[clang][Interp] Implement __builtin_assume

[tbaeder]

Summary

Just ignore it.

As part of this, move the Ret and RetVoid implementation to Interp.h, so
they can be shared with InterpBuiltin.cpp.

[aaron.ballman]

LGTM; I verified that constant expression evaluation ignores failed assumptions: http://eel.is/c++draft/expr.const#5.8 so this is in line with the C++2b feature.

clang/lib/AST/Interp/Interp.h

 
 /// Checks if the result is a floating-point operation is valid
 /// in the current context.
 bool CheckFloatResult(InterpState &S, CodePtr OpPC, APFloat::opStatus Status);
 
 /// Interpreter entry point.
 bool Interpret(InterpState &S, APValue &Result);
 
-bool InterpretBuiltin(InterpState &S, CodePtr PC, unsigned BuiltinID);
+/// Interpret a builtin function.
+bool InterpretBuiltin(InterpState &S, CodePtr &PC, unsigned BuiltinID);
 
 enum class ArithOp { Add, Sub };
 
 //===----------------------------------------------------------------------===//
+// Returning values
+//===----------------------------------------------------------------------===//
+
+template <PrimType Name, bool Builtin = false,
+          class T = typename PrimConv<Name>::T>
+bool Ret(InterpState &S, CodePtr &PC, APValue &Result) {
+  S.CallStackDepth--;
+  const T &Ret = S.Stk.pop<T>();
+
+  assert(S.Current->getFrameOffset() == S.Stk.size() && "Invalid frame");
+  if (Builtin || !S.checkingPotentialConstantExpression())
+    S.Current->popArgs();
+
+  if (InterpFrame *Caller = S.Current->Caller) {
+    PC = S.Current->getRetPC();
+    delete S.Current;
+    S.Current = Caller;
+    S.Stk.push<T>(Ret);
+  } else {
+    delete S.Current;
+    S.Current = nullptr;
+    if (!ReturnValue<T>(Ret, Result))
+      return false;
+  }
+  return true;
+}
+
+template <bool Builtin = false>
+inline bool RetVoid(InterpState &S, CodePtr &PC, APValue &Result) {
+  S.CallStackDepth--;
+
+  assert(S.Current->getFrameOffset() == S.Stk.size() && "Invalid frame");
+  if (Builtin || !S.checkingPotentialConstantExpression())
+    S.Current->popArgs();
+
+  if (InterpFrame *Caller = S.Current->Caller) {
+    PC = S.Current->getRetPC();
+    delete S.Current;
+    S.Current = Caller;
+  } else {
+    delete S.Current;
+    S.Current = nullptr;
+  }
+  return true;
+}
+
+//===----------------------------------------------------------------------===//
 // Add, Sub, Mul
 //===----------------------------------------------------------------------===//
 
 template <typename T, bool (*OpFW)(T, T, unsigned, T *),
           template <typename U> class OpAP>
 bool AddSubMulHelper(InterpState &S, CodePtr OpPC, unsigned Bits, const T &LHS,
                      const T &RHS) {
   // Fast path - add the numbers with fixed width.

clang/lib/AST/Interp/Interp.cpp

 #include "clang/AST/CXXInheritance.h"
 #include "clang/AST/Expr.h"
 #include "clang/AST/ExprCXX.h"
 #include "llvm/ADT/APSInt.h"
 
 using namespace clang;
 using namespace clang::interp;
 
-//===----------------------------------------------------------------------===//
-// Ret
-//===----------------------------------------------------------------------===//
-
-template <PrimType Name, class T = typename PrimConv<Name>::T>
-static bool Ret(InterpState &S, CodePtr &PC, APValue &Result) {
-  S.CallStackDepth--;
-  const T &Ret = S.Stk.pop<T>();
-
-  assert(S.Current->getFrameOffset() == S.Stk.size() && "Invalid frame");
-  if (!S.checkingPotentialConstantExpression())
-    S.Current->popArgs();
-
-  if (InterpFrame *Caller = S.Current->Caller) {
-    PC = S.Current->getRetPC();
-    delete S.Current;
-    S.Current = Caller;
-    S.Stk.push<T>(Ret);
-  } else {
-    delete S.Current;
-    S.Current = nullptr;
-    if (!ReturnValue<T>(Ret, Result))
-      return false;
-  }
-  return true;
-}
-
-static bool RetVoid(InterpState &S, CodePtr &PC, APValue &Result) {
-  S.CallStackDepth--;
-
-  assert(S.Current->getFrameOffset() == S.Stk.size() && "Invalid frame");
-  if (!S.checkingPotentialConstantExpression())
-    S.Current->popArgs();
-
-  if (InterpFrame *Caller = S.Current->Caller) {
-    PC = S.Current->getRetPC();
-    delete S.Current;
-    S.Current = Caller;
-  } else {
-    delete S.Current;
-    S.Current = nullptr;
-  }
-  return true;
-}
-
 static bool RetValue(InterpState &S, CodePtr &Pt, APValue &Result) {
   llvm::report_fatal_error("Interpreter cannot return values");
 }
 
 //===----------------------------------------------------------------------===//
 // Jmp, Jt, Jf
 //===----------------------------------------------------------------------===//
 

clang/lib/AST/Interp/InterpBuiltin.cpp

 //===--- InterpBuiltin.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 "Boolean.h"
 #include "Interp.h"
 #include "PrimType.h"
 #include "clang/Basic/Builtins.h"
 
 namespace clang {
 namespace interp {
 
-/// This is a slightly simplified version of the Ret() we have in Interp.cpp
-/// If they end up diverging in the future, we should get rid of the code
-/// duplication.
-template <PrimType Name, class T = typename PrimConv<Name>::T>
-static bool Ret(InterpState &S, CodePtr &PC) {
-  S.CallStackDepth--;
-  const T &Ret = S.Stk.pop<T>();
+bool InterpretBuiltin(InterpState &S, CodePtr &PC, unsigned BuiltinID) {
+  APValue Dummy;
 
-  assert(S.Current->getFrameOffset() == S.Stk.size() && "Invalid frame");
-  if (!S.checkingPotentialConstantExpression())
-    S.Current->popArgs();
-
-  InterpFrame *Caller = S.Current->Caller;
-  assert(Caller);
-
-  PC = S.Current->getRetPC();
-  delete S.Current;
-  S.Current = Caller;
-  S.Stk.push<T>(Ret);
-
-  return true;
-}
-
-bool InterpretBuiltin(InterpState &S, CodePtr PC, unsigned BuiltinID) {
   switch (BuiltinID) {
   case Builtin::BI__builtin_is_constant_evaluated:
     S.Stk.push<Boolean>(Boolean::from(S.inConstantContext()));
-    Ret<PT_Bool>(S, PC);
-    return true;
+    return Ret<PT_Bool, true>(S, PC, Dummy);
+  case Builtin::BI__builtin_assume:
+    return RetVoid<true>(S, PC, Dummy);
+  default:
+    return false;
   }
 
   return false;
 }
 
 } // namespace interp
 } // namespace clang

clang/test/AST/Interp/builtins.cpp

 constexpr bool b = std::is_constant_evaluated();
 static_assert(b, "");
 static_assert(std::is_constant_evaluated() , "");
 
 
 bool is_this_constant() {
   return __builtin_is_constant_evaluated(); // CHECK: ret i1 false
 }
+
+constexpr bool assume() {
+  __builtin_assume(true);
+  __builtin_assume(false);
+  return true;
+}
+static_assert(assume(), "");