[Clang][CodeGen] Fix use of CXXThisValue with StrictVTablePointers

When emitting non-virtual base initializers for the constructor prologue,
we would potentially use a re-laundered this pointer value from a
previous block, which subsequently would not dominate this use.

With this fix, we always launder the original CXXThisValue.

This fixes #67937

Author: mizvekov

clang/lib/CodeGen/CGClass.cpp

@@ -28,6 +28,7 @@
 #include "clang/CodeGen/CGFunctionInfo.h"
 #include "llvm/IR/Intrinsics.h"
 #include "llvm/IR/Metadata.h"
+#include "llvm/Support/SaveAndRestore.h"
 #include "llvm/Transforms/Utils/SanitizerStats.h"
 #include <optional>
 
@@ -1291,10 +1292,10 @@ void CodeGenFunction::EmitCtorPrologue(const CXXConstructorDecl *CD,
     assert(BaseCtorContinueBB);
   }
 
-  llvm::Value *const OldThis = CXXThisValue;
   for (; B != E && (*B)->isBaseInitializer() && (*B)->isBaseVirtual(); B++) {
     if (!ConstructVBases)
       continue;
+    SaveAndRestore ThisRAII(CXXThisValue);
     if (CGM.getCodeGenOpts().StrictVTablePointers &&
         CGM.getCodeGenOpts().OptimizationLevel > 0 &&
         isInitializerOfDynamicClass(*B))
@@ -1311,16 +1312,14 @@ void CodeGenFunction::EmitCtorPrologue(const CXXConstructorDecl *CD,
   // Then, non-virtual base initializers.
   for (; B != E && (*B)->isBaseInitializer(); B++) {
     assert(!(*B)->isBaseVirtual());
-
+    SaveAndRestore ThisRAII(CXXThisValue);
     if (CGM.getCodeGenOpts().StrictVTablePointers &&
         CGM.getCodeGenOpts().OptimizationLevel > 0 &&
         isInitializerOfDynamicClass(*B))
       CXXThisValue = Builder.CreateLaunderInvariantGroup(LoadCXXThis());
     EmitBaseInitializer(*this, ClassDecl, *B);
   }
 
-  CXXThisValue = OldThis;
-
   InitializeVTablePointers(ClassDecl);
 
   // And finally, initialize class members.

clang/test/CodeGenCXX/strict-vtable-pointers-GH67937.cpp

@@ -1,6 +1,5 @@
-// RUN: %clang_cc1 %s -I%S -triple=x86_64-pc-windows-msvc -fstrict-vtable-pointers -disable-llvm-passes -disable-llvm-verifier -O1 -emit-llvm -o %t.ll
+// RUN: %clang_cc1 %s -I%S -triple=x86_64-pc-windows-msvc -fstrict-vtable-pointers -disable-llvm-passes -O1 -emit-llvm -o %t.ll
 // RUN: FileCheck %s < %t.ll
-// RUN: not llvm-as < %t.ll -o /dev/null 2>&1 | FileCheck %s --check-prefix=CHECK-VERIFIER
 
 struct A {
   virtual ~A();
@@ -9,11 +8,6 @@ struct B : virtual A {};
 class C : B {};
 C foo;
 
-// FIXME: This is not supposed to generate invalid IR!
-// CHECK-VERIFIER: Instruction does not dominate all uses!
-// CHECK-VERIFIER-NEXT: %1 = call ptr @llvm.launder.invariant.group.p0(ptr %this1)
-// CHECK-VERIFIER-NEXT: %3 = call ptr @llvm.launder.invariant.group.p0(ptr %1)
-
 // CHECK-LABEL: define {{.*}} @"??0C@@QEAA@XZ"(ptr {{.*}} %this, i32 {{.*}} %is_most_derived)
 // CHECK: ctor.init_vbases:
 // CHECK-NEXT: %0 = getelementptr inbounds i8, ptr %this1, i64 0
@@ -24,6 +18,5 @@ C foo;
 // CHECK-NEXT: br label %ctor.skip_vbases
 // CHECK-EMPTY:
 // CHECK-NEXT: ctor.skip_vbases:
-// FIXME: Should be using '%this1' instead of %1 below.
-// CHECK-NEXT: %3 = call ptr @llvm.launder.invariant.group.p0(ptr %1)
+// CHECK-NEXT: %3 = call ptr @llvm.launder.invariant.group.p0(ptr %this1)
 // CHECK-NEXT: %call3 = call noundef ptr @"??0B@@QEAA@XZ"(ptr {{.*}} %3, i32 noundef 0) #2