ThinLTOBitcodeWriter: Split modules with __cfi_check and no type metadata.

Eli Friedman found a case that was not handled correctly by #154833 where
we failed to split the module if it contained a __cfi_check function but
no type metadata. Handle this case correctly by checking for __cfi_check
when deciding whether to split.

Reviewers: teresajohnson, efriedma-quic

Reviewed By: efriedma-quic

Pull Request: #155930

Author: pcc

llvm/lib/Transforms/IPO/ThinLTOBitcodeWriter.cpp

@@ -269,6 +269,12 @@ static bool enableUnifiedLTO(Module &M) {
 }
 #endif
 
+bool 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";
+}
+
 // If it's possible to split M into regular and thin LTO parts, do so and write
 // a multi-module bitcode file with the two parts to OS. Otherwise, write only a
 // regular LTO bitcode file to OS.
@@ -350,19 +356,13 @@ 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))
+        if (mustEmitToMergedModule(GV))
           return true;
         if (auto *F = dyn_cast<Function>(GV))
           return EligibleVirtualFns.count(F);
@@ -380,7 +380,7 @@ void splitAndWriteThinLTOBitcode(
   cloneUsedGlobalVariables(M, *MergedM, /*CompilerUsed*/ true);
 
   for (Function &F : *MergedM)
-    if (!F.isDeclaration() && !MustEmitToMergedModule(&F)) {
+    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.
@@ -406,7 +406,7 @@ void splitAndWriteThinLTOBitcode(
     if (const auto *C = GV->getComdat())
       if (MergedMComdats.count(C))
         return false;
-    if (MustEmitToMergedModule(GV))
+    if (mustEmitToMergedModule(GV))
       return false;
     return true;
   });
@@ -529,11 +529,13 @@ bool enableSplitLTOUnit(Module &M) {
   return EnableSplitLTOUnit;
 }
 
-// Returns whether this module needs to be split because it uses type metadata.
-bool hasTypeMetadata(Module &M) {
+// Returns whether this module needs to be split (if splitting is enabled).
+bool requiresSplit(Module &M) {
   for (auto &GO : M.global_objects()) {
     if (GO.hasMetadata(LLVMContext::MD_type))
       return true;
+    if (mustEmitToMergedModule(&GO))
+      return true;
   }
   return false;
 }
@@ -543,9 +545,9 @@ bool writeThinLTOBitcode(raw_ostream &OS, raw_ostream *ThinLinkOS,
                          Module &M, const ModuleSummaryIndex *Index,
                          const bool ShouldPreserveUseListOrder) {
   std::unique_ptr<ModuleSummaryIndex> NewIndex = nullptr;
-  // See if this module has any type metadata. If so, we try to split it
+  // See if this module needs to be split. If so, we try to split it
   // or at least promote type ids to enable WPD.
-  if (hasTypeMetadata(M)) {
+  if (requiresSplit(M)) {
     if (enableSplitLTOUnit(M)) {
       splitAndWriteThinLTOBitcode(OS, ThinLinkOS, AARGetter, M,
                                   ShouldPreserveUseListOrder);

llvm/test/Transforms/ThinLTOBitcodeWriter/cfi-check-no-metadata.ll

@@ -0,0 +1,18 @@
+; 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, even if nothing has !type metadata
+; on the ThinLTO side.
+
+; M0: @g = global i32 0
+@g = global i32 0
+
+; M1: define void @__cfi_check() #0
+define void @__cfi_check() #0 {
+  ret void
+}
+
+; M1: attributes #0 = { "branch-target-enforcement" }
+attributes #0 = { "branch-target-enforcement" }