diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
--- a/clang/docs/ReleaseNotes.rst
+++ b/clang/docs/ReleaseNotes.rst
@@ -94,7 +94,8 @@
 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
@@ -3823,3 +3823,12 @@
   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/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
@@ -1212,6 +1212,7 @@
 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/include/clang/Basic/DiagnosticSemaKinds.td b/clang/include/clang/Basic/DiagnosticSemaKinds.td
--- a/clang/include/clang/Basic/DiagnosticSemaKinds.td
+++ b/clang/include/clang/Basic/DiagnosticSemaKinds.td
@@ -11169,6 +11169,9 @@
   "'%0' qualifier on omitted return type %1 has no effect">,
   InGroup<IgnoredQualifiers>;
 
+def err_attribute_removed_on_redeclaration : Error<
+  "'%0' attribute removed from redeclared variable">;
+
 def warn_shadow_field : Warning<
   "%select{parameter|non-static data member}3 %0 %select{|of %1 }3shadows "
   "member inherited from type %2">, InGroup<ShadowField>, DefaultIgnore;
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 +759,33 @@
   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());
+
+        // FIXME: we can't yet diagnose indirect calls. When/if we can, we
+        // should instead assert that LocCookie.isValid().
+        if (!LocCookie.isValid())
+          return;
+
+        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 +861,9 @@
   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
@@ -2188,6 +2188,11 @@
                                                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/SemaDecl.cpp b/clang/lib/Sema/SemaDecl.cpp
--- a/clang/lib/Sema/SemaDecl.cpp
+++ b/clang/lib/Sema/SemaDecl.cpp
@@ -2979,6 +2979,15 @@
     }
   }
 
+  // If redeclaration removes error or warning attribute...
+  if (auto *EA = New->getAttr<ErrorAttr>())
+    if (!Old->hasAttr<ErrorAttr>()) {
+      Diag(New->getLocation(), diag::err_attribute_removed_on_redeclaration)
+          << EA->getSpelling();
+      Diag(Old->getLocation(), diag::note_previous_declaration);
+      return;
+    }
+
   // Redeclaration adds code-seg attribute.
   const auto *NewCSA = New->getAttr<CodeSegAttr>();
   if (NewCSA && !Old->hasAttr<CodeSegAttr>() &&
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
@@ -947,6 +947,34 @@
     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;
+    }
+    if (EA->getUserDiagnostic() != Str) {
+      S.Diag(AL.getLoc(), diag::warn_duplicate_attribute) << EA->getSpelling();
+      S.Diag(EA->getLoc(), diag::note_previous_attribute);
+    }
+    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).
@@ -7941,6 +7969,9 @@
   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 +1,23 @@
+// 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();
+}
+
+// FIXME: indirect call detection not yet supported.
+void (*quux)(void);
+
+void indirect(void) {
+  quux = foo;
+  quux();
+}
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 +1,32 @@
+// 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 {{attribute error is already applied with different arguments}}
+void                                        // expected-note@-1 {{previous attribute is here}}
+duplicate_errors(void);
+
+__attribute__((warning("one"), warning("two"))) // expected-warning {{attribute warning is already applied with different arguments}}
+void                                            // expected-note@-1 {{previous attribute is here}}
+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}}
+  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 +1,45 @@
+// 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}}
+
+/*
+ * Note: we differ from GCC here; rather than support redeclarations that add
+ * or remove this fn attr, we diagnose such differences.
+ */
+
+void foo(void); // expected-note {{previous declaration is here}}
+
+void x0(void) { foo(); }
+
+__attribute__((error("oh no foo")))
+void foo(void); // expected-error {{'error' attribute removed from redeclared variable}}
+
+void x1(void) { foo(); }
+
+void foo(void);
+
+void x2(void) { foo(); }
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 +1,45 @@
+// 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}}
+
+/*
+ * Note: we differ from GCC here; rather than support redeclarations that add
+ * or remove this fn attr, we diagnose such differences.
+ */
+
+void foo(void); // expected-note {{previous declaration is here}}
+
+void x0(void) { foo(); }
+
+__attribute__((warning("oh no foo")))
+void foo(void); // expected-error {{'warning' attribute removed from redeclared variable}}
+
+void x1(void) { foo(); }
+
+void foo(void);
+
+void x2(void) { foo(); }
diff --git a/llvm/docs/LangRef.rst b/llvm/docs/LangRef.rst
--- a/llvm/docs/LangRef.rst
+++ b/llvm/docs/LangRef.rst
@@ -1582,6 +1582,12 @@
     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"
diff --git a/llvm/test/ThinLTO/X86/dontcall.ll b/llvm/test/ThinLTO/X86/dontcall.ll
new file mode 100644
--- /dev/null
+++ b/llvm/test/ThinLTO/X86/dontcall.ll
@@ -0,0 +1,33 @@
+; RUN: split-file %s %t
+; RUN: opt -module-summary %t/a.s -o %t/a.bc
+; RUN: opt -module-summary %t/b.s -o %t/b.bc
+; RUN: not llvm-lto2 run %t/a.bc %t/b.bc -o %t/out -save-temps 2>&1 \
+; RUN:   -r=%t/a.bc,callee,px \
+; RUN:   -r=%t/b.bc,callee,x  \
+; RUN:   -r=%t/b.bc,caller,px
+
+; TODO: As part of LTO, we check that types match, but *we don't yet check that
+; attributes match!!! What should happen if we remove "dontcall" from the
+; definition or declaration of @callee?
+
+; CHECK: call to callee marked "dontcall"
+
+;--- a.s
+target datalayout = "e-m:e-p270:32:32-p271:32:32-p272:64:64-i64:64-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-unknown-linux-gnu"
+
+define i32 @callee() "dontcall" noinline {
+  ret i32 42
+}
+
+;--- b.s
+target datalayout = "e-m:e-p270:32:32-p271:32:32-p272:64:64-i64:64-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-unknown-linux-gnu"
+
+declare i32 @callee() "dontcall"
+
+define i32 @caller() {
+entry:
+  %0 = call i32 @callee()
+  ret i32 %0
+}