[cfi] Fix one -fno-sanitize-merge case, and add two TODOs (#135438)

-fno-sanitize-merge (introduced in
llvm/llvm-project#120464) nearly works for CFI:
code that calls EmitCheck will already check the merge options. This
patch fixes one EmitTrapCheck call, which did not check the merge
options, and for two other EmitTrapChecks, adds two TODOs that explain
why it is difficult to fix them.

Author: thurstond

clang/lib/CodeGen/CGClass.cpp

@@ -2895,7 +2895,8 @@ void CodeGenFunction::EmitVTablePtrCheck(const CXXRecordDecl *RD,
   }
 
   if (CGM.getCodeGenOpts().SanitizeTrap.has(M)) {
-    EmitTrapCheck(TypeTest, SanitizerHandler::CFICheckFail);
+    bool NoMerge = !CGM.getCodeGenOpts().SanitizeMergeHandlers.has(M);
+    EmitTrapCheck(TypeTest, SanitizerHandler::CFICheckFail, NoMerge);
     return;
   }
 

clang/lib/CodeGen/CGExpr.cpp

@@ -3923,7 +3923,11 @@ void CodeGenFunction::EmitCfiCheckFail() {
   // Data == nullptr means the calling module has trap behaviour for this check.
   llvm::Value *DataIsNotNullPtr =
       Builder.CreateICmpNE(Data, llvm::ConstantPointerNull::get(Int8PtrTy));
-  EmitTrapCheck(DataIsNotNullPtr, SanitizerHandler::CFICheckFail);
+  // TODO: since there is no data, we don't know the CheckKind, and therefore
+  // cannot inspect CGM.getCodeGenOpts().SanitizeMergeHandlers. We default to
+  // NoMerge = false. Users can disable merging by disabling optimization.
+  EmitTrapCheck(DataIsNotNullPtr, SanitizerHandler::CFICheckFail,
+                /*NoMerge=*/false);
 
   llvm::StructType *SourceLocationTy =
       llvm::StructType::get(VoidPtrTy, Int32Ty, Int32Ty);
@@ -3962,7 +3966,11 @@ void CodeGenFunction::EmitCfiCheckFail() {
       EmitCheck(std::make_pair(Cond, Ordinal), SanitizerHandler::CFICheckFail,
                 {}, {Data, Addr, ValidVtable});
     else
-      EmitTrapCheck(Cond, SanitizerHandler::CFICheckFail);
+      // TODO: we can't rely on CGM.getCodeGenOpts().SanitizeMergeHandlers.
+      // Although the compiler allows SanitizeMergeHandlers to be set
+      // independently of CGM.getLangOpts().Sanitize, Driver/SanitizerArgs.cpp
+      // requires that SanitizeMergeHandlers is a subset of Sanitize.
+      EmitTrapCheck(Cond, SanitizerHandler::CFICheckFail, /*NoMerge=*/false);
   }
 
   FinishFunction();