.
+  BlockScope OuterScope(this);
+  if (auto *Init = FS->getInit())
+    if (auto E = compile(Init))
+      return E;
+
+  auto LabelStart = getLabel();
+  auto LabelEnd = getLabel();
+
+  // Compile the condition, body and increment in the loop scope.
+  emitLabel(LabelStart);
+  {
+    BlockScope LoopScope(this);
+
+    if (auto *Cond = FS->getCond()) {
+      if (auto *CondDecl = FS->getConditionVariableDeclStmt())
+        if (auto E = compileDeclStmt(CondDecl))
+          return E;
+
+      if (auto E = Visit(Cond))
+        return E;
+
+      emitJF(LabelEnd);
+    }
+
+    if (auto *Body = FS->getBody())
+      if (auto E = compile(Body))
+        return E;
+
+    if (auto *Inc = FS->getInc()) {
+      ExprScope IncScope(this);
+      if (auto E = discard(Inc))
+        return E;
+    }
+  }
+  emitJMP(LabelStart);
+  emitLabel(LabelEnd);
+  return Error::success();
+}
+
+Error Compiler::compileWhileStmt(const WhileStmt *WS) {
+  auto LabelStart = getLabel();
+  auto LabelEnd = getLabel();
+  emitLabel(LabelStart);
+  {
+    BlockScope LoopScope(this);
+    if (auto *CondDecl = WS->getConditionVariableDeclStmt())
+      if (auto E = compileDeclStmt(CondDecl))
+        return E;
+
+    if (auto E = Visit(WS->getCond()))
+      return E;
+    emitJF(LabelEnd);
+
+    if (auto E = compile(WS->getBody()))
+      return E;
+  }
+  emitJMP(LabelStart);
+  emitLabel(LabelEnd);
+  return Error::success();
+}
+
+Error Compiler::compileReturnStmt(const ReturnStmt *RS) {
+  if (auto *RetExpr = RS->getRetValue()) {
+    if (auto T = Ctx.classify(RetExpr->getType())) {
+      ExprScope RetScope(this);
+      if (auto E = Visit(RetExpr))
+        return E;
+      for (auto *C = Top; C; C = C->getParent())
+        C->emitDestructor();
+      emitRET(*T, RS);
+      return Error::success();
+    }
+    return bail(RS);
+  } else {
+    for (auto *C = Top; C; C = C->getParent())
+      C->emitDestructor();
+    emitOp(EO_RET_VOID, RS);
+    return Error::success();
+  }
+}
+
+Error Compiler::compileIfStmt(const IfStmt *IS) {
+  BlockScope IfScope(this);
+  if (auto *CondInit = IS->getInit())
+    if (auto E = compile(IS->getInit()))
+      return E;
+
+  if (auto *CondDecl = IS->getConditionVariableDeclStmt())
+    if (auto E = compileDeclStmt(CondDecl))
+      return E;
+
+  if (auto E = Visit(IS->getCond()))
+    return E;
+
+  if (auto *Else = IS->getElse()) {
+    auto LabelElse = getLabel();
+    auto LabelEnd = getLabel();
+    emitJF(LabelElse);
+    if (auto E = compile(IS->getThen()))
+      return E;
+    emitJMP(LabelEnd);
+    emitLabel(LabelElse);
+    if (auto E = compile(Else)) {
+      return E;
+    }
+    emitLabel(LabelEnd);
+  } else {
+    auto LabelEnd = getLabel();
+    emitJF(LabelEnd);
+    if (auto E = compile(IS->getThen()))
+      return E;
+    emitLabel(LabelEnd);
+  }
+
+  return Error::success();
+}
+
+Error Compiler::VisitCastExpr(const CastExpr *CE) {
+  auto *SubExpr = CE->getSubExpr();
+  switch (CE->getCastKind()) {
+  case CK_LValueToRValue: {
+    if (auto T = Ctx.classify(CE->getType())) {
+      if (auto *DE = dyn_cast<DeclRefExpr>(SubExpr)) {
+        bool IsReference = DE->getDecl()->getType()->isReferenceType();
+        if (!IsReference) {
+          if (auto *PD = dyn_cast<ParmVarDecl>(DE->getDecl())) {
+            return withParam(PD, [this, T, DE](unsigned Idx) -> Error {
+              emitGET_PARAM(*T, Idx, DE);
+              return Error::success();
+            });
+          }
+
+          if (auto *VD = dyn_cast<VarDecl>(DE->getDecl())) {
+            return withLocal(VD, [this, T, DE](const Local &L) -> Error {
+              emitGET_LOCAL(*T, L.Location, DE);
+              return Error::success();
+            });
+          }
+        }
+      }
+
+      return bail(CE);
+    } else {
+      // TODO: implement a fallback.
+      return bail(CE);
+    }
+    break;
+  }
+  case CK_IntegralToBoolean: {
+    if (auto T = Ctx.classify(SubExpr->getType())) {
+      if (auto E = Visit(SubExpr)) {
+        return E;
+      }
+
+      emitTEST(*T, CE);
+      return Error::success();
+    } else {
+      return bail(CE);
+    }
+    break;
+  }
+  case CK_NoOp: {
+    return Visit(SubExpr);
+  }
+  default: {
+    // TODO: implement other casts.
+    return bail(CE);
+  }
+  }
+}
+
+Error Compiler::VisitIntegerLiteral(const IntegerLiteral *E) {
+  if (auto T = Ctx.classify(E->getType())) {
+    switch (*T) {
+    case ET_SINT_8: {
+      emitOp<ET_SINT_8>(EO_CONST_SINT_8, E->getValue().getSExtValue(), E);
+      return Error::success();
+    }
+    case ET_UINT_8: {
+      emitOp<ET_UINT_8>(EO_CONST_UINT_8, E->getValue().getZExtValue(), E);
+      return Error::success();
+    }
+    case ET_SINT_16: {
+      emitOp<ET_SINT_16>(EO_CONST_SINT_16, E->getValue().getSExtValue(), E);
+      return Error::success();
+    }
+    case ET_UINT_16: {
+      emitOp<ET_UINT_16>(EO_CONST_UINT_16, E->getValue().getZExtValue(), E);
+      return Error::success();
+    }
+    case ET_SINT_32: {
+      emitOp<ET_SINT_32>(EO_CONST_SINT_32, E->getValue().getSExtValue(), E);
+      return Error::success();
+    }
+    case ET_UINT_32: {
+      emitOp<ET_UINT_32>(EO_CONST_UINT_32, E->getValue().getZExtValue(), E);
+      return Error::success();
+    }
+    case ET_SINT_64: {
+      emitOp<ET_SINT_64>(EO_CONST_SINT_64, E->getValue().getSExtValue(), E);
+      return Error::success();
+    }
+    case ET_UINT_64: {
+      emitOp<ET_UINT_64>(EO_CONST_UINT_64, E->getValue().getZExtValue(), E);
+      return Error::success();
+    }
+    case ET_BOOL:
+      return bail(E);
+    }
+  } else {
+    // TODO: implement other literal types.
+    return bail(E);
+  }
+}
+
+Error Compiler::VisitParenExpr(const ParenExpr *PE) {
+  return Visit(PE->getSubExpr());
+}
+
+Error Compiler::VisitBinaryOperator(const BinaryOperator *BO) {
+  // Typecheck the args.
+  auto *LHS = BO->getLHS();
+  auto TypeLHS = Ctx.classify(LHS->getType());
+  if (!TypeLHS) {
+    return bail(BO);
+  }
+  auto *RHS = BO->getRHS();
+  auto TypeRHS = Ctx.classify(RHS->getType());
+  if (!TypeRHS) {
+    return bail(BO);
+  }
+
+  if (auto T = Ctx.classify(BO->getType())) {
+    // If the LHS is a local variable, set it directly.
+    if (BO->getOpcode() == BO_Assign) {
+      if (auto *DE = dyn_cast<DeclRefExpr>(BO->getLHS())) {
+        bool IsReference = DE->getDecl()->getType()->isReferenceType();
+        if (*TypeLHS == *T && *TypeRHS == *T && !IsReference) {
+          if (auto *PD = dyn_cast<ParmVarDecl>(DE->getDecl())) {
+            return withParam(PD, [this, RHS, BO](unsigned Idx) -> Error {
+              if (auto E = Visit(RHS))
+                return E;
+              return bail(BO);
+            });
+          }
+
+          if (auto *VD = dyn_cast<VarDecl>(DE->getDecl())) {
+            return withLocal(VD, [this, T, RHS, DE](const Local &L) -> Error {
+              if (auto E = Visit(RHS))
+                return E;
+              emitSET_LOCAL(*T, L.Location, DE);
+              // TODO: return pointer
+              return Error::success();
+            });
+          }
+        }
+      }
+    }
+
+    if (*TypeLHS != *TypeRHS)
+      return bail(BO);
+    if (auto E = Visit(LHS))
+      return E;
+    if (auto E = Visit(RHS))
+      return E;
+
+    switch (BO->getOpcode()) {
+    case BO_EQ: emitEQ(*TypeLHS, BO); break;
+    case BO_NE: emitNE(*TypeLHS, BO); break;
+    case BO_LT: emitLT(*TypeLHS, BO); break;
+    case BO_LE: emitLE(*TypeLHS, BO); break;
+    case BO_GT: emitGT(*TypeLHS, BO); break;
+    case BO_GE: emitGE(*TypeLHS, BO); break;
+    case BO_Rem: emitREM(*T, BO); break;
+    case BO_Div: emitDIV(*T, BO); break;
+    case BO_Sub: emitSUB(*T, BO); break;
+    case BO_Add: emitADD(*T, BO); break;
+    case BO_Mul: emitMUL(*T, BO); break;
+    default:
+      return bail(BO);
+    }
+
+    return Error::success();
+  }
+
+  return bail(BO);
+}
+
+Error Compiler::discard(const Expr *DE) {
+  switch (DE->getStmtClass()) {
+  case Stmt::BinaryOperatorClass:
+    return discardBinaryOperator(static_cast<const BinaryOperator *>(DE));
+  default:
+    if (auto E = Visit(DE))
+      return E;
+    if (auto T = Ctx.classify(DE)) {
+      emitPOP(*T, DE);
+      return Error::success();
+    }
+    break;
+  }
+
+  return bail(DE);
+}
+
+Error Compiler::discardBinaryOperator(const BinaryOperator *BO) {
+  // Typecheck the args.
+  auto *LHS = BO->getLHS();
+  auto TypeLHS = Ctx.classify(LHS->getType());
+  if (!TypeLHS) {
+    return bail(BO);
+  }
+  auto *RHS = BO->getRHS();
+  auto TypeRHS = Ctx.classify(RHS->getType());
+  if (!TypeRHS) {
+    return bail(BO);
+  }
+
+  // Peephole optimisations.
+  if (auto T = Ctx.classify(BO->getType())) {
+    switch (BO->getOpcode()) {
+    case BO_Assign:
+      if (auto *DE = dyn_cast<DeclRefExpr>(BO->getLHS())) {
+        bool IsReference = DE->getDecl()->getType()->isReferenceType();
+        if (*TypeLHS == *T && *TypeRHS == *T && !IsReference) {
+          if (auto *PD = dyn_cast<ParmVarDecl>(DE->getDecl())) {
+            return withParam(PD, [this, RHS, BO](unsigned Idx) -> Error {
+              if (auto E = Visit(RHS))
+                return E;
+              return bail(BO);
+            });
+          }
+
+          if (auto *VD = dyn_cast<VarDecl>(DE->getDecl())) {
+            return withLocal(VD, [this, T, RHS, DE](const Local &L) -> Error {
+              if (auto E = Visit(RHS))
+                return E;
+              emitSET_LOCAL(*T, L.Location, DE);
+              return Error::success();
+            });
+          }
+        }
+      }
+      break;
+    case BO_Comma: {
+      if (auto E = discard(LHS))
+        return E;
+      if (auto E = discard(RHS))
+        return E;
+      return Error::success();
+    }
+    default:
+      break;
+    }
+  }
+
+  if (auto E = Visit(BO))
+    return E;
+
+  emitPOP(*Ctx.classify(BO), BO);
+  return Error::success();
+}
+
+Error Compiler::withParam(const ParmVarDecl *P,
+                          std::function<Error(unsigned)> &&f) {
+  auto It = Params.find(P);
+  if (It == Params.end()) {
+    return bail(P);
+  }
+  return f(It->second);
+}
+
+const Compiler::Local &Compiler::createLocal(const VarDecl *V, unsigned Size,
+                                             unsigned ElemSize,
+                                             PrimType ElemTy) {
+  auto Offset = NextLocalOffset;
+  // TODO: reserve space for block metadata
+  auto Location = NextLocalOffset;
+  NextLocalOffset += align(Size);
+  Local L{Location, Size};
+  bool IsMutable = !V->getType().isConstQualified();
+  bool IsArray = V->getType()->isArrayType();
+  Top->addDescriptor({V, Offset, Size, ElemSize, ElemTy, IsMutable, IsArray});
+  return Locals.insert({V, L}).first->second;
+}
+
+Error Compiler::withLocal(const VarDecl *V,
+                          std::function<Error(const Local &)> &&f) {
+  auto It = Locals.find(V);
+  if (It == Locals.end()) {
+    return bail(V);
+  }
+  return f(It->second);
+}
+
+void Compiler::emitLabel(LabelTy Label) {
+  size_t Offset = P.getOffset();
+  LabelOffsets.insert({Label, Offset});
+  auto It = LabelRelocs.find(Label);
+  if (It != LabelRelocs.end()) {
+    for (unsigned Reloc : It->second) {
+      P.write<int32_t>(Reloc, Offset - static_cast<int32_t>(Reloc));
+    }
+    LabelRelocs.erase(It);
+  }
+}
+
+void Compiler::emitJT(LabelTy Label) {
+  emitOp(EO_JT, {});
+  emitReloc(Label);
+}
+
+void Compiler::emitJF(LabelTy Label) {
+  emitOp(EO_JF, {});
+  emitReloc(Label);
+}
+
+void Compiler::emitJMP(LabelTy Label) {
+  emitOp(EO_JMP, {});
+  emitReloc(Label);
+}
+
+template <PrimType... Tys>
+void Compiler::emitOp(Opcode Op, const typename PrimConv<Tys>::T &... Args,
+                      const SourceInfo &L) {
+  if (L)
+    P.emitSource(L);
+  P.emitBytes(reinterpret_cast<const char *>(&Op), sizeof(Op));
+  (void)std::initializer_list<int>{
+      (P.emitBytes(reinterpret_cast<const char *>(&Args), sizeof(Args)), 0)...};
+}
+
+void Compiler::emitReloc(LabelTy Label) {
+  uint32_t Offset;
+
+  auto Position = P.getOffset();
+  auto It = LabelOffsets.find(Label);
+  if (It != LabelOffsets.end()) {
+    Offset = It->second - static_cast<int32_t>(Position);
+  } else {
+    Offset = 0;
+    LabelRelocs[Label].push_back(Position);
+  }
+
+  P.emitBytes(reinterpret_cast<const char *>(&Offset), sizeof(Offset));
+}
+
+//===----------------------------------------------------------------------===//
+// Opcode emitters
+//===----------------------------------------------------------------------===//
+
+#define GET_EMITTER_IMPL
+#include "Opcodes.inc"
+#undef GET_EMITTER_IMPL
diff --git a/clang/lib/AST/ExprVM/Context.h b/clang/lib/AST/ExprVM/Context.h
new file mode 100644
--- /dev/null
+++ b/clang/lib/AST/ExprVM/Context.h
@@ -0,0 +1,78 @@
+//===--- Context.h - Context for the constexpr VM ---------------*- C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+//
+// Defines the constexpr execution context.
+//
+// The execution context manages cached bytecode and the global context.
+// It invokes the compiler and interpreter, propagating errors.
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_CLANG_AST_EXPRVM_CONTEXT_H
+#define LLVM_CLANG_AST_EXPRVM_CONTEXT_H
+
+#include "Context.h"
+#include "clang/AST/APValue.h"
+#include "llvm/Support/Error.h"
+
+namespace clang {
+class ASTContext;
+class Stmt;
+class FunctionDecl;
+
+namespace vm {
+class Function;
+class Program;
+class State;
+enum PrimType : unsigned;
+
+/// Wrapper around interpreter termination results.
+enum class InterpResult {
+  /// Interpreter successfully computed a value.
+  SUCCESS,
+  /// Interpreter encountered an error and quit.
+  FAIL,
+  /// Interpreter encountered an unimplemented feature, AST fallback.
+  BAIL,
+};
+
+/// Holds all information required to evaluate constexpr code in a module.
+class Context {
+public:
+  /// Initialises the constexpr VM.
+  Context(ASTContext &Ctx);
+
+  /// Cleans up the constexpr VM.
+  ~Context();
+
+  /// Compiles a function and links it to the constexpr program.
+  llvm::Expected<Function *> compileFunction(const FunctionDecl *F);
+
+  /// Evaluats a function call.
+  InterpResult executeFunction(State &Parent, Function *Func,
+                               llvm::ArrayRef<APValue> Args, APValue &Result);
+
+  /// Returns the AST context.
+  ASTContext &getASTContext() const { return Ctx; }
+
+  /// Classifies an expression.
+  llvm::Optional<PrimType> classify(QualType T);
+  /// Classifies a type.
+  llvm::Optional<PrimType> classify(const Expr *E);
+
+private:
+  /// Current compilation context.
+  ASTContext &Ctx;
+  /// Constexpr program.
+  std::unique_ptr<Program> P;
+};
+
+} // namespace vm
+} // namespace clang
+
+#endif
diff --git a/clang/lib/AST/ExprVM/Context.cpp b/clang/lib/AST/ExprVM/Context.cpp
new file mode 100644
--- /dev/null
+++ b/clang/lib/AST/ExprVM/Context.cpp
@@ -0,0 +1,124 @@
+//===--- Context.cpp - Context for the constexpr VM -------------*- C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include "Context.h"
+#include "Compiler.h"
+#include "Interp.h"
+#include "InterpFrame.h"
+#include "InterpStack.h"
+#include "Program.h"
+#include "Type.h"
+#include "clang/AST/Expr.h"
+
+using namespace clang;
+using namespace clang::vm;
+
+Context::Context(ASTContext &Ctx) : Ctx(Ctx), P(new Program()) {}
+
+Context::~Context() {}
+
+Expected<Function *> Context::compileFunction(const FunctionDecl *F) {
+  if (auto *Func = P->getFunction(F))
+    return Func;
+  return Compiler(*this, *P, F).compile();
+}
+
+InterpResult Context::executeFunction(State &Parent, Function *Func,
+                                      llvm::ArrayRef<APValue> Args,
+                                      APValue &Result) {
+
+  InterpState State(Parent, *P, *this);
+
+  // Convert the arguments and store them on the stack.
+  if (!Parent.checkingPotentialConstantExpression()) {
+    auto *F = Func->getDecl();
+    auto &Stk = State.Stk;
+
+    assert(Args.size() == F->getNumParams() && "Missing arguments");
+
+    for (unsigned I = 0, N = F->getNumParams(); I < N; ++I) {
+      auto *ParamDecl = F->getParamDecl(I);
+      auto &Value = Args[I];
+
+      switch (*classify(ParamDecl->getType())) {
+      case ET_SINT_8:
+        Stk.push<PrimConv<ET_SINT_8>::T>(Value.getInt().getSExtValue());
+        break;
+      case ET_UINT_8:
+        Stk.push<PrimConv<ET_UINT_8>::T>(Value.getInt().getZExtValue());
+        break;
+      case ET_SINT_16:
+        Stk.push<PrimConv<ET_SINT_16>::T>(Value.getInt().getSExtValue());
+        break;
+      case ET_UINT_16:
+        Stk.push<PrimConv<ET_UINT_16>::T>(Value.getInt().getZExtValue());
+        break;
+      case ET_SINT_32:
+        Stk.push<PrimConv<ET_SINT_32>::T>(Value.getInt().getSExtValue());
+        break;
+      case ET_UINT_32:
+        Stk.push<PrimConv<ET_UINT_32>::T>(Value.getInt().getZExtValue());
+        break;
+      case ET_SINT_64:
+        Stk.push<PrimConv<ET_SINT_64>::T>(Value.getInt().getSExtValue());
+        break;
+      case ET_UINT_64:
+        Stk.push<PrimConv<ET_UINT_64>::T>(Value.getInt().getZExtValue());
+        break;
+      case ET_BOOL:
+        Stk.push<PrimConv<ET_BOOL>::T>(Value.getInt().getBoolValue());
+        break;
+      }
+    }
+  }
+
+  State.Current = new InterpFrame(State, Func, nullptr);
+  return Interpret(State, Result) ? InterpResult::SUCCESS : InterpResult::FAIL;
+}
+
+llvm::Optional<PrimType> Context::classify(QualType T) {
+  if (T->isBooleanType()) {
+    return ET_BOOL;
+  }
+
+  if (T->isSignedIntegerType()) {
+    switch (Ctx.getIntWidth(T)) {
+    case 64:
+      return ET_SINT_64;
+    case 32:
+      return ET_SINT_32;
+    case 16:
+      return ET_SINT_16;
+    case 8:
+      return ET_SINT_8;
+    default:
+      return {};
+    }
+  }
+
+  if (T->isUnsignedIntegerType()) {
+    switch (Ctx.getIntWidth(T)) {
+    case 64:
+      return ET_UINT_64;
+    case 32:
+      return ET_UINT_32;
+    case 16:
+      return ET_SINT_16;
+    case 8:
+      return ET_SINT_8;
+    default:
+      return {};
+    }
+  }
+
+  return {};
+}
+
+llvm::Optional<PrimType> Context::classify(const Expr *E) {
+  return classify(E->getType());
+}
diff --git a/clang/lib/AST/ExprVM/Disasm.cpp b/clang/lib/AST/ExprVM/Disasm.cpp
new file mode 100644
--- /dev/null
+++ b/clang/lib/AST/ExprVM/Disasm.cpp
@@ -0,0 +1,58 @@
+//===--- Disasm.cpp - Disassembler for bytecode functions -------*- C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+//
+// Dump method for Function which disassembles the bytecode.
+//
+//===----------------------------------------------------------------------===//
+
+#include "Function.h"
+#include "Opcode.h"
+#include "Program.h"
+#include "Type.h"
+#include "llvm/Support/Compiler.h"
+
+using namespace clang;
+using namespace vm;
+
+namespace {
+
+template <typename T> T Get(const char *&PC) {
+  auto Value =
+      llvm::support::endian::read<T, llvm::support::endianness::native, 1>(PC);
+  PC += sizeof(T);
+  return Value;
+}
+
+} // namespace
+
+LLVM_DUMP_METHOD void Function::dump() const { dump(llvm::errs()); }
+
+LLVM_DUMP_METHOD void Function::dump(llvm::raw_ostream &OS) const {
+  OS << F->getName() << ":\n";
+
+  const char *PC = P.getCodeBegin() + Start;
+
+  auto PrintName = [&OS](const char *Name) {
+    OS << Name;
+    for (long I = 0, N = strlen(Name); I < 30 - N; ++I) {
+      OS << ' ';
+    }
+  };
+
+  while (PC != P.getCodeBegin() + End) {
+    size_t Addr = PC - P.getCodeBegin();
+    Opcode Op = *reinterpret_cast<const Opcode *>(PC);
+    PC += sizeof(Opcode);
+    OS << llvm::format("%8d", Addr) << " ";
+    switch (Op) {
+#define GET_DISASM
+#include "Opcodes.inc"
+#undef GET_DISASM
+    }
+  }
+}
diff --git a/clang/lib/AST/ExprVM/Frame.h b/clang/lib/AST/ExprVM/Frame.h
new file mode 100644
--- /dev/null
+++ b/clang/lib/AST/ExprVM/Frame.h
@@ -0,0 +1,41 @@
+//===--- Frame.h - Call frame for the VM and AST Walker ---------*- C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+//
+// Defines the base class of interpreter and evaluator stack frames.
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_CLANG_AST_EXPRVM_FRAME_H
+#define LLVM_CLANG_AST_EXPRVM_FRAME_H
+
+#include "clang/Basic/SourceLocation.h"
+#include "llvm/Support/raw_ostream.h"
+
+namespace clang {
+class FunctionDecl;
+
+namespace vm {
+
+/// Base class for stack frames, shared between VM and walker.
+class Frame {
+public:
+  virtual ~Frame();
+
+  virtual void describe(llvm::raw_ostream &OS) = 0;
+
+  virtual Frame *getCaller() const = 0;
+
+  virtual SourceLocation getLoc() const = 0;
+
+  virtual const FunctionDecl *getCallee() const = 0;
+};
+
+} // namespace vm
+} // namespace clang
+
+#endif
diff --git a/clang/lib/AST/ExprVM/Frame.cpp b/clang/lib/AST/ExprVM/Frame.cpp
new file mode 100644
--- /dev/null
+++ b/clang/lib/AST/ExprVM/Frame.cpp
@@ -0,0 +1,14 @@
+//===--- Frame.cpp - Call frame for the VM and AST Walker -------*- C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include "Frame.h"
+
+using namespace clang;
+using namespace clang::vm;
+
+Frame::~Frame() {}
diff --git a/clang/lib/AST/ExprVM/Function.h b/clang/lib/AST/ExprVM/Function.h
new file mode 100644
--- /dev/null
+++ b/clang/lib/AST/ExprVM/Function.h
@@ -0,0 +1,126 @@
+//===--- Function.h - Bytecode function for the VM --------------*- C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+//
+// Defines the Function class which holds all bytecode function-specific data.
+//
+// The scope class which describes local variables is also defined here.
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_CLANG_AST_EXPRVM_FUNCTION_H
+#define LLVM_CLANG_AST_EXPRVM_FUNCTION_H
+
+#include "Pointer.h"
+#include "clang/AST/Decl.h"
+#include "llvm/Support/raw_ostream.h"
+
+namespace clang {
+namespace vm {
+class Program;
+enum PrimType : uint32_t;
+
+using CodePtr = const char *;
+
+/// Describes a scope block.
+///
+/// The block gathers all the descriptors of the locals defined in this block.
+class Scope {
+public:
+  using DescVector = llvm::SmallVector<Descriptor, 8>;
+
+  Scope(DescVector &&Descriptors) : Descriptors(std::move(Descriptors)) {}
+
+  llvm::iterator_range<DescVector::iterator> descriptors() {
+    return llvm::make_range(Descriptors.begin(), Descriptors.end());
+  }
+
+private:
+  /// Object descriptors in this block.
+  DescVector Descriptors;
+};
+
+/// Bytecode function.
+///
+/// Contains links to the bytecode of the function, as well as metadata
+/// describing all arguments and stack-local variables.
+class Function {
+public:
+  /// Returns the size of the function's local stack.
+  unsigned getFrameSize() const { return FrameSize; }
+  /// Returns the size of the argument stack.
+  unsigned getArgSize() const { return ArgSize; }
+
+  /// Returns a pointer to the start of the code.
+  CodePtr getCodeBegin() const;
+
+  /// Returns the address of the function.
+  uint32_t getAddress() const { return Start; }
+
+  /// Returns the function's name.
+  StringRef getName() const { return F->getName(); }
+
+  /// Returns the original FunctionDecl.
+  const FunctionDecl *getDecl() const { return F; }
+
+  /// Returns a parameter descriptor.
+  Descriptor *getParamDescriptor(unsigned Offset) const;
+
+  /// Range over the scope blocks.
+  llvm::iterator_range<llvm::SmallVector<Scope, 2>::iterator> scopes() {
+    return llvm::make_range(Scopes.begin(), Scopes.end());
+  }
+
+  /// Range over argument types.
+  using arg_reverse_iterator = SmallVectorImpl<PrimType>::reverse_iterator;
+  llvm::iterator_range<arg_reverse_iterator> args_reverse() {
+    return llvm::make_range(Args.rbegin(), Args.rend());
+  }
+
+  /// Returns a specific scope.
+  Scope &getScope(unsigned Idx) { return Scopes[Idx]; }
+
+private:
+  /// Only Program is allowed to construct this.
+  Function(Program &P, const FunctionDecl *F, unsigned Start, unsigned End,
+           unsigned FrameSize, unsigned ArgSize,
+           llvm::SmallVector<Scope, 2> &&Scopes,
+           llvm::SmallVector<PrimType, 8> &&Args,
+           llvm::DenseMap<unsigned, std::unique_ptr<Descriptor>> &&Params);
+
+private:
+  friend class Program;
+
+  /// Program reference.
+  Program &P;
+  /// Declaration this function was compiled from.
+  const FunctionDecl *F;
+  /// Function start index.
+  unsigned Start;
+  /// Function end index.
+  unsigned End;
+  /// Local area size: storage + metadata.
+  unsigned FrameSize;
+  /// Size of the argument stack.
+  unsigned ArgSize;
+  /// List of block descriptors.
+  llvm::SmallVector<Scope, 2> Scopes;
+  /// List of argument types.
+  llvm::SmallVector<PrimType, 8> Args;
+  /// Map from byte offset to parameter descriptor.
+  llvm::DenseMap<unsigned, std::unique_ptr<Descriptor>> Params;
+
+public:
+  /// Dumps the disassembled bytecode to \c llvm::errs().
+  void dump() const;
+  void dump(llvm::raw_ostream &OS) const;
+};
+
+} // namespace vm
+} // namespace clang
+
+#endif
diff --git a/clang/lib/AST/ExprVM/Function.cpp b/clang/lib/AST/ExprVM/Function.cpp
new file mode 100644
--- /dev/null
+++ b/clang/lib/AST/ExprVM/Function.cpp
@@ -0,0 +1,30 @@
+//===--- Function.h - Bytecode function for the VM --------------*- C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include "Function.h"
+#include "Program.h"
+#include "clang/AST/Decl.h"
+
+using namespace clang;
+using namespace clang::vm;
+
+Function::Function(
+    Program &P, const FunctionDecl *F, unsigned Start, unsigned End,
+    unsigned FrameSize, unsigned ArgSize, llvm::SmallVector<Scope, 2> &&Scopes,
+    llvm::SmallVector<PrimType, 8> &&Args,
+    llvm::DenseMap<unsigned, std::unique_ptr<Descriptor>> &&Params)
+    : P(P), F(F), Start(Start), End(End), FrameSize(FrameSize),
+      ArgSize(ArgSize), Scopes(std::move(Scopes)), Args(std::move(Args)),
+      Params(std::move(Params)) {}
+
+CodePtr Function::getCodeBegin() const { return P.getCodeBegin() + Start; }
+
+Descriptor *Function::getParamDescriptor(unsigned Offset) const {
+  auto It = Params.find(Offset);
+  return It == Params.end() ? nullptr : It->second.get();
+}
diff --git a/clang/lib/AST/ExprVM/Interp.h b/clang/lib/AST/ExprVM/Interp.h
new file mode 100644
--- /dev/null
+++ b/clang/lib/AST/ExprVM/Interp.h
@@ -0,0 +1,101 @@
+//===--- Interp.h - Interpreter for the constexpr VM ------------*- C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+//
+// Definition of the interpreter state and entry point.
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_CLANG_AST_EXPRVM_INTERP_H
+#define LLVM_CLANG_AST_EXPRVM_INTERP_H
+
+#include "Context.h"
+#include "Function.h"
+#include "InterpStack.h"
+#include "State.h"
+#include "clang/AST/APValue.h"
+#include "clang/AST/ASTDiagnostic.h"
+#include "clang/AST/Expr.h"
+#include "clang/Basic/OptionalDiagnostic.h"
+
+namespace clang {
+namespace vm {
+class Context;
+class Function;
+class InterpStack;
+class InterpFrame;
+
+/// Interpreter context.
+class InterpState final : public State {
+public:
+  InterpState(State &Parent, Program &P, Context &Ctx);
+
+  ~InterpState();
+
+  // Stack frame accessors.
+  Frame *getSplitFrame() { return Parent.getCurrentFrame(); }
+  Frame *getCurrentFrame() override;
+  unsigned getCallStackDepth() override { return CallStackDepth; }
+  const Frame *getBottomFrame() const override {
+    return Parent.getBottomFrame();
+  }
+
+  // Acces objects from the walker context.
+  Expr::EvalStatus &getEvalStatus() const override {
+    return Parent.getEvalStatus();
+  }
+  ASTContext &getCtx() const override { return Parent.getCtx(); }
+
+  // Forward status checks and updates to the walker.
+  bool checkingForOverflow() const override {
+    return Parent.checkingForOverflow();
+  }
+  bool checkingPotentialConstantExpression() const override {
+    return Parent.checkingPotentialConstantExpression();
+  }
+  bool noteUndefinedBehavior() override {
+    return Parent.noteUndefinedBehavior();
+  }
+  bool hasActiveDiagnostic() override { return Parent.hasActiveDiagnostic(); }
+  void setActiveDiagnostic(bool Flag) override {
+    Parent.setActiveDiagnostic(Flag);
+  }
+  void setFoldFailureDiagnostic(bool Flag) override {
+    Parent.setFoldFailureDiagnostic(Flag);
+  }
+  bool hasPriorDiagnostic() override { return Parent.hasPriorDiagnostic(); }
+
+  /// Reports overflow and return true if evaluation should continue.
+  bool reportOverflow(const Expr *E, const llvm::APSInt &Value);
+
+  /// Notes the location of a descriptor.
+  void noteLocation(const Descriptor *Desc);
+
+private:
+  /// AST Walker state.
+  State &Parent;
+
+public:
+  /// Reference to the module containing all bytecode.
+  Program &P;
+  /// Interpreter Context.
+  Context &Ctx;
+  /// Temporary stack.
+  InterpStack Stk;
+  /// The current frame.
+  InterpFrame *Current = nullptr;
+  /// Call stack depth.
+  unsigned CallStackDepth;
+};
+
+/// Interpreter entry point.
+bool Interpret(InterpState &S, APValue &Result);
+
+} // namespace vm
+} // namespace clang
+
+#endif
diff --git a/clang/lib/AST/ExprVM/Interp.cpp b/clang/lib/AST/ExprVM/Interp.cpp
new file mode 100644
--- /dev/null
+++ b/clang/lib/AST/ExprVM/Interp.cpp
@@ -0,0 +1,400 @@
+//===--- Interp.cpp - Interpreter for the constexpr VM ----------*- C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include "Interp.h"
+#include "Function.h"
+#include "InterpFrame.h"
+#include "InterpStack.h"
+#include "Opcode.h"
+#include "Program.h"
+#include "State.h"
+#include "Type.h"
+#include "clang/AST/ASTContext.h"
+#include "clang/AST/ASTDiagnostic.h"
+#include "clang/AST/CXXInheritance.h"
+#include "clang/AST/Expr.h"
+#include "llvm/ADT/APSInt.h"
+#include "llvm/Support/Endian.h"
+#include <limits>
+#include <vector>
+
+using namespace clang;
+using namespace clang::vm;
+
+using APSInt = llvm::APSInt;
+
+InterpState::InterpState(State &Parent, Program &P, Context &Ctx)
+    : Parent(Parent), P(P), Ctx(Ctx), Current(nullptr),
+      CallStackDepth(Parent.getCallStackDepth() + 1) {}
+
+InterpState::~InterpState() {
+  while (Current) {
+    InterpFrame *Next = Current->Caller;
+    delete Current;
+    Current = Next;
+  }
+}
+
+Frame *InterpState::getCurrentFrame() {
+  return Current->Caller ? Current : Parent.getCurrentFrame();
+}
+
+bool InterpState::reportOverflow(const Expr *E, const llvm::APSInt &Value) {
+  QualType Type = E->getType();
+  CCEDiag(E, diag::note_constexpr_overflow) << Value << Type;
+  return noteUndefinedBehavior();
+}
+
+void InterpState::noteLocation(const Descriptor *Desc) {
+  if (auto *D = Desc->Source.dyn_cast<const ValueDecl *>())
+    Note(D->getLocation(), diag::note_declared_at);
+  if (auto *E = Desc->Source.dyn_cast<const Expr *>())
+    Note(E->getExprLoc(), diag::note_constexpr_temporary_here);
+}
+
+namespace {
+
+/// Reads a primitive and advances the program counter.
+template <typename T> T read(CodePtr &PC) {
+  T Value =
+      llvm::support::endian::read<T, llvm::support::endianness::native, 1>(PC);
+  PC += sizeof(T);
+  return Value;
+}
+
+/// Converts an integer into an APSInt.
+template <typename T>
+APSInt MakeAPSInt(T Value, unsigned Digits = sizeof(T) * 8) {
+  llvm::APInt Boxed(Digits, Value, std::is_signed<T>::value);
+  return APSInt(Boxed, std::is_unsigned<T>::value);
+}
+
+/// Convers a value to an APValue.
+template <typename T> APValue MakeValue(const T &Value) {
+  return APValue(MakeAPSInt<T>(Value));
+}
+
+template <> APValue MakeValue<bool>(const bool &V) {
+  return {APValue(APSInt(llvm::APInt(1, V, false), true))};
+}
+
+//===----------------------------------------------------------------------===//
+// ADD, SUB, MUL
+//===----------------------------------------------------------------------===//
+
+template <typename T, unsigned BitWidth, bool (*OpFW)(T, T, T *),
+          template <typename U> class OpAP>
+bool AddSubMul(InterpState &S, CodePtr OpPC) {
+  const T RHS = S.Stk.pop<T>();
+  const T LHS = S.Stk.pop<T>();
+
+  // Unsigned overflows are not UB - defined as modular arithmetic.
+  // Signed path must handle overflows as they are UB.
+  if (std::is_unsigned<T>::value) {
+    S.Stk.push<T>(OpAP<T>()(LHS, RHS));
+    return true;
+  } else {
+    // Fast path - add the numbers with fixed width.
+    T Result;
+    if (!OpFW(LHS, RHS, &Result)) {
+      S.Stk.push<T>(Result);
+      return true;
+    }
+
+    // If for some reason evaluation continues, use the truncated results.
+    S.Stk.push<T>(Result);
+
+    // Slow path - compute the result using another bit of precision.
+    APSInt APLHS = MakeAPSInt(LHS, BitWidth);
+    APSInt APRHS = MakeAPSInt(RHS, BitWidth);
+    APSInt Value = OpAP<APSInt>()(APLHS, APRHS);
+
+    // Report undefined behaviour, stopping if required.
+    const Expr *E = S.P.getExpr(OpPC);
+    QualType Type = E->getType();
+    if (S.checkingForOverflow()) {
+      auto Trunc = Value.trunc(APLHS.getBitWidth()).toString(10);
+      auto Loc = E->getExprLoc();
+      S.report(Loc, diag::warn_integer_constant_overflow) << Trunc << Type;
+      return true;
+    } else {
+      S.CCEDiag(E, diag::note_constexpr_overflow) << Value << Type;
+      return S.noteUndefinedBehavior();
+    }
+  }
+};
+
+template <typename T> bool AddFn(T a, T b, T *r) {
+  return __builtin_add_overflow(a, b, r);
+}
+
+template <typename T> bool SubFn(T a, T b, T *r) {
+  return __builtin_sub_overflow(a, b, r);
+}
+
+template <typename T> bool MulFn(T a, T b, T *r) {
+  return __builtin_mul_overflow(a, b, r);
+}
+
+template <typename T> bool ADD(InterpState &S, CodePtr OpPC) {
+  constexpr auto BitWidth = sizeof(T) * CHAR_BIT + 1;
+  return AddSubMul<T, BitWidth, AddFn<T>, std::plus>(S, OpPC);
+}
+
+template <typename T> bool SUB(InterpState &S, CodePtr OpPC) {
+  constexpr auto BitWidth = sizeof(T) * CHAR_BIT + 1;
+  return AddSubMul<T, BitWidth, SubFn<T>, std::minus>(S, OpPC);
+}
+
+template <typename T> bool MUL(InterpState &S, CodePtr OpPC) {
+  constexpr auto BitWidth = sizeof(T) * CHAR_BIT * 2;
+  return AddSubMul<T, BitWidth, MulFn<T>, std::multiplies>(S, OpPC);
+}
+
+//===----------------------------------------------------------------------===//
+// DIV, REM
+//===----------------------------------------------------------------------===//
+
+template <typename T, T (*OpFW)(T, T), typename OpAP>
+bool DivRem(InterpState &S, CodePtr OpPC) {
+  const T RHS = S.Stk.pop<T>();
+  const T LHS = S.Stk.pop<T>();
+
+  // Bail on division by zero.
+  if (RHS == 0) {
+    S.FFDiag(S.P.getSource(OpPC), diag::note_expr_divide_by_zero);
+    return false;
+  }
+
+  if (std::is_signed<T>::value) {
+    // (-MAX - 1) / -1 = MAX + 1 overflows.
+    if (LHS == std::numeric_limits<T>::min() && RHS == static_cast<T>(-1)) {
+      // Push the truncated value in case the interpreter continues.
+      S.Stk.push<T>(std::numeric_limits<T>::min());
+
+      // Compute the actual value for the diagnostic.
+      constexpr size_t BitWidth = sizeof(T) * CHAR_BIT + 1;
+      APSInt Value =
+          OpAP()(MakeAPSInt(LHS, BitWidth), MakeAPSInt(RHS, BitWidth));
+      return S.reportOverflow(S.P.getExpr(OpPC), Value);
+    }
+  }
+
+  // Safe to execute division here.
+  S.Stk.push<T>(OpFW(LHS, RHS));
+  return true;
+}
+
+template <typename T> T DivFn(T a, T b) { return a / b; }
+
+template <typename T> T RemFn(T a, T b) { return a % b; }
+
+template <typename T> bool DIV(InterpState &S, CodePtr OpPC) {
+  return DivRem<T, DivFn<T>, std::divides<APSInt>>(S, OpPC);
+}
+
+template <typename T> bool REM(InterpState &S, CodePtr OpPC) {
+  return DivRem<T, RemFn<T>, std::modulus<APSInt>>(S, OpPC);
+}
+
+//===----------------------------------------------------------------------===//
+// EQ, NE, GT, GE, LT, LE
+//===----------------------------------------------------------------------===//
+
+template <template <typename U> class CmpFn, typename T>
+bool CmpValues(InterpState &S, CodePtr OpPC, const T &LHS, const T &RHS) {
+  S.Stk.push<PrimConv<ET_BOOL>::T>(CmpFn<T>()(LHS, RHS));
+  return true;
+}
+
+template <typename T, template <typename U> class CmpFn>
+bool Cmp(InterpState &S, CodePtr OpPC) {
+  const T RHS = S.Stk.pop<T>();
+  const T LHS = S.Stk.pop<T>();
+  return CmpValues<CmpFn>(S, OpPC, LHS, RHS);
+}
+
+template <typename T> bool EQ(InterpState &S, CodePtr OpPC) {
+  return Cmp<T, std::equal_to>(S, OpPC);
+}
+template <typename T> bool NE(InterpState &S, CodePtr OpPC) {
+  return Cmp<T, std::not_equal_to>(S, OpPC);
+}
+template <typename T> bool LT(InterpState &S, CodePtr OpPC) {
+  return Cmp<T, std::less>(S, OpPC);
+}
+template <typename T> bool LE(InterpState &S, CodePtr OpPC) {
+  return Cmp<T, std::less_equal>(S, OpPC);
+}
+template <typename T> bool GT(InterpState &S, CodePtr OpPC) {
+  return Cmp<T, std::greater>(S, OpPC);
+}
+template <typename T> bool GE(InterpState &S, CodePtr OpPC) {
+  return Cmp<T, std::greater_equal>(S, OpPC);
+}
+
+//===----------------------------------------------------------------------===//
+// MINUS
+//===----------------------------------------------------------------------===//
+
+template <typename T> bool MINUS(InterpState &S, CodePtr OpPC) {
+  const auto &Arg = S.Stk.pop<T>();
+
+  if (std::is_signed<T>::value) {
+    if (Arg == std::numeric_limits<T>::min()) {
+      // Push the truncated value in case the interpreter continues.
+      S.Stk.push<T>(std::numeric_limits<T>::min());
+
+      // Compute the actual value for the diagnostic.
+      constexpr size_t BitWidth = sizeof(T) * CHAR_BIT + 1;
+      APSInt Value = std::negate<APSInt>()(MakeAPSInt(Arg, BitWidth));
+      return S.reportOverflow(S.P.getExpr(OpPC), Value);
+    }
+  }
+
+  S.Stk.push<T>(std::negate<T>()(Arg));
+  return true;
+}
+
+//===----------------------------------------------------------------------===//
+// DUP, POP, TEST
+//===----------------------------------------------------------------------===//
+
+template <typename T> bool DUP(InterpState &S, CodePtr OpPC) {
+  S.Stk.push<T>(S.Stk.peek<T>());
+  return true;
+}
+template <typename T> bool POP(InterpState &S, CodePtr OpPC) {
+  S.Stk.pop<T>();
+  return true;
+}
+
+template <typename T> bool IsTrue(const T &V) { return V != 0; }
+
+template <typename T> bool TEST(InterpState &S, CodePtr OpPC) {
+  S.Stk.push<bool>(IsTrue<T>(S.Stk.pop<T>()));
+  return true;
+}
+
+//===----------------------------------------------------------------------===//
+// CONST
+//===----------------------------------------------------------------------===//
+
+template <typename T> bool CONST(InterpState &S, CodePtr OpPC, const T &Arg) {
+  S.Stk.push<T>(Arg);
+  return true;
+}
+
+//===----------------------------------------------------------------------===//
+// GET_LOCAL, SET_LOCAL, GET_PARAM, GET_GLOBAL
+//===----------------------------------------------------------------------===//
+
+template <typename T>
+bool GET_LOCAL(InterpState &S, CodePtr OpPC, PrimConv<ET_UINT_32>::T I) {
+  S.Stk.push<T>(S.Current->getLocal<T>(I));
+  return true;
+}
+
+template <typename T>
+bool SET_LOCAL(InterpState &S, CodePtr OpPC, PrimConv<ET_UINT_32>::T I) {
+  S.Current->setLocal<T>(I, S.Stk.pop<T>());
+  return true;
+}
+
+template <typename T>
+bool GET_PARAM(InterpState &S, CodePtr OpPC, PrimConv<ET_UINT_32>::T I) {
+  if (S.checkingPotentialConstantExpression()) {
+    return false;
+  }
+  S.Stk.push<T>(S.Current->getArg<T>(I));
+  return true;
+}
+
+//===----------------------------------------------------------------------===//
+// RET
+//===----------------------------------------------------------------------===//
+
+template <typename T> bool RET(InterpState &S, CodePtr &PC, APValue &Result) {
+  S.CallStackDepth--;
+  const T &Ret = S.Stk.pop<T>();
+  if (InterpFrame *Caller = S.Current->Caller) {
+    S.Current->popArgs();
+    PC = S.Current->RetPC;
+    delete S.Current;
+    S.Current = Caller;
+    S.Stk.push<T>(Ret);
+  } else {
+    delete S.Current;
+    S.Current = nullptr;
+    Result = MakeValue<T>(Ret);
+  }
+  return true;
+}
+
+bool RET(InterpState &S, CodePtr &PC, APValue &Result) {
+  S.CallStackDepth--;
+  if (InterpFrame *Caller = S.Current->Caller) {
+    S.Current->popArgs();
+    PC = S.Current->RetPC;
+    delete S.Current;
+    S.Current = Caller;
+  } else {
+    delete S.Current;
+    S.Current = nullptr;
+  }
+  return true;
+}
+
+bool NORET(InterpState &S, CodePtr &PC, APValue &Ret) {
+  SourceLocation EndLoc = S.Current->getCallee()->getEndLoc();
+  S.FFDiag(EndLoc, diag::note_constexpr_no_return);
+  return false;
+}
+
+} // namespace
+
+namespace clang {
+namespace vm {
+
+bool Interpret(InterpState &S, APValue &Result) {
+  CodePtr PC = S.Current->Func->getCodeBegin();
+
+  for (;;) {
+    CodePtr OpPC = PC;
+    auto Op = read<Opcode>(PC);
+
+    switch (Op) {
+    case EO_JMP: {
+      auto Offset = read<int32_t>(PC);
+      PC = PC - sizeof(int32_t) + Offset;
+      continue;
+    }
+    case EO_JT: {
+      auto Offset = read<int32_t>(PC);
+      if (S.Stk.pop<bool>()) {
+        PC = PC - sizeof(int32_t) + Offset;
+      }
+      continue;
+    }
+    case EO_JF: {
+      auto Offset = read<int32_t>(PC);
+      if (!S.Stk.pop<bool>()) {
+        PC = PC - sizeof(int32_t) + Offset;
+      }
+      continue;
+    }
+
+#define GET_INTERP
+#include "Opcodes.inc"
+#undef GET_INTERP
+    }
+  }
+}
+
+} // namespace vm
+} // namespace clang
diff --git a/clang/lib/AST/ExprVM/InterpFrame.h b/clang/lib/AST/ExprVM/InterpFrame.h
new file mode 100644
--- /dev/null
+++ b/clang/lib/AST/ExprVM/InterpFrame.h
@@ -0,0 +1,96 @@
+//===--- InterpFrame.h - Call Frame implementation for the VM ---*- C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+//
+// Defines the class storing information about stack frames in the interpreter.
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_CLANG_AST_EXPRVM_INTERPFRAME_H
+#define LLVM_CLANG_AST_EXPRVM_INTERPFRAME_H
+
+#include "Frame.h"
+#include "Pointer.h"
+#include "Program.h"
+#include "State.h"
+#include <cstdint>
+#include <vector>
+
+namespace clang {
+namespace vm {
+class Function;
+class InterpState;
+
+/// Frame storing local variables.
+class InterpFrame final : public Frame {
+public:
+  /// Reference to the interpreter state.
+  InterpState &S;
+  /// Reference to the function being executed.
+  Function *Func;
+  /// The frame of the previous function.
+  InterpFrame *Caller;
+  /// Return address.
+  CodePtr RetPC = nullptr;
+  /// Size of the arguments.
+  unsigned ArgSize;
+
+  /// Creates a new frame.
+  InterpFrame(InterpState &S, Function *Func, InterpFrame *Caller);
+
+  /// Pops the arguments off the stack.
+  void popArgs();
+
+  /// Describes the frame with arguments for diagnostic purposes.
+  void describe(llvm::raw_ostream &OS);
+
+  /// Returns the parent frame object.
+  Frame *getCaller() const;
+
+  /// Returns the location of the call to the frame.
+  SourceLocation getLoc() const;
+
+  /// Returns the caller.
+  const FunctionDecl *getCallee() const;
+
+  /// Accessors to locals and parameters.
+  template <typename T> const T &getLocal(unsigned Offset) {
+    return localRef<T>(Offset);
+  }
+
+  /// Returns non-mutable reference to an argument.
+  template <typename T> const T &getArg(unsigned Offset) {
+    return stackRef<T>(Offset);
+  }
+
+  /// Mutates a local.
+  template <typename T> void setLocal(unsigned Offset, const T &Value) {
+    localRef<T>(Offset) = Value;
+  }
+
+private:
+  /// Returns an original argument from the stack.
+  template <typename T> const T &stackRef(unsigned Offset) {
+    return *reinterpret_cast<const T *>(Args - ArgSize + Offset);
+  }
+
+  /// Returns an offset to a local.
+  template <typename T> T &localRef(unsigned Offset) {
+    return *reinterpret_cast<T *>(Locals.get() + Offset);
+  }
+
+private:
+  /// Pointer to the arguments in the callee's frame.
+  char *Args = nullptr;
+  /// Fixed, initial storage for known local variables.
+  std::unique_ptr<char[]> Locals;
+};
+
+} // namespace vm
+} // namespace clang
+
+#endif
diff --git a/clang/lib/AST/ExprVM/InterpFrame.cpp b/clang/lib/AST/ExprVM/InterpFrame.cpp
new file mode 100644
--- /dev/null
+++ b/clang/lib/AST/ExprVM/InterpFrame.cpp
@@ -0,0 +1,60 @@
+//===--- InterpFrame.cpp - Call Frame implementation for the VM -*- C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include "InterpFrame.h"
+#include "Function.h"
+#include "Interp.h"
+#include "InterpStack.h"
+#include "Program.h"
+#include "Type.h"
+
+using namespace clang;
+using namespace clang::vm;
+
+InterpFrame::InterpFrame(InterpState &S, Function *Func, InterpFrame *Caller)
+    : S(S), Func(Func), Caller(Caller), ArgSize(Func->getArgSize()) {
+
+  if (auto FrameSize = Func->getFrameSize()) {
+    Locals = llvm::make_unique<char[]>(FrameSize);
+  }
+
+  Args = static_cast<char *>(S.Stk.top());
+}
+
+void InterpFrame::popArgs() {
+  for (PrimType Ty : Func->args_reverse()) {
+    TYPE_SWITCH(Ty, S.Stk.pop<T>());
+  }
+}
+
+void InterpFrame::describe(llvm::raw_ostream &OS) {
+  OS << Func->getName() << "(";
+
+  auto *F = Func->getDecl();
+  for (unsigned I = 0, N = F->getNumParams(), Off = 0; I < N; ++I) {
+    auto Ty = *S.Ctx.classify(F->getParamDecl(I)->getType());
+    TYPE_SWITCH(Ty, OS << stackRef<T>(Off));
+    Off += primSize(Ty);
+    if (I + 1 != N)
+      OS << ", ";
+  }
+
+  OS << ")";
+}
+
+Frame *InterpFrame::getCaller() const {
+  if (Caller->Caller) {
+    return Caller;
+  } else {
+    return S.getSplitFrame();
+  }
+}
+
+SourceLocation InterpFrame::getLoc() const { return S.P.getLocation(RetPC); }
+
+const FunctionDecl *InterpFrame::getCallee() const { return Func->getDecl(); }
diff --git a/clang/lib/AST/ExprVM/InterpStack.h b/clang/lib/AST/ExprVM/InterpStack.h
new file mode 100644
--- /dev/null
+++ b/clang/lib/AST/ExprVM/InterpStack.h
@@ -0,0 +1,97 @@
+//===--- InterpStack.h - Stack implementation for the VM --------*- C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+//
+// Defines the upwards-growing stack used by the interpreter.
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_CLANG_AST_EXPRVM_INTERPSTACK_H
+#define LLVM_CLANG_AST_EXPRVM_INTERPSTACK_H
+
+#include <memory>
+
+namespace clang {
+namespace vm {
+
+/// Stack frame storing temporaries and parameters.
+class InterpStack final {
+public:
+  InterpStack() {}
+
+  ~InterpStack();
+
+  template <typename T, typename... Tys> void push(Tys &&... Args) {
+    new (grow(aligned_size<T>())) T(std::forward<Tys>(Args)...);
+  }
+
+  template <typename T> T pop() {
+    auto *Ptr = &peek<T>();
+    auto Value = std::move(*Ptr);
+    Ptr->~T();
+    shrink(aligned_size<T>());
+    return Value;
+  }
+
+  template <typename T> T &peek() {
+    return *reinterpret_cast<T *>(peek(aligned_size<T>()));
+  }
+
+  void *top() { return Chunk ? peek(0) : nullptr; }
+
+  size_t size() const { return StackSize; }
+
+private:
+  /// All stack slots are aligned to the native pointer alignment for storage.
+  template <typename T> constexpr size_t aligned_size() const {
+    static_assert(alignof(void *) % alignof(T) == 0, "invalid alignment");
+    return ((sizeof(T) + alignof(void *) - 1) / alignof(void *)) *
+           alignof(void *);
+  }
+
+  /// Grows the stack to accomodate a value and returns a pointer to it.
+  void *grow(size_t Size);
+  /// Returns a pointer from the top of the stack.
+  void *peek(size_t Size);
+  /// Shrinks the stack.
+  void shrink(size_t Size);
+
+  /// Allocate stack space in 1Mb chunks.
+  static constexpr size_t ChunkSize = 1024 * 1024;
+
+  /// Metadata for each stack chunk.
+  ///
+  /// The stack is composed of a linked list of chunks. Whenever an allocation
+  /// is out of bounds, a new chunk is linked. When a chunk becomes empty,
+  /// it is not immediately freed: a chunk is deallocated only when the
+  /// predecessor becomes empty.
+  struct StackChunk {
+    StackChunk *Next;
+    StackChunk *Prev;
+    char *End;
+
+    StackChunk(StackChunk *Prev)
+        : Next(nullptr), Prev(Prev), End(reinterpret_cast<char *>(this + 1)) {}
+
+    /// Returns the size of the chunk, minus the header.
+    size_t size() { return End - start(); }
+
+    /// Returns a pointer to the start of the data region.
+    char *start() { return reinterpret_cast<char *>(this + 1); }
+  };
+  static_assert(sizeof(StackChunk) < ChunkSize, "Invalid chunk size");
+
+  /// First chunk on the stack.
+  StackChunk *Chunk = nullptr;
+  /// Total size of the stack.
+  size_t StackSize = 0;
+};
+
+} // namespace vm
+} // namespace clang
+
+#endif
diff --git a/clang/lib/AST/ExprVM/InterpStack.cpp b/clang/lib/AST/ExprVM/InterpStack.cpp
new file mode 100644
--- /dev/null
+++ b/clang/lib/AST/ExprVM/InterpStack.cpp
@@ -0,0 +1,70 @@
+//===--- InterpStack.cpp - Stack implementation for the VM ------*- C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include "InterpStack.h"
+
+using namespace clang;
+using namespace clang::vm;
+
+InterpStack::~InterpStack() {
+  if (Chunk && Chunk->Next)
+    free(Chunk->Next);
+  if (Chunk)
+    free(Chunk);
+}
+
+void *InterpStack::grow(size_t Size) {
+  assert(Size < ChunkSize - sizeof(StackChunk) && "Object too large");
+
+  if (!Chunk || sizeof(StackChunk) + Chunk->size() + Size > ChunkSize) {
+    if (Chunk && Chunk->Next) {
+      Chunk = Chunk->Next;
+    } else {
+      StackChunk *Next = new (malloc(ChunkSize)) StackChunk(Chunk);
+      if (Chunk)
+        Chunk->Next = Next;
+      Chunk = Next;
+    }
+  }
+
+  auto Object = Chunk->End;
+  Chunk->End += Size;
+  StackSize += Size;
+  return reinterpret_cast<void *>(Object);
+}
+
+void *InterpStack::peek(size_t Size) {
+  assert(Chunk && "Stack is empty!");
+
+  StackChunk *Ptr = Chunk;
+  while (Size > Ptr->size()) {
+    Size -= Ptr->size();
+    Ptr = Ptr->Prev;
+    assert(Ptr && "Offset too large");
+  }
+
+  return reinterpret_cast<void *>(Ptr->End - Size);
+}
+
+void InterpStack::shrink(size_t Size) {
+  assert(Chunk && "Chunk is empty!");
+
+  while (Size > Chunk->size()) {
+    Size -= Chunk->size();
+    if (Chunk->Next) {
+      free(Chunk->Next);
+      Chunk->Next = nullptr;
+    }
+    Chunk->End = Chunk->start();
+    Chunk = Chunk->Prev;
+    assert(Chunk && "Offset too large");
+  }
+
+  Chunk->End -= Size;
+  StackSize -= Size;
+}
diff --git a/clang/lib/AST/ExprVM/Opcode.h b/clang/lib/AST/ExprVM/Opcode.h
new file mode 100644
--- /dev/null
+++ b/clang/lib/AST/ExprVM/Opcode.h
@@ -0,0 +1,30 @@
+//===--- Opcode.h - Opcodes for the constexpr VM ----------------*- C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+//
+// Defines all opcodes executed by the VM and emitted by the compiler.
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_CLANG_AST_EXPRVM_OPCODE_H
+#define LLVM_CLANG_AST_EXPRVM_OPCODE_H
+
+#include <cstdint>
+
+namespace clang {
+namespace vm {
+
+enum Opcode : uint32_t {
+#define GET_OPCODE_NAMES
+#include "Opcodes.inc"
+#undef GET_OPCODE_NAMES
+};
+
+} // namespace vm
+} // namespace clang
+
+#endif
diff --git a/clang/lib/AST/ExprVM/Opcodes.inc b/clang/lib/AST/ExprVM/Opcodes.inc
new file mode 100644
--- /dev/null
+++ b/clang/lib/AST/ExprVM/Opcodes.inc
@@ -0,0 +1,415 @@
+//===--- Opcodes.inc - Opcode defitions for the constexpr VM ----*- C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+//
+// Helper file used to generate opcodes, the interpreter and the disassembler.
+//
+//===----------------------------------------------------------------------===//
+
+
+//===----------------------------------------------------------------------===//
+
+#ifdef GET_OPCODE_NAMES
+# define OPCODE_DEFINE(Name) EO_ ## Name,
+#else
+# define OPCODE_DEFINE(Name)
+#endif
+
+//===----------------------------------------------------------------------===//
+// Disassemblers
+//===----------------------------------------------------------------------===//
+
+#ifdef GET_DISASM
+# define OPCODE_DISASM(Name, Args)      \
+    case EO_ ## Name: {                 \
+      PrintName(#Name);                 \
+      OS << "\t" << Args;               \
+      continue;                         \
+    }
+#else
+# define OPCODE_DISASM(Name, Args)
+#endif
+
+#define OPCODE_DISASM_0(Name) \
+    OPCODE_DISASM(Name, "\n")
+#define OPCODE_DISASM_1(Name, A0) \
+    OPCODE_DISASM(Name, A0 << "\n")
+#define OPCODE_DISASM_2(Name, A0, A1) \
+    OPCODE_DISASM(Name, A0 << " " << A1 << "\n")
+
+#define DISASM_SINT_8   (Get<PrimConv<ET_SINT_8>::T>(PC))
+#define DISASM_UINT_8   (Get<PrimConv<ET_UINT_8>::T>(PC))
+#define DISASM_SINT_16  (Get<PrimConv<ET_SINT_16>::T>(PC))
+#define DISASM_UINT_16  (Get<PrimConv<ET_UINT_16>::T>(PC))
+#define DISASM_SINT_32  (Get<PrimConv<ET_SINT_32>::T>(PC))
+#define DISASM_UINT_32  (Get<PrimConv<ET_UINT_32>::T>(PC))
+#define DISASM_SINT_64  (Get<PrimConv<ET_SINT_64>::T>(PC))
+#define DISASM_UINT_64  (Get<PrimConv<ET_UINT_64>::T>(PC))
+#define DISASM_BOOL     (Get<PrimConv<ET_BOOL>::T>(PC))
+#define DISASM_OFF      (Addr + Get<PrimConv<ET_SINT_32>::T>(PC) + 4)
+
+//===----------------------------------------------------------------------===//
+// Interpreter
+//===----------------------------------------------------------------------===//
+
+#ifdef GET_INTERP
+# define OPCODE_INTERP_1(Name, Arg0)                                           \
+  case EO_ ## Name: {                                                          \
+    auto V = read<PrimConv<Arg0>::T>(PC);                                      \
+    if (!Name(S, OpPC, V))                                                     \
+      return false;                                                            \
+    continue;                                                                  \
+  }
+# define OPCODE_INTERP_T_0(Name, T0)                                           \
+  case EO_ ## Name ## _ ## T0: {                                               \
+    if (!Name<PrimConv<ET_ ## T0>::T>(S, OpPC))                                \
+      return false;                                                            \
+    continue;                                                                  \
+  }
+# define OPCODE_INTERP_T_T_0(Name, T0, T1)                                     \
+  case EO_ ## Name ## _ ## T0 ## _ ## T1: {                                    \
+    if (!Name<PrimConv<ET_ ## T0>::T, PrimConv<ET_ ## T1>::T>(S, OpPC))        \
+      return false;                                                            \
+    continue;                                                                  \
+  }
+# define OPCODE_INTERP_T_1(Name, T0, Arg0)                                     \
+  case EO_ ## Name ## _ ## T0: {                                               \
+    auto V = read<PrimConv<Arg0>::T>(PC);                                      \
+    if (!Name<PrimConv<ET_ ## T0>::T>(S, OpPC, V))                             \
+      return false;                                                            \
+    continue;                                                                  \
+  }
+# define OPCODE_INTERP_RET_T(Name, T0)                                         \
+  case EO_ ## Name ## _ ## T0: {                                               \
+    if (!Name<PrimConv<ET_ ## T0>::T>(S, PC, Result))                          \
+      return false;                                                            \
+    if (!S.Current)                                                            \
+      return true;                                                             \
+    continue;                                                                  \
+  }
+# define OPCODE_INTERP_RET(Name)                                               \
+  case EO_ ## Name ## _VOID: {                                                 \
+    if (!Name(S, PC, Result))                                                  \
+      return false;                                                            \
+    if (!S.Current)                                                            \
+      return true;                                                             \
+    continue;                                                                  \
+  }
+#define OPCODE_INTERP_CALL_1(Name, Arg0)                                       \
+  case EO_ ## Name: {                                                          \
+    auto V = read<PrimConv<Arg0>::T>(PC);                                      \
+    if (!Name(S, PC, V))                                                       \
+      return false;                                                            \
+    continue;                                                                  \
+  }
+#else
+# define OPCODE_INTERP_1(Name, T0)
+# define OPCODE_INTERP_T_0(Name, T0)
+# define OPCODE_INTERP_T_T_0(Name, T0, T1)
+# define OPCODE_INTERP_T_1(Name, T0, Arg0)
+# define OPCODE_INTERP_RET_T(Name, T0)
+# define OPCODE_INTERP_RET(Name)
+# define OPCODE_INTERP_CALL_1(Name, Arg0)
+#endif
+
+//===----------------------------------------------------------------------===//
+// Emitter implementation
+//===----------------------------------------------------------------------===//
+
+#define _CASE_ALU(Name, Args,...)                                              \
+  case ET_BOOL:    emitOp<Args>(EO_##Name##_BOOL,    __VA_ARGS__); break;      \
+  case ET_SINT_8:  emitOp<Args>(EO_##Name##_SINT_8,  __VA_ARGS__); break;      \
+  case ET_UINT_8:  emitOp<Args>(EO_##Name##_UINT_8,  __VA_ARGS__); break;      \
+  case ET_SINT_16: emitOp<Args>(EO_##Name##_SINT_16, __VA_ARGS__); break;      \
+  case ET_UINT_16: emitOp<Args>(EO_##Name##_UINT_16, __VA_ARGS__); break;      \
+  case ET_SINT_32: emitOp<Args>(EO_##Name##_SINT_32, __VA_ARGS__); break;      \
+  case ET_UINT_32: emitOp<Args>(EO_##Name##_UINT_32, __VA_ARGS__); break;      \
+  case ET_SINT_64: emitOp<Args>(EO_##Name##_SINT_64, __VA_ARGS__); break;      \
+  case ET_UINT_64: emitOp<Args>(EO_##Name##_UINT_64, __VA_ARGS__); break;      \
+
+#define _CASE_ALL(Name, Args, ...)                                             \
+  _CASE_ALU(Name, Args, __VA_ARGS__)
+
+#define _BRANCH(Name, T)                                                       \
+  switch (T0) {                                                                \
+  _CASE_ALU(Name##_##T, , L)                                                   \
+  }
+
+#ifdef GET_EMITTER_IMPL
+# define OPCODE_EMITTER_IMPL_ALL_ARG(Name, ArgTy)                              \
+void Compiler::emit##Name(PrimType T, const PrimConv<ArgTy>::T &Arg,           \
+                          const SourceInfo &L) {                               \
+  switch (T) {                                                                 \
+  _CASE_ALL(Name, ArgTy, Arg, L)                                               \
+  }                                                                            \
+}
+#define OPCODE_EMITTER_IMPL_ALL(Name)                                          \
+void Compiler::emit##Name(PrimType T, const SourceInfo &L) {                   \
+  switch (T) {                                                                 \
+  _CASE_ALL(Name, , L)                                                         \
+  }                                                                            \
+}
+#define OPCODE_EMITTER_IMPL_ALU(Name)                                          \
+  void Compiler::emit##Name(PrimType T, const SourceInfo &L) {                 \
+    switch (T) {                                                               \
+    _CASE_ALU(Name, , L)                                                       \
+    }                                                                          \
+  }
+#define OPCODE_EMITTER_IMPL_ALU_ALU(Name)                                      \
+void Compiler::emit ## Name(PrimType T0, PrimType T1, const SourceInfo &L) {   \
+  switch (T1) {                                                                \
+  case ET_BOOL:    _BRANCH(Name, BOOL);    break;                              \
+  case ET_SINT_8:  _BRANCH(Name, SINT_8);  break;                              \
+  case ET_UINT_8:  _BRANCH(Name, UINT_8);  break;                              \
+  case ET_SINT_16: _BRANCH(Name, SINT_16); break;                              \
+  case ET_UINT_16: _BRANCH(Name, UINT_16); break;                              \
+  case ET_SINT_32: _BRANCH(Name, SINT_32); break;                              \
+  case ET_UINT_32: _BRANCH(Name, UINT_32); break;                              \
+  case ET_SINT_64: _BRANCH(Name, SINT_64); break;                              \
+  case ET_UINT_64: _BRANCH(Name, UINT_64); break;                              \
+  }                                                                            \
+}
+#else
+# define OPCODE_EMITTER_IMPL_ALL_ARG(Name, ArgTy)
+# define OPCODE_EMITTER_IMPL_ALL(Name)
+# define OPCODE_EMITTER_IMPL_ALU(Name)
+# define OPCODE_EMITTER_IMPL_ALU_ALU(Name)
+#endif
+
+//===----------------------------------------------------------------------===//
+// Emitter header
+//===----------------------------------------------------------------------===//
+
+#ifdef GET_EMITTER_PROTO
+# define OPCODE_EMITTER_PROTO_T_ARG(Name, ArgTy)                               \
+  void emit##Name(PrimType, const PrimConv<ArgTy>::T &, const SourceInfo &);
+# define OPCODE_EMITTER_PROTO_T(Name)                                          \
+  void emit##Name(PrimType, const SourceInfo &);
+# define OPCODE_EMITTER_PROTO_T_T(Name)                                        \
+  void emit##Name(PrimType, PrimType, const SourceInfo &);
+#else
+# define OPCODE_EMITTER_PROTO_T_ARG(Name, ArgTy)
+# define OPCODE_EMITTER_PROTO_T(Name)
+# define OPCODE_EMITTER_PROTO_T_T(Name)
+#endif
+
+//===----------------------------------------------------------------------===//
+
+#define OP_ARG(Name, Arg)                               \
+  OPCODE_DEFINE(Name)                                   \
+  OPCODE_DISASM_1(Name, DISASM_ ## Arg)                 \
+  OPCODE_INTERP_1(Name, ET_ ## Arg)
+
+#define OP_ARG_TYPE(Name, Type, Arg)                    \
+  OPCODE_DEFINE(Name ## _ ## Type)                      \
+  OPCODE_DISASM_1(Name ## _ ## Type, DISASM_ ## Arg)    \
+  OPCODE_INTERP_T_1(Name, Type, ET_ ## Arg)
+
+#define OP_TYPE(Name, Type)                         \
+  OPCODE_DEFINE(Name ## _ ## Type)                  \
+  OPCODE_DISASM_0(Name ## _ ## Type)                \
+  OPCODE_INTERP_T_0(Name, Type)
+
+#define OP_TYPE_TYPE(Name, T0, T1)                  \
+  OPCODE_DEFINE(Name ## _ ## T0 ## _ ## T1)         \
+  OPCODE_DISASM_0(Name ## _ ## T0 ## _ ## T1)       \
+  OPCODE_INTERP_T_T_0(Name, T0, T1)
+
+#define OP_VOID(Name)                               \
+  OPCODE_DEFINE(Name ## _VOID)                      \
+  OPCODE_DISASM_0(Name ## _VOID)
+
+#define OP_OFF(Name)                                \
+  OPCODE_DEFINE(Name)                               \
+  OPCODE_DISASM_1(Name, DISASM_OFF)
+
+#define OP_UINT_32_UINT_32(Name)                          \
+  OPCODE_DEFINE(Name)                                     \
+  OPCODE_DISASM_2(Name, DISASM_UINT_32, DISASM_UINT_32)
+
+#define OP_CALL(Name)                                  \
+  OPCODE_DEFINE(Name)                                  \
+  OPCODE_DISASM_1(Name, DISASM_UINT_64)                \
+  OPCODE_INTERP_CALL_1(Name, ET_NATIVE_PTR)
+
+#define OP_RET_TYPE(Name, Type)                     \
+  OPCODE_DEFINE(Name ## _ ## Type)                  \
+  OPCODE_DISASM_0(Name ## _ ## Type)                \
+  OPCODE_INTERP_RET_T(Name, Type)
+
+#define OP_RET_VOID(Name)                           \
+  OPCODE_DEFINE(Name ## _VOID)                      \
+  OPCODE_DISASM_0(Name ## _VOID)                    \
+  OPCODE_INTERP_RET(Name)
+
+#define OP_UINT_8_TYPE(Name, Type)  OP_ARG_TYPE(Name, Type, UINT_8)
+#define OP_SINT_8_TYPE(Name, Type)  OP_ARG_TYPE(Name, Type, SINT_8)
+#define OP_UINT_16_TYPE(Name, Type) OP_ARG_TYPE(Name, Type, UINT_16)
+#define OP_SINT_16_TYPE(Name, Type) OP_ARG_TYPE(Name, Type, SINT_16)
+#define OP_UINT_32_TYPE(Name, Type) OP_ARG_TYPE(Name, Type, UINT_32)
+#define OP_SINT_32_TYPE(Name, Type) OP_ARG_TYPE(Name, Type, SINT_32)
+#define OP_UINT_64_TYPE(Name, Type) OP_ARG_TYPE(Name, Type, UINT_64)
+#define OP_SINT_64_TYPE(Name, Type) OP_ARG_TYPE(Name, Type, SINT_64)
+#define OP_BOOL_TYPE(Name, Type)    OP_ARG_TYPE(Name, Type, BOOL)
+
+#define OP_UINT_32(Name)            OP_ARG(Name, UINT_32)
+#define OP_SINT_32(Name)            OP_ARG(Name, SINT_32)
+
+//===----------------------------------------------------------------------===//
+
+#define OP_GROUP_UINT_32_ALL(Name)               \
+  OP_UINT_32_TYPE(Name, SINT_8)                  \
+  OP_UINT_32_TYPE(Name, UINT_8)                  \
+  OP_UINT_32_TYPE(Name, SINT_16)                 \
+  OP_UINT_32_TYPE(Name, UINT_16)                 \
+  OP_UINT_32_TYPE(Name, SINT_32)                 \
+  OP_UINT_32_TYPE(Name, UINT_32)                 \
+  OP_UINT_32_TYPE(Name, SINT_64)                 \
+  OP_UINT_32_TYPE(Name, UINT_64)                 \
+  OP_UINT_32_TYPE(Name, BOOL)                    \
+  OPCODE_EMITTER_IMPL_ALL_ARG(Name, ET_UINT_32)  \
+  OPCODE_EMITTER_PROTO_T_ARG(Name, ET_UINT_32)
+
+#define OP_GROUP_RET_ALL(Name)      \
+  OP_RET_TYPE(Name, SINT_8)         \
+  OP_RET_TYPE(Name, UINT_8)         \
+  OP_RET_TYPE(Name, SINT_16)        \
+  OP_RET_TYPE(Name, UINT_16)        \
+  OP_RET_TYPE(Name, SINT_32)        \
+  OP_RET_TYPE(Name, UINT_32)        \
+  OP_RET_TYPE(Name, SINT_64)        \
+  OP_RET_TYPE(Name, UINT_64)        \
+  OP_RET_TYPE(Name, BOOL)           \
+  OPCODE_EMITTER_IMPL_ALL(Name)     \
+  OPCODE_EMITTER_PROTO_T(Name)
+
+#define OP_GROUP_ALU(Name)          \
+  OP_TYPE(Name, SINT_8)             \
+  OP_TYPE(Name, UINT_8)             \
+  OP_TYPE(Name, SINT_16)            \
+  OP_TYPE(Name, UINT_16)            \
+  OP_TYPE(Name, SINT_32)            \
+  OP_TYPE(Name, UINT_32)            \
+  OP_TYPE(Name, SINT_64)            \
+  OP_TYPE(Name, UINT_64)            \
+  OP_TYPE(Name, BOOL)               \
+  OPCODE_EMITTER_IMPL_ALU(Name)     \
+  OPCODE_EMITTER_PROTO_T(Name)
+
+#define OP_GROUP_ALL(Name)          \
+  OP_TYPE(Name, SINT_8)             \
+  OP_TYPE(Name, UINT_8)             \
+  OP_TYPE(Name, SINT_16)            \
+  OP_TYPE(Name, UINT_16)            \
+  OP_TYPE(Name, SINT_32)            \
+  OP_TYPE(Name, UINT_32)            \
+  OP_TYPE(Name, SINT_64)            \
+  OP_TYPE(Name, UINT_64)            \
+  OP_TYPE(Name, BOOL)               \
+  OPCODE_EMITTER_IMPL_ALL(Name)     \
+  OPCODE_EMITTER_PROTO_T(Name)
+
+#define _OP_GROUP_ALU(Name, Type)    \
+  OP_TYPE_TYPE(Name, Type, SINT_8)   \
+  OP_TYPE_TYPE(Name, Type, UINT_8)   \
+  OP_TYPE_TYPE(Name, Type, SINT_16)  \
+  OP_TYPE_TYPE(Name, Type, UINT_16)  \
+  OP_TYPE_TYPE(Name, Type, SINT_32)  \
+  OP_TYPE_TYPE(Name, Type, UINT_32)  \
+  OP_TYPE_TYPE(Name, Type, SINT_64)  \
+  OP_TYPE_TYPE(Name, Type, UINT_64)  \
+  OP_TYPE_TYPE(Name, Type, BOOL)
+
+#define OP_GROUP_ALU_ALU(Name)       \
+  _OP_GROUP_ALU(Name, SINT_8)        \
+  _OP_GROUP_ALU(Name, UINT_8)        \
+  _OP_GROUP_ALU(Name, SINT_16)       \
+  _OP_GROUP_ALU(Name, UINT_16)       \
+  _OP_GROUP_ALU(Name, SINT_32)       \
+  _OP_GROUP_ALU(Name, UINT_32)       \
+  _OP_GROUP_ALU(Name, SINT_64)       \
+  _OP_GROUP_ALU(Name, UINT_64)       \
+  _OP_GROUP_ALU(Name, BOOL)          \
+  OPCODE_EMITTER_IMPL_ALU_ALU(Name)  \
+  OPCODE_EMITTER_PROTO_T_T(Name)
+
+//===----------------------------------------------------------------------===//
+// Control Flow
+//===----------------------------------------------------------------------===//
+
+OP_OFF(JMP)
+OP_OFF(JT)
+OP_OFF(JF)
+
+OP_GROUP_RET_ALL(RET)
+OP_RET_VOID(RET)
+OP_RET_VOID(NORET)
+
+//===----------------------------------------------------------------------===//
+// Constants
+//===----------------------------------------------------------------------===//
+
+OP_SINT_8_TYPE(CONST, SINT_8)
+OP_UINT_8_TYPE(CONST, UINT_8)
+OP_SINT_16_TYPE(CONST, SINT_16)
+OP_UINT_16_TYPE(CONST, UINT_16)
+OP_SINT_32_TYPE(CONST, SINT_32)
+OP_UINT_32_TYPE(CONST, UINT_32)
+OP_SINT_64_TYPE(CONST, SINT_64)
+OP_UINT_64_TYPE(CONST, UINT_64)
+OP_BOOL_TYPE(CONST, BOOL)
+
+//===----------------------------------------------------------------------===//
+// Frame and parameters
+//===----------------------------------------------------------------------===//
+
+OP_GROUP_UINT_32_ALL(GET_LOCAL)
+OP_GROUP_UINT_32_ALL(SET_LOCAL)
+OP_GROUP_UINT_32_ALL(GET_PARAM)
+
+//===----------------------------------------------------------------------===//
+// Generic Operations
+//===----------------------------------------------------------------------===//
+
+OP_GROUP_ALU(SUB)
+OP_GROUP_ALU(ADD)
+OP_GROUP_ALU(MUL)
+OP_GROUP_ALU(REM)
+OP_GROUP_ALU(DIV)
+
+OP_GROUP_ALL(EQ)
+OP_GROUP_ALL(NE)
+OP_GROUP_ALL(LT)
+OP_GROUP_ALL(LE)
+OP_GROUP_ALL(GT)
+OP_GROUP_ALL(GE)
+OP_GROUP_ALL(TEST)
+OP_GROUP_ALL(POP)
+OP_GROUP_ALL(DUP)
+
+//===----------------------------------------------------------------------===//
+
+#undef OPCODE_DEFINE
+#undef OPCODE_DISASM
+#undef OPCODE_DISASM_0
+#undef OPCODE_DISASM_1
+#undef OPCODE_DISASM_2
+#undef OPCODE_INTERP
+#undef OPCODE_INTERP_1
+#undef OPCODE_INTERP_T_0
+#undef OPCODE_INTERP_T_T_0
+#undef OPCODE_INTERP_T_1
+#undef OPCODE_INTERP_RET_T
+#undef OPCODE_INTERP_RET
+#undef OPCODE_INTERP_CALL_1
+#undef OPCODE_EMITTER_IMPL_ALL_ARG
+#undef OPCODE_EMITTER_IMPL_ALL
+#undef OPCODE_EMITTER_IMPL_ALU
+#undef OPCODE_EMITTER_IMPL_ALU_ALU
+#undef OPCODE_EMITTER_PROTO_T_ARG
+#undef OPCODE_EMITTER_PROTO_T
+#undef OPCODE_EMITTER_PROTO_T_T
diff --git a/clang/lib/AST/ExprVM/Pointer.h b/clang/lib/AST/ExprVM/Pointer.h
new file mode 100644
--- /dev/null
+++ b/clang/lib/AST/ExprVM/Pointer.h
@@ -0,0 +1,72 @@
+//===--- Pointer.h - Types for the constexpr VM -----------------*- C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+//
+// Defines the classes responsible for pointer tracking.
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_CLANG_AST_EXPRVM_POINTER_H
+#define LLVM_CLANG_AST_EXPRVM_POINTER_H
+
+#include "clang/AST/Decl.h"
+#include "clang/AST/Expr.h"
+#include "llvm/ADT/PointerUnion.h"
+#include "llvm/Support/raw_ostream.h"
+
+namespace clang {
+class ValueDecl;
+class Expr;
+
+namespace vm {
+class Context;
+struct Pointer;
+enum PrimType : unsigned;
+
+/// Describes a memory block - generated once by the compiler.
+struct Descriptor {
+  /// Original declaration, used to emit the error message.
+  llvm::PointerUnion<const ValueDecl *, const Expr *> Source;
+  /// Offset on the stack for a local.
+  unsigned Offset = 0;
+  /// Size of the object.
+  unsigned Size;
+  /// Size of an element.
+  unsigned ElemSize;
+  /// Type of the elements.
+  PrimType ElemType;
+  /// Flag indicating if the descriptor is mutable.
+  bool IsMutable = false;
+  /// Flag indicating if the object is an array.
+  bool IsArray = false;
+  /// Flag indicating if the object is a global.
+  bool IsGlobal = false;
+  /// Flag indicating if the descriptor is a temporary.
+  bool IsTemporary = false;
+
+  Descriptor(const ValueDecl *D, unsigned Size, unsigned ElemSize,
+             PrimType ElemType, bool IsMutable, bool IsArray, bool IsGlobal)
+      : Source(D), Size(Size), ElemSize(ElemSize), ElemType(ElemType),
+        IsMutable(IsMutable), IsArray(IsArray), IsGlobal(IsGlobal) {}
+
+  Descriptor(const Expr *E, unsigned Size, unsigned ElemSize, PrimType ElemType,
+             bool IsMutable, bool IsArray, bool IsGlobal)
+      : Source(E), Size(Size), ElemSize(ElemSize), ElemType(ElemType),
+        IsMutable(IsMutable), IsArray(IsArray), IsGlobal(IsGlobal) {}
+
+  Descriptor(const ValueDecl *D, unsigned Offset, unsigned Size,
+             unsigned ElemSize, PrimType ElemType, bool IsMutable, bool IsArray)
+      : Source(D), Offset(Offset), Size(Size), ElemSize(ElemSize),
+        ElemType(ElemType), IsMutable(IsMutable), IsArray(IsArray) {}
+
+  QualType getType() const;
+};
+
+} // namespace vm
+} // namespace clang
+
+#endif
diff --git a/clang/lib/AST/ExprVM/Pointer.cpp b/clang/lib/AST/ExprVM/Pointer.cpp
new file mode 100644
--- /dev/null
+++ b/clang/lib/AST/ExprVM/Pointer.cpp
@@ -0,0 +1,21 @@
+//===--- Pointer.cpp - Types for the constexpr VM ---------------*- C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include "Pointer.h"
+#include "Type.h"
+
+using namespace clang;
+using namespace vm;
+
+QualType Descriptor::getType() const {
+  if (auto *D = Source.dyn_cast<const ValueDecl *>())
+    return D->getType();
+  if (auto *E = Source.dyn_cast<const Expr *>())
+    return E->getType();
+  llvm_unreachable("Invalid descriptor type");
+}
diff --git a/clang/lib/AST/ExprVM/Program.h b/clang/lib/AST/ExprVM/Program.h
new file mode 100644
--- /dev/null
+++ b/clang/lib/AST/ExprVM/Program.h
@@ -0,0 +1,113 @@
+//===--- Program.h - Bytecode for the constexpr VM --------------*- C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+//
+// Defines a program which organises and links multiple bytecode functions.
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_CLANG_AST_EXPRVM_PROGRAM_H
+#define LLVM_CLANG_AST_EXPRVM_PROGRAM_H
+
+#include "Function.h"
+#include "Pointer.h"
+#include "Type.h"
+#include "llvm/ADT/DenseMap.h"
+#include "llvm/ADT/PointerUnion.h"
+#include "llvm/ADT/StringRef.h"
+#include "llvm/Support/Allocator.h"
+#include <map>
+#include <vector>
+
+namespace clang {
+class Expr;
+class FunctionDecl;
+class Stmt;
+class StringLiteral;
+class VarDecl;
+
+namespace vm {
+class Context;
+class Scope;
+
+using CodePtr = const char *;
+
+/// Describes the statement/declaration an opcode was generated from.
+class SourceInfo {
+public:
+  SourceInfo() {}
+  SourceInfo(const Stmt *E) : Source(E) {}
+  SourceInfo(const VarDecl *D) : Source(D) {}
+
+  SourceLocation getLoc() const;
+
+  const Stmt *asStmt() const { return Source.dyn_cast<const Stmt *>(); }
+  const VarDecl *asDecl() const { return Source.dyn_cast<const VarDecl *>(); }
+  const Expr *asExpr() const;
+
+  operator bool() const { return !Source.isNull(); }
+
+private:
+  llvm::PointerUnion<const VarDecl *, const Stmt *> Source;
+};
+
+/// The program contains and links the bytecode for all functions.
+class Program {
+public:
+  /// Program code emitter.
+  void emitSource(const SourceInfo &L);
+  void emitBytes(const char *Bytes, unsigned Count);
+  void emitSpace(unsigned Size);
+
+  /// Returns the offset into the code segment.
+  unsigned getOffset();
+
+  // Reads a value from the code.
+  template <typename T> T read(unsigned Offset) {
+    return llvm::support::endian::read<T, llvm::support::endianness::native, 1>(
+        Code.data() + Offset);
+  }
+  /// Rewrites a bit of code with a new value.
+  template <typename T> void write(unsigned Offset, const T &Value) {
+    return llvm::support::endian::write<T, llvm::support::endianness::native,
+                                        1>(Code.data() + Offset, Value);
+  }
+
+  /// Returns a pointer to the program code.
+  CodePtr getCodeBegin() const { return Code.data(); }
+
+  /// Returns source information for an offset.
+  SourceInfo getSource(CodePtr PC) const;
+  const Expr *getExpr(CodePtr PC) const;
+
+  /// Returns location information for an offset.
+  SourceLocation getLocation(CodePtr PC) const;
+
+  /// Creates a new function from a code range.
+  template <typename... Ts>
+  Function *createFunction(const FunctionDecl *Def, Ts &&... Args) {
+    auto *Func = new Function(*this, Def, std::forward<Ts>(Args)...);
+    Funcs.insert({Def, std::unique_ptr<Function>(Func)});
+    return Func;
+  }
+
+  /// Returns or creates a function.
+  Function *getFunction(const FunctionDecl *F);
+
+private:
+  /// Mapping from decls to cached bytecode functions.
+  llvm::DenseMap<const FunctionDecl *, std::unique_ptr<Function>> Funcs;
+  /// Program bytecode.
+  std::vector<char> Code;
+  /// Mapping from bytecode offsets to source locations.
+  std::map<unsigned, SourceInfo> Source;
+};
+
+} // namespace vm
+} // namespace clang
+
+#endif
diff --git a/clang/lib/AST/ExprVM/Program.cpp b/clang/lib/AST/ExprVM/Program.cpp
new file mode 100644
--- /dev/null
+++ b/clang/lib/AST/ExprVM/Program.cpp
@@ -0,0 +1,68 @@
+//===--- Program.cpp - Bytecode for the constexpr VM ------------*- C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include "Program.h"
+#include "Context.h"
+#include "Function.h"
+#include "Opcode.h"
+#include "Type.h"
+#include "clang/AST/Decl.h"
+
+using namespace clang;
+using namespace clang::vm;
+
+SourceLocation SourceInfo::getLoc() const {
+  if (auto *E = asExpr())
+    return E->getExprLoc();
+  if (auto *S = asStmt())
+    return S->getBeginLoc();
+  if (auto *D = asDecl())
+    return D->getBeginLoc();
+  return SourceLocation();
+}
+
+const Expr *SourceInfo::asExpr() const {
+  if (auto *S = Source.dyn_cast<const Stmt *>())
+    return dyn_cast<Expr>(S);
+  return nullptr;
+}
+
+void Program::emitSource(const SourceInfo &L) {
+  assert(L && "invalid source");
+  Source.emplace(getOffset(), L);
+}
+
+void Program::emitBytes(const char *Bytes, unsigned Count) {
+  Code.insert(Code.end(), Bytes, Bytes + Count);
+}
+
+void Program::emitSpace(unsigned Size) { Code.resize(Code.size() + Size); }
+
+unsigned Program::getOffset() { return Code.size(); }
+
+SourceInfo Program::getSource(CodePtr PC) const {
+  auto It = Source.find(PC - getCodeBegin());
+  if (It == Source.end())
+    llvm::report_fatal_error("missing source location");
+  return It->second;
+}
+
+const Expr *Program::getExpr(CodePtr PC) const {
+  if (auto *E = getSource(PC).asExpr())
+    return E;
+  llvm::report_fatal_error("missing source expression");
+}
+
+SourceLocation Program::getLocation(CodePtr PC) const {
+  return getSource(PC).getLoc();
+}
+
+Function *Program::getFunction(const FunctionDecl *F) {
+  auto It = Funcs.find(F);
+  return It == Funcs.end() ? nullptr : It->second.get();
+}
diff --git a/clang/lib/AST/ExprVM/State.h b/clang/lib/AST/ExprVM/State.h
new file mode 100644
--- /dev/null
+++ b/clang/lib/AST/ExprVM/State.h
@@ -0,0 +1,118 @@
+//===--- State.h - State chain for the VM and AST Walker --------*- C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+//
+// Defines the base class of the interpreter and evaluator state.
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_CLANG_AST_EXPRVM_STATE_H
+#define LLVM_CLANG_AST_EXPRVM_STATE_H
+
+#include "clang/AST/ASTDiagnostic.h"
+#include "clang/AST/Expr.h"
+#include "clang/Basic/OptionalDiagnostic.h"
+
+namespace clang {
+
+/// Kinds of access we can perform on an object, for diagnostics. Note that
+/// we consider a member function call to be a kind of access, even though
+/// it is not formally an access of the object, because it has (largely) the
+/// same set of semantic restrictions.
+enum AccessKinds {
+  AK_Read,
+  AK_Assign,
+  AK_Increment,
+  AK_Decrement,
+  AK_MemberCall,
+  AK_DynamicCast,
+  AK_TypeId,
+};
+
+namespace vm {
+class Frame;
+class SourceInfo;
+
+/// Interface for the VM to interact with the AST walker's context.
+class State {
+public:
+  virtual ~State();
+
+  virtual bool checkingForOverflow() const = 0;
+  virtual bool checkingPotentialConstantExpression() const = 0;
+  virtual bool noteUndefinedBehavior() = 0;
+  virtual Frame *getCurrentFrame() = 0;
+  virtual const Frame *getBottomFrame() const = 0;
+  virtual bool hasActiveDiagnostic() = 0;
+  virtual void setActiveDiagnostic(bool Flag) = 0;
+  virtual void setFoldFailureDiagnostic(bool Flag) = 0;
+  virtual Expr::EvalStatus &getEvalStatus() const = 0;
+  virtual ASTContext &getCtx() const = 0;
+  virtual bool hasPriorDiagnostic() = 0;
+  virtual unsigned getCallStackDepth() = 0;
+
+public:
+  // Diagnose that the evaluation could not be folded (FF => FoldFailure)
+  OptionalDiagnostic
+  FFDiag(SourceLocation Loc,
+         diag::kind DiagId = diag::note_invalid_subexpr_in_const_expr,
+         unsigned ExtraNotes = 0);
+
+  OptionalDiagnostic
+  FFDiag(const Expr *E,
+         diag::kind DiagId = diag::note_invalid_subexpr_in_const_expr,
+         unsigned ExtraNotes = 0);
+
+  OptionalDiagnostic
+  FFDiag(const SourceInfo &SI,
+         diag::kind DiagId = diag::note_invalid_subexpr_in_const_expr,
+         unsigned ExtraNotes = 0);
+
+  /// Diagnose that the evaluation does not produce a C++11 core constant
+  /// expression.
+  ///
+  /// FIXME: Stop evaluating if we're in EM_ConstantExpression or
+  /// EM_PotentialConstantExpression mode and we produce one of these.
+  OptionalDiagnostic
+  CCEDiag(SourceLocation Loc,
+          diag::kind DiagId = diag::note_invalid_subexpr_in_const_expr,
+          unsigned ExtraNotes = 0);
+
+  OptionalDiagnostic
+  CCEDiag(const Expr *E,
+          diag::kind DiagId = diag::note_invalid_subexpr_in_const_expr,
+          unsigned ExtraNotes = 0);
+
+  OptionalDiagnostic
+  CCEDiag(const SourceInfo &SI,
+          diag::kind DiagId = diag::note_invalid_subexpr_in_const_expr,
+          unsigned ExtraNotes = 0);
+
+  /// Add a note to a prior diagnostic.
+  OptionalDiagnostic Note(SourceLocation Loc, diag::kind DiagId);
+
+  /// Add a stack of notes to a prior diagnostic.
+  void addNotes(ArrayRef<PartialDiagnosticAt> Diags);
+
+  /// Directly reports a diagnostic message.
+  DiagnosticBuilder report(SourceLocation Loc, diag::kind DiagId);
+
+  const LangOptions &getLangOpts() const;
+
+private:
+  void addCallStack(unsigned Limit);
+
+  PartialDiagnostic &addDiag(SourceLocation Loc, diag::kind DiagId);
+
+  OptionalDiagnostic diag(SourceLocation Loc, diag::kind DiagId,
+                          unsigned ExtraNotes, bool IsCCEDiag);
+};
+
+} // namespace vm
+} // namespace clang
+
+#endif
diff --git a/clang/lib/AST/ExprVM/State.cpp b/clang/lib/AST/ExprVM/State.cpp
new file mode 100644
--- /dev/null
+++ b/clang/lib/AST/ExprVM/State.cpp
@@ -0,0 +1,156 @@
+//===--- State.cpp - State chain for the VM and AST Walker ------*- C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include "State.h"
+#include "Frame.h"
+#include "Program.h"
+#include "clang/AST/ASTContext.h"
+#include "clang/AST/CXXInheritance.h"
+
+using namespace clang;
+using namespace clang::vm;
+
+State::~State() {}
+
+OptionalDiagnostic State::FFDiag(SourceLocation Loc, diag::kind DiagId,
+                                 unsigned ExtraNotes) {
+  return diag(Loc, DiagId, ExtraNotes, false);
+}
+
+OptionalDiagnostic State::FFDiag(const Expr *E, diag::kind DiagId,
+                                 unsigned ExtraNotes) {
+  if (getEvalStatus().Diag)
+    return diag(E->getExprLoc(), DiagId, ExtraNotes, false);
+  setActiveDiagnostic(false);
+  return OptionalDiagnostic();
+}
+
+OptionalDiagnostic State::FFDiag(const SourceInfo &SI, diag::kind DiagId,
+                                 unsigned ExtraNotes) {
+  if (getEvalStatus().Diag)
+    return diag(SI.getLoc(), DiagId, ExtraNotes, false);
+  setActiveDiagnostic(false);
+  return OptionalDiagnostic();
+}
+
+OptionalDiagnostic State::CCEDiag(SourceLocation Loc, diag::kind DiagId,
+                                  unsigned ExtraNotes) {
+  // Don't override a previous diagnostic. Don't bother collecting
+  // diagnostics if we're evaluating for overflow.
+  if (!getEvalStatus().Diag || !getEvalStatus().Diag->empty()) {
+    setActiveDiagnostic(false);
+    return OptionalDiagnostic();
+  }
+  return diag(Loc, DiagId, ExtraNotes, true);
+}
+
+OptionalDiagnostic State::CCEDiag(const Expr *E, diag::kind DiagId,
+                                  unsigned ExtraNotes) {
+  return CCEDiag(E->getExprLoc(), DiagId, ExtraNotes);
+}
+
+OptionalDiagnostic State::CCEDiag(const SourceInfo &SI, diag::kind DiagId,
+                                  unsigned ExtraNotes) {
+  return CCEDiag(SI.getLoc(), DiagId, ExtraNotes);
+}
+
+OptionalDiagnostic State::Note(SourceLocation Loc, diag::kind DiagId) {
+  if (!hasActiveDiagnostic())
+    return OptionalDiagnostic();
+  return OptionalDiagnostic(&addDiag(Loc, DiagId));
+}
+
+void State::addNotes(ArrayRef<PartialDiagnosticAt> Diags) {
+  if (hasActiveDiagnostic()) {
+    getEvalStatus().Diag->insert(getEvalStatus().Diag->end(), Diags.begin(),
+                                 Diags.end());
+  }
+}
+
+DiagnosticBuilder State::report(SourceLocation Loc, diag::kind DiagId) {
+  return getCtx().getDiagnostics().Report(Loc, DiagId);
+}
+
+/// Add a diagnostic to the diagnostics list.
+PartialDiagnostic &State::addDiag(SourceLocation Loc, diag::kind DiagId) {
+  PartialDiagnostic PD(DiagId, getCtx().getDiagAllocator());
+  getEvalStatus().Diag->push_back(std::make_pair(Loc, PD));
+  return getEvalStatus().Diag->back().second;
+}
+
+OptionalDiagnostic State::diag(SourceLocation Loc, diag::kind DiagId,
+                               unsigned ExtraNotes, bool IsCCEDiag) {
+  auto &EvalStatus = getEvalStatus();
+  if (EvalStatus.Diag) {
+    if (hasPriorDiagnostic()) {
+      return OptionalDiagnostic();
+    }
+
+    unsigned CallStackNotes = getCallStackDepth() - 1;
+    unsigned Limit = getCtx().getDiagnostics().getConstexprBacktraceLimit();
+    if (Limit)
+      CallStackNotes = std::min(CallStackNotes, Limit + 1);
+    if (checkingPotentialConstantExpression())
+      CallStackNotes = 0;
+
+    setActiveDiagnostic(true);
+    setFoldFailureDiagnostic(!IsCCEDiag);
+    EvalStatus.Diag->clear();
+    EvalStatus.Diag->reserve(1 + ExtraNotes + CallStackNotes);
+    addDiag(Loc, DiagId);
+    if (!checkingPotentialConstantExpression()) {
+      addCallStack(Limit);
+    }
+    return OptionalDiagnostic(&(*EvalStatus.Diag)[0].second);
+  }
+  setActiveDiagnostic(false);
+  return OptionalDiagnostic();
+}
+
+const LangOptions &State::getLangOpts() const { return getCtx().getLangOpts(); }
+
+void State::addCallStack(unsigned Limit) {
+  // Determine which calls to skip, if any.
+  unsigned ActiveCalls = getCallStackDepth() - 1;
+  unsigned SkipStart = ActiveCalls, SkipEnd = SkipStart;
+  if (Limit && Limit < ActiveCalls) {
+    SkipStart = Limit / 2 + Limit % 2;
+    SkipEnd = ActiveCalls - Limit / 2;
+  }
+
+  // Walk the call stack and add the diagnostics.
+  unsigned CallIdx = 0;
+  auto *CurrentCall = getCurrentFrame();
+  auto *BottomFrame = getBottomFrame();
+  for (auto *F = CurrentCall; F != BottomFrame; F = F->getCaller(), ++CallIdx) {
+    // Skip this call?
+    if (CallIdx >= SkipStart && CallIdx < SkipEnd) {
+      if (CallIdx == SkipStart) {
+        // Note that we're skipping calls.
+        addDiag(F->getLoc(), diag::note_constexpr_calls_suppressed)
+            << unsigned(ActiveCalls - Limit);
+      }
+      continue;
+    }
+
+    // Use a different note for an inheriting constructor, because from the
+    // user's perspective it's not really a function at all.
+    if (auto *CD = dyn_cast_or_null<CXXConstructorDecl>(F->getCallee())) {
+      if (CD->isInheritingConstructor()) {
+        addDiag(F->getLoc(), diag::note_constexpr_inherited_ctor_call_here)
+            << CD->getParent();
+        continue;
+      }
+    }
+
+    SmallVector<char, 128> Buffer;
+    llvm::raw_svector_ostream Out(Buffer);
+    F->describe(Out);
+    addDiag(F->getLoc(), diag::note_constexpr_call_here) << Out.str();
+  }
+}
diff --git a/clang/lib/AST/ExprVM/Type.h b/clang/lib/AST/ExprVM/Type.h
new file mode 100644
--- /dev/null
+++ b/clang/lib/AST/ExprVM/Type.h
@@ -0,0 +1,77 @@
+//===--- Type.h - Types for the constexpr VM --------------------*- C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+//
+// Defines the VM types and helpers operating on types.
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_CLANG_AST_EXPRVM_TYPE_H
+#define LLVM_CLANG_AST_EXPRVM_TYPE_H
+
+#include "Pointer.h"
+#include <cstddef>
+#include <cstdint>
+
+namespace clang {
+namespace vm {
+
+/// Enumeration of the primitive types of the VM.
+enum PrimType : unsigned {
+  ET_SINT_8,
+  ET_UINT_8,
+  ET_SINT_16,
+  ET_UINT_16,
+  ET_SINT_32,
+  ET_UINT_32,
+  ET_SINT_64,
+  ET_UINT_64,
+  ET_BOOL
+};
+
+/// Mapping from primitive types to their representation.
+template <PrimType T> struct PrimConv;
+template <> struct PrimConv<ET_SINT_8> { typedef int8_t T; };
+template <> struct PrimConv<ET_UINT_8> { typedef uint8_t T; };
+template <> struct PrimConv<ET_SINT_16> { typedef int16_t T; };
+template <> struct PrimConv<ET_UINT_16> { typedef uint16_t T; };
+template <> struct PrimConv<ET_SINT_32> { typedef int32_t T; };
+template <> struct PrimConv<ET_UINT_32> { typedef uint32_t T; };
+template <> struct PrimConv<ET_SINT_64> { typedef int64_t T; };
+template <> struct PrimConv<ET_UINT_64> { typedef uint64_t T; };
+template <> struct PrimConv<ET_BOOL> { typedef bool T; };
+
+/// Returns the size of a primitive type in bytes.
+size_t primSize(PrimType Type);
+
+/// An integer which is as wide as the native pointer size to encode pointers.
+constexpr PrimType ET_NATIVE_PTR =
+    sizeof(size_t) == sizeof(PrimConv<ET_UINT_64>::T) ? ET_UINT_64 : ET_UINT_32;
+
+/// Aligns a size to the pointer alignment.
+constexpr size_t align(size_t Size) {
+  return ((Size + alignof(void *) - 1) / alignof(void *)) * alignof(void *);
+}
+
+} // namespace vm
+} // namespace clang
+
+/// Helper macro to simplify type switches.
+#define TYPE_SWITCH(Expr, B)                                                   \
+  switch (Expr) {                                                              \
+  case ET_SINT_8:  {using T = PrimConv<ET_SINT_8>::T;  do{B;}while(0); break; }\
+  case ET_UINT_8:  {using T = PrimConv<ET_UINT_8>::T;  do{B;}while(0); break; }\
+  case ET_SINT_16: {using T = PrimConv<ET_SINT_16>::T; do{B;}while(0); break; }\
+  case ET_UINT_16: {using T = PrimConv<ET_UINT_16>::T; do{B;}while(0); break; }\
+  case ET_SINT_32: {using T = PrimConv<ET_SINT_32>::T; do{B;}while(0); break; }\
+  case ET_UINT_32: {using T = PrimConv<ET_UINT_32>::T; do{B;}while(0); break; }\
+  case ET_SINT_64: {using T = PrimConv<ET_SINT_64>::T; do{B;}while(0); break; }\
+  case ET_UINT_64: {using T = PrimConv<ET_UINT_64>::T; do{B;}while(0); break; }\
+  case ET_BOOL:    {using T = PrimConv<ET_BOOL>::T;    do{B;}while(0); break; }\
+  }
+
+#endif
diff --git a/clang/lib/AST/ExprVM/Type.cpp b/clang/lib/AST/ExprVM/Type.cpp
new file mode 100644
--- /dev/null
+++ b/clang/lib/AST/ExprVM/Type.cpp
@@ -0,0 +1,20 @@
+//===--- Type.cpp - Types for the constexpr VM ------------------*- C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include "Type.h"
+
+using namespace clang;
+using namespace clang::vm;
+
+namespace clang {
+namespace vm {
+
+size_t primSize(PrimType Type) { TYPE_SWITCH(Type, return sizeof(T)); }
+
+} // namespace vm
+} // namespace clang
diff --git a/clang/lib/Driver/ToolChains/Clang.cpp b/clang/lib/Driver/ToolChains/Clang.cpp
--- a/clang/lib/Driver/ToolChains/Clang.cpp
+++ b/clang/lib/Driver/ToolChains/Clang.cpp
@@ -4376,6 +4376,12 @@
     CmdArgs.push_back(A->getValue());
   }
 
+  if (Args.hasArg(options::OPT_fexperimental_constexpr_interpreter))
+    CmdArgs.push_back("-fexperimental-constexpr-interpreter");
+
+  if (Args.hasArg(options::OPT_fexperimental_constexpr_force_interpreter))
+    CmdArgs.push_back("-fexperimental-constexpr-force-interpreter");
+
   if (Arg *A = Args.getLastArg(options::OPT_fbracket_depth_EQ)) {
     CmdArgs.push_back("-fbracket-depth");
     CmdArgs.push_back(A->getValue());
diff --git a/clang/lib/Frontend/CompilerInvocation.cpp b/clang/lib/Frontend/CompilerInvocation.cpp
--- a/clang/lib/Frontend/CompilerInvocation.cpp
+++ b/clang/lib/Frontend/CompilerInvocation.cpp
@@ -2723,6 +2723,10 @@
       getLastArgIntValue(Args, OPT_fconstexpr_depth, 512, Diags);
   Opts.ConstexprStepLimit =
       getLastArgIntValue(Args, OPT_fconstexpr_steps, 1048576, Diags);
+  Opts.ConstexprInterpreter =
+      Args.hasArg(OPT_fexperimental_constexpr_interpreter);
+  Opts.ConstexprForceInterpreter =
+      Args.hasArg(OPT_fexperimental_constexpr_force_interpreter);
   Opts.BracketDepth = getLastArgIntValue(Args, OPT_fbracket_depth, 256, Diags);
   Opts.DelayedTemplateParsing = Args.hasArg(OPT_fdelayed_template_parsing);
   Opts.NumLargeByValueCopy =
diff --git a/clang/test/ExprVM/cond.cpp b/clang/test/ExprVM/cond.cpp
new file mode 100644
--- /dev/null
+++ b/clang/test/ExprVM/cond.cpp
@@ -0,0 +1,50 @@
+// RUN: %clang_cc1 -std=c++17 -fsyntax-only -fexperimental-constexpr-force-interpreter %s
+// RUN: %clang_cc1 -std=c++17 -fsyntax-only %s
+
+constexpr int cond_then_else(int a, int b) {
+  if (a < b) {
+    return b - a;
+  } else {
+    return a - b;
+  }
+}
+
+static_assert(cond_then_else(10, 20) == 10);
+static_assert(cond_then_else(30, 10) == 20);
+
+constexpr int cond_empty_then(int a) {
+  if (a < 0)
+    ;
+  return 1;
+}
+
+static_assert(cond_empty_then(0) == 1);
+
+constexpr int cond_else(int a) {
+  if (a < 0)
+    return 10;
+  return 1;
+}
+
+static_assert(cond_else(-1) == 10);
+static_assert(cond_else(1) == 1);
+
+constexpr int cond_decl(int a) {
+  if (int b = a + 1)
+    return b - 1;
+  else
+    return b + 1;
+}
+
+static_assert(cond_decl(-1) == 1);
+static_assert(cond_decl(5) == 5);
+
+constexpr int cond_decl_init(int a) {
+  if (int b = a + 1; b != 0)
+    return b - 1;
+  else
+    return b + 1;
+}
+
+static_assert(cond_decl_init(-1) == 1);
+static_assert(cond_decl_init(5) == 5);