[clang] 2ba1cc8 - [Clang][CodeGen] Fix bad codegen when building Clang with latest MSVC (#102681)

[via cfe-commits]

Author: Alexandre Ganea
Date: 2024-08-10T16:26:10-04:00
New Revision: 2ba1cc8ea53cd76fbeac79dad837dcae32cc7e59

URL: https://github.com/llvm/llvm-project/commit/2ba1cc8ea53cd76fbeac79dad837dcae32cc7e59
DIFF: https://github.com/llvm/llvm-project/commit/2ba1cc8ea53cd76fbeac79dad837dcae32cc7e59.diff

LOG: [Clang][CodeGen] Fix bad codegen when building Clang with latest MSVC (#102681)

Before this PR, when using the latest MSVC `Microsoft (R) C/C++
Optimizing Compiler Version 19.40.33813 for x64` one of the Clang unit
test used to fail: `CodeGenObjC/gnustep2-direct-method.m`, see full
failure log:
[here](https://github.com/llvm/llvm-project/pull/100517#issuecomment-2266269490).

This PR temporarily shuffles around the code to make the MSVC inliner/
optimizer happy and avoid the bug.

MSVC bug report:
https://developercommunity.visualstudio.com/t/Bad-code-generation-when-building-LLVM-w/10719589?port=1025&fsid=e572244a-cde7-4d75-a73d-9b8cd94204dd

Added: 
    

Modified: 
    clang/lib/CodeGen/CGObjCGNU.cpp

Removed: 
    


################################################################################
diff  --git a/clang/lib/CodeGen/CGObjCGNU.cpp b/clang/lib/CodeGen/CGObjCGNU.cpp
index 948b10954ebbe..ca5804018227e 100644
--- a/clang/lib/CodeGen/CGObjCGNU.cpp
+++ b/clang/lib/CodeGen/CGObjCGNU.cpp
@@ -2092,10 +2092,15 @@ class CGObjCGNUstep2 : public CGObjCGNUstep {
         auto *classStart =
             llvm::StructType::get(PtrTy, PtrTy, PtrTy, LongTy, LongTy);
         auto &astContext = CGM.getContext();
-        auto flags = Builder.CreateLoad(
-            Address{Builder.CreateStructGEP(classStart, selfValue, 4), LongTy,
-                    CharUnits::fromQuantity(
-                        astContext.getTypeAlign(astContext.UnsignedLongTy))});
+        // FIXME: The following few lines up to and including the call to
+        // `CreateLoad` were known to miscompile when MSVC 19.40.33813 is used
+        // to build Clang. When the bug is fixed in future MSVC releases, we
+        // should revert these lines to their previous state. See discussion in
+        // https://github.com/llvm/llvm-project/pull/102681
+        llvm::Value *Val = Builder.CreateStructGEP(classStart, selfValue, 4);
+        auto Align = CharUnits::fromQuantity(
+            astContext.getTypeAlign(astContext.UnsignedLongTy));
+        auto flags = Builder.CreateLoad(Address{Val, LongTy, Align});
         auto isInitialized =
             Builder.CreateAnd(flags, ClassFlags::ClassFlagInitialized);
         llvm::BasicBlock *notInitializedBlock =