diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
--- a/clang/docs/ReleaseNotes.rst
+++ b/clang/docs/ReleaseNotes.rst
@@ -122,7 +122,8 @@
 C Language Changes in Clang
 ---------------------------
 
-- ...
+- Support for ``__attribute__((error("")))`` and
+  ``__attribute__((warning("")))`` function attributes have been added.
 
 C++ Language Changes in Clang
 -----------------------------
diff --git a/clang/include/clang/Basic/Attr.td b/clang/include/clang/Basic/Attr.td
--- a/clang/include/clang/Basic/Attr.td
+++ b/clang/include/clang/Basic/Attr.td
@@ -3817,3 +3817,12 @@
   let Documentation = [EnforceTCBLeafDocs];
   bit InheritEvenIfAlreadyPresent = 1;
 }
+
+def Error : Attr {
+  let Spellings = [GCC<"error">, GCC<"warning">];
+  let Accessors = [Accessor<"isError", [GCC<"error">]>,
+                   Accessor<"isWarning", [GCC<"warning">]>];
+  let Args = [StringArgument<"UserDiagnostic">];
+  let Subjects = SubjectList<[Function], ErrorDiag>;
+  let Documentation = [ErrorAttrDocs];
+}
diff --git a/clang/include/clang/Basic/AttrDocs.td b/clang/include/clang/Basic/AttrDocs.td
--- a/clang/include/clang/Basic/AttrDocs.td
+++ b/clang/include/clang/Basic/AttrDocs.td
@@ -6045,3 +6045,15 @@
   - ``enforce_tcb_leaf(Name)`` indicates that this function is a part of the TCB named ``Name``
   }];
 }
+
+def ErrorAttrDocs : Documentation {
+  let Category = DocCatFunction;
+  let Heading = "error, warning";
+  let Content = [{
+    The ``error`` and ``warning`` function attributes can be used to specify a
+    custom diagnostic to be emitted when a call to such a function is not
+    eliminated via optimizations. This can be used to create compile time
+    assertions that depend on optimizations, while providing diagnostics
+    pointing to precise locations of the call site in the source.
+  }];
+}
diff --git a/clang/include/clang/Basic/DiagnosticCommonKinds.td b/clang/include/clang/Basic/DiagnosticCommonKinds.td
--- a/clang/include/clang/Basic/DiagnosticCommonKinds.td
+++ b/clang/include/clang/Basic/DiagnosticCommonKinds.td
@@ -371,4 +371,9 @@
   "options %0 and %1 are set to different values">;
 def err_opencl_feature_requires : Error<
   "feature %0 requires support of %1 feature">;
+
+def warn_changed_error_warning_attr_text : Warning<
+  "duplicate attribute changes text from %0 to %1">,
+  InGroup<DuplicateErrorWarningAttr>;
+
 }
diff --git a/clang/include/clang/Basic/DiagnosticFrontendKinds.td b/clang/include/clang/Basic/DiagnosticFrontendKinds.td
--- a/clang/include/clang/Basic/DiagnosticFrontendKinds.td
+++ b/clang/include/clang/Basic/DiagnosticFrontendKinds.td
@@ -72,6 +72,12 @@
 def err_fe_backend_unsupported : Error<"%0">, BackendInfo;
 def warn_fe_backend_unsupported : Warning<"%0">, BackendInfo;
 
+def err_fe_backend_error_attr :
+  Error<"call to %0 declared with attribute error: %1">, BackendInfo;
+def warn_fe_backend_warning_attr :
+  Warning<"call to %0 declared with attribute warning: %1">, BackendInfo,
+  InGroup<BackendWarningAttributes>;
+
 def err_fe_invalid_code_complete_file : Error<
     "cannot locate code-completion file %0">, DefaultFatal;
 def err_fe_dependency_file_requires_MT : Error<
diff --git a/clang/include/clang/Basic/DiagnosticGroups.td b/clang/include/clang/Basic/DiagnosticGroups.td
--- a/clang/include/clang/Basic/DiagnosticGroups.td
+++ b/clang/include/clang/Basic/DiagnosticGroups.td
@@ -803,6 +803,7 @@
 def CharSubscript : DiagGroup<"char-subscripts">;
 def LargeByValueCopy : DiagGroup<"large-by-value-copy">;
 def DuplicateArgDecl : DiagGroup<"duplicate-method-arg">;
+def DuplicateErrorWarningAttr : DiagGroup<"duplicate-error-warning-attr">;
 def SignedEnumBitfield : DiagGroup<"signed-enum-bitfield">;
 
 def ReservedIdentifier : DiagGroup<"reserved-identifier",
@@ -1208,6 +1209,7 @@
 def BackendOptimizationRemarkMissed : DiagGroup<"pass-missed">;
 def BackendOptimizationRemarkAnalysis : DiagGroup<"pass-analysis">;
 def BackendOptimizationFailure : DiagGroup<"pass-failed">;
+def BackendWarningAttributes : DiagGroup<"warning-attributes">;
 
 // Instrumentation based profiling warnings.
 def ProfileInstrMissing : DiagGroup<"profile-instr-missing">;
diff --git a/clang/lib/CodeGen/CGCall.cpp b/clang/lib/CodeGen/CGCall.cpp
--- a/clang/lib/CodeGen/CGCall.cpp
+++ b/clang/lib/CodeGen/CGCall.cpp
@@ -5306,6 +5306,15 @@
       TargetDecl->hasAttr<MSAllocatorAttr>())
     getDebugInfo()->addHeapAllocSiteMetadata(CI, RetTy->getPointeeType(), Loc);
 
+  // Add metadata if calling an __attribute__((error(""))) or warning fn.
+  if (TargetDecl && TargetDecl->hasAttr<ErrorAttr>()) {
+    llvm::ConstantInt *Line =
+        llvm::ConstantInt::get(Int32Ty, Loc.getRawEncoding());
+    llvm::ConstantAsMetadata *MD = llvm::ConstantAsMetadata::get(Line);
+    llvm::MDTuple *MDT = llvm::MDNode::get(getLLVMContext(), {MD});
+    CI->setMetadata("srcloc", MDT);
+  }
+
   // 4. Finish the call.
 
   // If the call doesn't return, finish the basic block and clear the
diff --git a/clang/lib/CodeGen/CodeGenAction.cpp b/clang/lib/CodeGen/CodeGenAction.cpp
--- a/clang/lib/CodeGen/CodeGenAction.cpp
+++ b/clang/lib/CodeGen/CodeGenAction.cpp
@@ -401,6 +401,7 @@
         const llvm::OptimizationRemarkAnalysisAliasing &D);
     void OptimizationFailureHandler(
         const llvm::DiagnosticInfoOptimizationFailure &D);
+    void DontCallDiagHandler(const DiagnosticInfoDontCall &D);
   };
 
   void BackendConsumer::anchor() {}
@@ -758,6 +759,29 @@
   EmitOptimizationMessage(D, diag::warn_fe_backend_optimization_failure);
 }
 
+void BackendConsumer::DontCallDiagHandler(const DiagnosticInfoDontCall &D) {
+  if (const Decl *DE = Gen->GetDeclForMangledName(D.getFunctionName()))
+    if (const auto *FD = dyn_cast<FunctionDecl>(DE)) {
+      assert(FD->hasAttr<ErrorAttr>() &&
+             "expected error or warning function attribute");
+
+      if (const auto *EA = FD->getAttr<ErrorAttr>()) {
+        assert((EA->isError() || EA->isWarning()) &&
+               "ErrorAttr neither error or warning");
+
+        SourceLocation LocCookie =
+            SourceLocation::getFromRawEncoding(D.getLocCookie());
+        assert(LocCookie.isValid() && "invalid location of dontcall call");
+
+        Diags.Report(LocCookie, EA->isError()
+                                    ? diag::err_fe_backend_error_attr
+                                    : diag::warn_fe_backend_warning_attr)
+            << FD->getDeclName() << EA->getUserDiagnostic();
+      }
+    }
+  // TODO: assert if DE or FD were nullptr?
+}
+
 /// This function is invoked when the backend needs
 /// to report something to the user.
 void BackendConsumer::DiagnosticHandlerImpl(const DiagnosticInfo &DI) {
@@ -833,6 +857,9 @@
   case llvm::DK_Unsupported:
     UnsupportedDiagHandler(cast<DiagnosticInfoUnsupported>(DI));
     return;
+  case llvm::DK_DontCall:
+    DontCallDiagHandler(cast<DiagnosticInfoDontCall>(DI));
+    return;
   default:
     // Plugin IDs are not bound to any value as they are set dynamically.
     ComputeDiagRemarkID(Severity, backend_plugin, DiagID);
diff --git a/clang/lib/CodeGen/CodeGenModule.cpp b/clang/lib/CodeGen/CodeGenModule.cpp
--- a/clang/lib/CodeGen/CodeGenModule.cpp
+++ b/clang/lib/CodeGen/CodeGenModule.cpp
@@ -2172,6 +2172,11 @@
                                                CalleeIdx, PayloadIndices,
                                                /* VarArgsArePassed */ false)}));
   }
+
+  // TODO: move to CodeGenModule::ConstructAttributeList() or
+  // CodeGenFunction::EmitCall() ???
+  if (FD->hasAttr<ErrorAttr>())
+    F->addFnAttr("dontcall");
 }
 
 void CodeGenModule::addUsedGlobal(llvm::GlobalValue *GV) {
diff --git a/clang/lib/Sema/SemaDeclAttr.cpp b/clang/lib/Sema/SemaDeclAttr.cpp
--- a/clang/lib/Sema/SemaDeclAttr.cpp
+++ b/clang/lib/Sema/SemaDeclAttr.cpp
@@ -946,6 +946,34 @@
     D->addAttr(::new (S.Context) EnableIfAttr(S.Context, AL, Cond, Msg));
 }
 
+static void handleErrorAttr(Sema &S, Decl *D, const ParsedAttr &AL) {
+  StringRef Str;
+  if (!S.checkStringLiteralArgumentAttr(AL, 0, Str))
+    return;
+
+  if (const auto *EA = D->getAttr<ErrorAttr>()) {
+    // TODO: something better than getNormalizedFullName to tell which spelling
+    // the new attribute we're trying to add is using?
+    std::string NewAttr = AL.getNormalizedFullName();
+    assert((NewAttr == "error" || NewAttr == "warning") &&
+           "unexpected normalized full name");
+
+    bool match = (EA->isError() && NewAttr == "error") ||
+                 (EA->isWarning() && NewAttr == "warning");
+    if (!match) {
+      S.Diag(AL.getLoc(), diag::err_attributes_are_not_compatible) << AL << EA;
+      S.Diag(EA->getLocation(), diag::note_conflicting_attribute);
+      return;
+    }
+    // TODO: wouldn't an additional note be nice?
+    if (EA->getUserDiagnostic() != Str)
+      S.Diag(AL.getLoc(), diag::warn_changed_error_warning_attr_text)
+          << EA->getUserDiagnostic() << Str;
+    D->dropAttr<ErrorAttr>();
+  }
+  D->addAttr(::new (S.Context) ErrorAttr(S.Context, AL, Str));
+}
+
 namespace {
 /// Determines if a given Expr references any of the given function's
 /// ParmVarDecls, or the function's implicit `this` parameter (if applicable).
@@ -7863,6 +7891,9 @@
   case ParsedAttr::AT_EnableIf:
     handleEnableIfAttr(S, D, AL);
     break;
+  case ParsedAttr::AT_Error:
+    handleErrorAttr(S, D, AL);
+    break;
   case ParsedAttr::AT_DiagnoseIf:
     handleDiagnoseIfAttr(S, D, AL);
     break;
diff --git a/clang/test/CodeGen/attr-error.c b/clang/test/CodeGen/attr-error.c
new file mode 100644
--- /dev/null
+++ b/clang/test/CodeGen/attr-error.c
@@ -0,0 +1,11 @@
+// RUN: %clang_cc1 -emit-llvm -o - %s | FileCheck %s
+__attribute__((error("oh no"))) void foo(void);
+
+void bar(void) {
+  foo();
+}
+
+// CHECK: call void @foo(), !srcloc [[SRCLOC:![0-9]+]]
+// CHECK: declare void @foo() [[ATTR:#[0-9]+]]
+// CHECK: attributes [[ATTR]] = {{{.*}}"dontcall"
+// CHECK: [[SRCLOC]] = !{i32 {{[0-9]+}}}
diff --git a/clang/test/CodeGen/attr-warning.c b/clang/test/CodeGen/attr-warning.c
new file mode 100644
--- /dev/null
+++ b/clang/test/CodeGen/attr-warning.c
@@ -0,0 +1,11 @@
+// RUN: %clang_cc1 -emit-llvm -o - %s | FileCheck %s
+__attribute__((warning("oh no"))) void foo(void);
+
+void bar(void) {
+  foo();
+}
+
+// CHECK: call void @foo(), !srcloc [[SRCLOC:![0-9]+]]
+// CHECK: declare void @foo() [[ATTR:#[0-9]+]]
+// CHECK: attributes [[ATTR]] = {{{.*}}"dontcall"
+// CHECK: [[SRCLOC]] = !{i32 {{[0-9]+}}}
diff --git a/clang/test/Frontend/backend-attribute-error-warning-optimize.c b/clang/test/Frontend/backend-attribute-error-warning-optimize.c
new file mode 100644
--- /dev/null
+++ b/clang/test/Frontend/backend-attribute-error-warning-optimize.c
@@ -0,0 +1,15 @@
+// REQUIRES: x86-registered-target
+// RUN: %clang_cc1 -O2 -verify -emit-codegen-only %s
+
+__attribute__((error("oh no foo"))) void foo(void);
+
+__attribute__((error("oh no bar"))) void bar(void);
+
+int x(void) {
+  return 8 % 2 == 1;
+}
+void baz(void) {
+  foo(); // expected-error {{call to 'foo' declared with attribute error: oh no foo}}
+  if (x())
+    bar();
+}
diff --git a/clang/test/Frontend/backend-attribute-error-warning.c b/clang/test/Frontend/backend-attribute-error-warning.c
new file mode 100644
--- /dev/null
+++ b/clang/test/Frontend/backend-attribute-error-warning.c
@@ -0,0 +1,31 @@
+// REQUIRES: x86-registered-target
+// RUN: %clang_cc1 -verify -emit-codegen-only %s
+
+__attribute__((error("oh no foo"))) void foo(void);
+
+__attribute__((error("oh no bar"))) void bar(void);
+
+int x(void) {
+  return 8 % 2 == 1;
+}
+
+__attribute__((warning("oh no quux"))) void quux(void);
+
+__attribute__((error("demangle me"))) void __compiletime_assert_455(void);
+
+__attribute__((error("one"),error("two"))) // expected-warning {{duplicate attribute changes text from one to two}}
+void duplicate_errors(void);
+
+__attribute__((warning("one"),warning("two"))) // expected-warning {{duplicate attribute changes text from one to two}}
+void duplicate_warnings(void);
+
+void baz(void) {
+  foo(); // expected-error {{call to 'foo' declared with attribute error: oh no foo}}
+  if (x())
+    bar();                    // expected-error {{call to 'bar' declared with attribute error: oh no bar}}
+  quux();                     // expected-warning {{call to 'quux' declared with attribute warning: oh no quux}}
+  __compiletime_assert_455(); // expected-error {{call to '__compiletime_assert_455' declared with attribute error: demangle me}}
+  duplicate_errors(); // expected-error {{call to 'duplicate_errors' declared with attribute error: two}}
+  // TODO: why is this a warning not an error????
+  duplicate_warnings(); // expected-warning {{call to 'duplicate_warnings' declared with attribute warning: two}}
+}
diff --git a/clang/test/Frontend/backend-attribute-error-warning.cpp b/clang/test/Frontend/backend-attribute-error-warning.cpp
new file mode 100644
--- /dev/null
+++ b/clang/test/Frontend/backend-attribute-error-warning.cpp
@@ -0,0 +1,37 @@
+// REQUIRES: x86-registered-target
+// RUN: %clang_cc1 -verify -emit-codegen-only %s
+
+__attribute__((error("oh no foo"))) void foo(void);
+
+__attribute__((error("oh no bar"))) void bar(void);
+
+int x(void) {
+  return 8 % 2 == 1;
+}
+
+__attribute__((warning("oh no quux"))) void quux(void);
+
+__attribute__((error("demangle me"))) void __compiletime_assert_455(void);
+
+template <typename T>
+__attribute__((error("demangle me, too")))
+T
+nocall(T t);
+
+struct Widget {
+  __attribute__((warning("don't call me!")))
+  operator int() { return 42; }
+};
+
+void baz(void) {
+  // Test that we demangle correctly in the diagnostic for C++.
+  foo(); // expected-error {{call to 'foo' declared with attribute error: oh no foo}}
+  if (x())
+    bar();                    // expected-error {{call to 'bar' declared with attribute error: oh no bar}}
+  quux();                     // expected-warning {{call to 'quux' declared with attribute warning: oh no quux}}
+  __compiletime_assert_455(); // expected-error {{call to '__compiletime_assert_455' declared with attribute error: demangle me}}
+  nocall<int>(42);            // expected-error {{call to 'nocall' declared with attribute error: demangle me, too}}
+
+  Widget W;
+  int w = W; // expected-warning {{'operator int' declared with attribute warning: don't call me!}}
+}
diff --git a/clang/test/Misc/pragma-attribute-supported-attributes-list.test b/clang/test/Misc/pragma-attribute-supported-attributes-list.test
--- a/clang/test/Misc/pragma-attribute-supported-attributes-list.test
+++ b/clang/test/Misc/pragma-attribute-supported-attributes-list.test
@@ -61,6 +61,7 @@
 // CHECK-NEXT: EnforceTCB (SubjectMatchRule_function)
 // CHECK-NEXT: EnforceTCBLeaf (SubjectMatchRule_function)
 // CHECK-NEXT: EnumExtensibility (SubjectMatchRule_enum)
+// CHECK-NEXT: Error (SubjectMatchRule_function)
 // CHECK-NEXT: ExcludeFromExplicitInstantiation (SubjectMatchRule_variable, SubjectMatchRule_function, SubjectMatchRule_record)
 // CHECK-NEXT: ExternalSourceSymbol ((SubjectMatchRule_record, SubjectMatchRule_enum, SubjectMatchRule_enum_constant, SubjectMatchRule_field, SubjectMatchRule_function, SubjectMatchRule_namespace, SubjectMatchRule_objc_category, SubjectMatchRule_objc_implementation, SubjectMatchRule_objc_interface, SubjectMatchRule_objc_method, SubjectMatchRule_objc_property, SubjectMatchRule_objc_protocol, SubjectMatchRule_record, SubjectMatchRule_type_alias, SubjectMatchRule_variable))
 // CHECK-NEXT: FlagEnum (SubjectMatchRule_enum)
diff --git a/clang/test/Sema/attr-error.c b/clang/test/Sema/attr-error.c
new file mode 100644
--- /dev/null
+++ b/clang/test/Sema/attr-error.c
@@ -0,0 +1,27 @@
+// RUN: %clang_cc1 -fsyntax-only -verify %s
+#if !__has_attribute(error)
+#error "error attribute missing"
+#endif
+
+__attribute__((error("don't call me!"))) int good0(void);
+
+__attribute__((error)) // expected-error {{'error' attribute takes one argument}}
+int
+bad0(void);
+
+int bad1(__attribute__((error("bad1"))) int param); // expected-error {{'error' attribute only applies to functions}}
+
+int bad2(void) {
+  __attribute__((error("bad2"))); // expected-error {{'error' attribute cannot be applied to a statement}}
+}
+
+__attribute__((error(3))) // expected-error {{'error' attribute requires a string}}
+int
+bad3(void);
+
+__attribute__((error("foo"), error("foo")))
+int good1(void);
+
+__attribute__((error("foo"), warning("foo"))) // expected-error {{'warning' and 'error' attributes are not compatible}}
+int bad4(void);
+// expected-note@-2 {{conflicting attribute is here}}
diff --git a/clang/test/Sema/attr-warning.c b/clang/test/Sema/attr-warning.c
new file mode 100644
--- /dev/null
+++ b/clang/test/Sema/attr-warning.c
@@ -0,0 +1,27 @@
+// RUN: %clang_cc1 -fsyntax-only -verify %s
+#if !__has_attribute(warning)
+#warning "warning attribute missing"
+#endif
+
+__attribute__((warning("don't call me!"))) int good0(void);
+
+__attribute__((warning)) // expected-error {{'warning' attribute takes one argument}}
+int
+bad0(void);
+
+int bad1(__attribute__((warning("bad1"))) int param); // expected-error {{'warning' attribute only applies to functions}}
+
+int bad2(void) {
+  __attribute__((warning("bad2"))); // expected-error {{'warning' attribute cannot be applied to a statement}}
+}
+
+__attribute__((warning(3))) // expected-error {{'warning' attribute requires a string}}
+int
+bad3(void);
+
+__attribute__((warning("foo"), warning("foo")))
+int good1(void);
+
+__attribute__((warning("foo"), error("foo"))) // expected-error {{'error' and 'warning' attributes are not compatible}}
+int bad4(void);
+// expected-note@-2 {{conflicting attribute is here}}
diff --git a/llvm/docs/LangRef.rst b/llvm/docs/LangRef.rst
--- a/llvm/docs/LangRef.rst
+++ b/llvm/docs/LangRef.rst
@@ -1576,6 +1576,12 @@
     can prove that the function does not execute any convergent operations.
     Similarly, the optimizer may remove ``convergent`` on calls/invokes when it
     can prove that the call/invoke cannot call a convergent function.
+``"dontcall"``
+    This attribute denotes that a diagnostic should be emitted when a call of a
+    function with this attribute is not eliminated via optimization. Front ends
+    can provide optional ``srcloc`` metadata nodes on call sites of such
+    callees to attach information about where in the source language such a
+    call came from.
 ``"frame-pointer"``
     This attribute tells the code generator whether the function
     should keep the frame pointer. The code generator may emit the frame pointer
diff --git a/llvm/include/llvm/IR/DiagnosticInfo.h b/llvm/include/llvm/IR/DiagnosticInfo.h
--- a/llvm/include/llvm/IR/DiagnosticInfo.h
+++ b/llvm/include/llvm/IR/DiagnosticInfo.h
@@ -79,6 +79,7 @@
   DK_PGOProfile,
   DK_Unsupported,
   DK_SrcMgr,
+  DK_DontCall,
   DK_FirstPluginKind // Must be last value to work with
                      // getNextAvailablePluginDiagnosticKind
 };
@@ -1070,6 +1071,23 @@
   }
 };
 
+class DiagnosticInfoDontCall : public DiagnosticInfo {
+  StringRef CalleeName;
+  unsigned LocCookie;
+
+public:
+  // TODO: make LocCookie optional
+  DiagnosticInfoDontCall(StringRef CalleeName, unsigned LocCookie)
+      : DiagnosticInfo(DK_DontCall, DS_Error), CalleeName(CalleeName),
+        LocCookie(LocCookie) {}
+  StringRef getFunctionName() const { return CalleeName; }
+  unsigned getLocCookie() const { return LocCookie; }
+  void print(DiagnosticPrinter &DP) const override;
+  static bool classof(const DiagnosticInfo *DI) {
+    return DI->getKind() == DK_DontCall;
+  }
+};
+
 } // end namespace llvm
 
 #endif // LLVM_IR_DIAGNOSTICINFO_H
diff --git a/llvm/lib/CodeGen/GlobalISel/IRTranslator.cpp b/llvm/lib/CodeGen/GlobalISel/IRTranslator.cpp
--- a/llvm/lib/CodeGen/GlobalISel/IRTranslator.cpp
+++ b/llvm/lib/CodeGen/GlobalISel/IRTranslator.cpp
@@ -47,6 +47,7 @@
 #include "llvm/IR/DataLayout.h"
 #include "llvm/IR/DebugInfo.h"
 #include "llvm/IR/DerivedTypes.h"
+#include "llvm/IR/DiagnosticInfo.h"
 #include "llvm/IR/Function.h"
 #include "llvm/IR/GetElementPtrTypeIterator.h"
 #include "llvm/IR/InlineAsm.h"
@@ -2321,6 +2322,15 @@
   if (CI.isInlineAsm())
     return translateInlineAsm(CI, MIRBuilder);
 
+  if (F && F->hasFnAttribute("dontcall")) {
+    unsigned LocCookie = 0;
+    if (MDNode *MD = CI.getMetadata("srcloc"))
+      LocCookie =
+          mdconst::extract<ConstantInt>(MD->getOperand(0))->getZExtValue();
+    DiagnosticInfoDontCall D(F->getName(), LocCookie);
+    F->getContext().diagnose(D);
+  }
+
   Intrinsic::ID ID = Intrinsic::not_intrinsic;
   if (F && F->isIntrinsic()) {
     ID = F->getIntrinsicID();
diff --git a/llvm/lib/CodeGen/SelectionDAG/FastISel.cpp b/llvm/lib/CodeGen/SelectionDAG/FastISel.cpp
--- a/llvm/lib/CodeGen/SelectionDAG/FastISel.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/FastISel.cpp
@@ -75,6 +75,7 @@
 #include "llvm/IR/DebugInfo.h"
 #include "llvm/IR/DebugLoc.h"
 #include "llvm/IR/DerivedTypes.h"
+#include "llvm/IR/DiagnosticInfo.h"
 #include "llvm/IR/Function.h"
 #include "llvm/IR/GetElementPtrTypeIterator.h"
 #include "llvm/IR/GlobalValue.h"
@@ -1151,6 +1152,16 @@
   CLI.setCallee(RetTy, FuncTy, CI->getCalledOperand(), std::move(Args), *CI)
       .setTailCall(IsTailCall);
 
+  if (const Function *F = CI->getCalledFunction())
+    if (F->hasFnAttribute("dontcall")) {
+      unsigned LocCookie = 0;
+      if (MDNode *MD = CI->getMetadata("srcloc"))
+        LocCookie =
+            mdconst::extract<ConstantInt>(MD->getOperand(0))->getZExtValue();
+      DiagnosticInfoDontCall D(F->getName(), LocCookie);
+      F->getContext().diagnose(D);
+    }
+
   return lowerCallTo(CLI);
 }
 
diff --git a/llvm/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp b/llvm/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp
--- a/llvm/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp
@@ -69,6 +69,7 @@
 #include "llvm/IR/DataLayout.h"
 #include "llvm/IR/DebugInfoMetadata.h"
 #include "llvm/IR/DerivedTypes.h"
+#include "llvm/IR/DiagnosticInfo.h"
 #include "llvm/IR/Function.h"
 #include "llvm/IR/GetElementPtrTypeIterator.h"
 #include "llvm/IR/InlineAsm.h"
@@ -7918,6 +7919,15 @@
   }
 
   if (Function *F = I.getCalledFunction()) {
+    if (F->hasFnAttribute("dontcall")) {
+      unsigned LocCookie = 0;
+      if (MDNode *MD = I.getMetadata("srcloc"))
+        LocCookie =
+            mdconst::extract<ConstantInt>(MD->getOperand(0))->getZExtValue();
+      DiagnosticInfoDontCall D(F->getName(), LocCookie);
+      DAG.getContext()->diagnose(D);
+    }
+
     if (F->isDeclaration()) {
       // Is this an LLVM intrinsic or a target-specific intrinsic?
       unsigned IID = F->getIntrinsicID();
diff --git a/llvm/lib/IR/DiagnosticInfo.cpp b/llvm/lib/IR/DiagnosticInfo.cpp
--- a/llvm/lib/IR/DiagnosticInfo.cpp
+++ b/llvm/lib/IR/DiagnosticInfo.cpp
@@ -401,3 +401,7 @@
 
 void OptimizationRemarkAnalysisFPCommute::anchor() {}
 void OptimizationRemarkAnalysisAliasing::anchor() {}
+
+void DiagnosticInfoDontCall::print(DiagnosticPrinter &DP) const {
+  DP << "call to " << getFunctionName() << " marked \"dontcall\"";
+}
diff --git a/llvm/test/CodeGen/X86/attr-dontcall.ll b/llvm/test/CodeGen/X86/attr-dontcall.ll
new file mode 100644
--- /dev/null
+++ b/llvm/test/CodeGen/X86/attr-dontcall.ll
@@ -0,0 +1,11 @@
+; RUN: not llc -global-isel=0 -fast-isel=0 -stop-after=finalize-isel %s 2>&1 | FileCheck %s
+; RUN: not llc -global-isel=0 -fast-isel=1 -stop-after=finalize-isel %s 2>&1 | FileCheck %s
+; RUN: not llc -global-isel=1 -fast-isel=0 -stop-after=irtranslator %s 2>&1 | FileCheck %s
+
+declare void @foo() "dontcall"
+define void @bar() {
+  call void @foo()
+  ret void
+}
+
+; CHECK: error: call to foo marked "dontcall"