[cfi] Emit __cfi_check stub in the frontend.

Previously __cfi_check was created in LTO optimization pipeline, which
means LLD has no way of knowing about the existence of this symbol
without rescanning the LTO output object. As a result, LLD fails to
export __cfi_check, even when given --export-dynamic-symbol flag.

llvm-svn: 299806

Author: eugenis

clang/lib/CodeGen/CGExpr.cpp

@@ -2783,6 +2783,24 @@ void CodeGenFunction::EmitCfiSlowPathCheck(
   EmitBlock(Cont);
 }
 
+// Emit a stub for __cfi_check function so that the linker knows about this
+// symbol in LTO mode.
+void CodeGenFunction::EmitCfiCheckStub() {
+  llvm::Module *M = &CGM.getModule();
+  auto &Ctx = M->getContext();
+  llvm::Function *F = llvm::Function::Create(
+      llvm::FunctionType::get(VoidTy, {Int64Ty, Int8PtrTy, Int8PtrTy}, false),
+      llvm::GlobalValue::WeakAnyLinkage, "__cfi_check", M);
+  llvm::BasicBlock *BB = llvm::BasicBlock::Create(Ctx, "entry", F);
+  // FIXME: consider emitting an intrinsic call like
+  // call void @llvm.cfi_check(i64 %0, i8* %1, i8* %2)
+  // which can be lowered in CrossDSOCFI pass to the actual contents of
+  // __cfi_check. This would allow inlining of __cfi_check calls.
+  llvm::CallInst::Create(
+      llvm::Intrinsic::getDeclaration(M, llvm::Intrinsic::trap), "", BB);
+  llvm::ReturnInst::Create(Ctx, nullptr, BB);
+}
+
 // This function is basically a switch over the CFI failure kind, which is
 // extracted from CFICheckFailData (1st function argument). Each case is either
 // llvm.trap or a call to one of the two runtime handlers, based on

clang/lib/CodeGen/CodeGenFunction.h

@@ -3524,6 +3524,9 @@ class CodeGenFunction : public CodeGenTypeCache {
   /// "trap-func-name" if specified.
   llvm::CallInst *EmitTrapCall(llvm::Intrinsic::ID IntrID);
 
+  /// \brief Emit a stub for the cross-DSO CFI check function.
+  void EmitCfiCheckStub();
+
   /// \brief Emit a cross-DSO CFI failure handling function.
   void EmitCfiCheckFail();
 

clang/lib/CodeGen/CodeGenModule.cpp

@@ -406,8 +406,10 @@ void CodeGenModule::Release() {
   EmitDeferredUnusedCoverageMappings();
   if (CoverageMapping)
     CoverageMapping->emit();
-  if (CodeGenOpts.SanitizeCfiCrossDso)
+  if (CodeGenOpts.SanitizeCfiCrossDso) {
     CodeGenFunction(*this).EmitCfiCheckFail();
+    CodeGenFunction(*this).EmitCfiCheckStub();
+  }
   emitAtAvailableLinkGuard();
   emitLLVMUsed();
   if (SanStats)

clang/test/CodeGen/cfi-check-fail.c

@@ -72,3 +72,8 @@ void caller(void (*f)()) {
 
 // CHECK: [[CONT5]]:
 // CHECK:   ret void
+
+// CHECK: define weak void @__cfi_check(i64, i8*, i8*)
+// CHECK-NOT: }
+// CHECK: call void @llvm.trap()
+// CHECK-NEXT: ret void