ThinLTOBitcodeWriter: Emit __cfi_check to full LTO part of bitcode file.

The CrossDSOCFI pass runs on the full LTO module and fills in the body of __cfi_check. This function must have the correct attributes in order to be compatible with the rest of the program. For example, when building with -mbranch-protection=standard, the function must have the branch-target-enforcement attribute, which is normally added by Clang. When __cfi_check is missing, CrossDSOCFI will give it the default set of attributes, which are likely incorrect. Therefore, emit __cfi_check to the full LTO part, where CrossDSOCFI will see it. Reviewers: efriedma-quic, vitalybuka, fmayer Reviewed By: efriedma-quic Pull Request: #154833

Author: pcc

llvm/lib/Transforms/IPO/ThinLTOBitcodeWriter.cpp

@@ -350,12 +350,20 @@ void splitAndWriteThinLTOBitcode(
  });
  }
 
+ auto MustEmitToMergedModule = [](const GlobalValue *GV) {
+ // The __cfi_check definition is filled in by the CrossDSOCFI pass which
+ // runs only in the merged module.
+ return GV->getName() == "__cfi_check";
+ };
+
  ValueToValueMapTy VMap;
  std::unique_ptr<Module> MergedM(
  CloneModule(M, VMap, [&](const GlobalValue *GV) -> bool {
  if (const auto *C = GV->getComdat())
  if (MergedMComdats.count(C))
  return true;
+ if (MustEmitToMergedModule(GV))
+ return true;
  if (auto *F = dyn_cast<Function>(GV))
  return EligibleVirtualFns.count(F);
  if (auto *GVar =
@@ -372,7 +380,7 @@ void splitAndWriteThinLTOBitcode(
  cloneUsedGlobalVariables(M, *MergedM, /*CompilerUsed*/ true);
 
  for (Function &F : *MergedM)
- if (!F.isDeclaration()) {
+ if (!F.isDeclaration() && !MustEmitToMergedModule(&F)) {
  // Reset the linkage of all functions eligible for virtual constant
  // propagation. The canonical definitions live in the thin LTO module so
  // that they can be imported.
@@ -398,6 +406,8 @@ void splitAndWriteThinLTOBitcode(
  if (const auto *C = GV->getComdat())
  if (MergedMComdats.count(C))
  return false;
+ if (MustEmitToMergedModule(GV))
+ return false;
  return true;
  });
 

llvm/test/Transforms/ThinLTOBitcodeWriter/cfi-check.ll

@@ -0,0 +1,19 @@
+; RUN: opt -thinlto-bc -thinlto-split-lto-unit -o %t %s
+; RUN: llvm-modextract -b -n 0 -o - %t | llvm-dis | FileCheck --check-prefix=M0 %s
+; RUN: llvm-modextract -b -n 1 -o - %t | llvm-dis | FileCheck --check-prefix=M1 %s
+
+; Check that __cfi_check is emitted on the full LTO side with
+; attributes preserved.
+
+; M0: define void @f()
+define void @f() !type !{!"f1", i32 0} {
+ ret void 
+}
+
+; M1: define void @__cfi_check() #0
+define void @__cfi_check() #0 {
+ ret void
+}
+
+; M1: attributes #0 = { "branch-target-enforcement" }
+attributes #0 = { "branch-target-enforcement" }