[clang] [clang][fatlto] Don't set ThinLTO module flag with FatLTO (PR #75079)

[Paul Kirth via cfe-commits]

https://github.com/ilovepi created https://github.com/llvm/llvm-project/pull/75079

Since FatLTO now uses the UnifiedLTO pipeline, we should not set the ThinLTO module flag to true, since it may cause an assertion failure. See https://github.com/llvm/llvm-project/issues/70703 for context.

>From 14805bacc7adce5ca41b6f064c219cd4a7c1fe89 Mon Sep 17 00:00:00 2001
From: Paul Kirth <paulkirth at google.com>
Date: Mon, 11 Dec 2023 09:38:44 -0800
Subject: [PATCH] [clang][fatlto] Don't set ThinLTO module flag with FatLTO

Since FatLTO now uses the UnifiedLTO pipeline, we should not set
the ThinLTO module flag to true, since it may cause an assertion
failure. See https://github.com/llvm/llvm-project/issues/70703 for
context.
---
 clang/lib/CodeGen/BackendUtil.cpp    | 5 +----
 clang/test/CodeGen/fat-lto-objects.c | 3 +--
 2 files changed, 2 insertions(+), 6 deletions(-)

diff --git a/clang/lib/CodeGen/BackendUtil.cpp b/clang/lib/CodeGen/BackendUtil.cpp
index 77455c075cab0d..5ffda8117db1b3 100644
--- a/clang/lib/CodeGen/BackendUtil.cpp
+++ b/clang/lib/CodeGen/BackendUtil.cpp
@@ -1063,11 +1063,8 @@ void EmitAssemblyHelper::RunOptimizationPipeline(
     }
   }
   if (CodeGenOpts.FatLTO) {
-    // Set module flags, like EnableSplitLTOUnit and UnifiedLTO, since FatLTO
+    // Set the EnableSplitLTOUnit and UnifiedLTO module flags, since FatLTO
     // uses a different action than Backend_EmitBC or Backend_EmitLL.
-    if (!TheModule->getModuleFlag("ThinLTO"))
-      TheModule->addModuleFlag(llvm::Module::Error, "ThinLTO",
-                               uint32_t(CodeGenOpts.PrepareForThinLTO));
     if (!TheModule->getModuleFlag("EnableSplitLTOUnit"))
       TheModule->addModuleFlag(llvm::Module::Error, "EnableSplitLTOUnit",
                                uint32_t(CodeGenOpts.EnableSplitLTOUnit));
diff --git a/clang/test/CodeGen/fat-lto-objects.c b/clang/test/CodeGen/fat-lto-objects.c
index 95207e77c244cc..5c8ad1fd93c4b3 100644
--- a/clang/test/CodeGen/fat-lto-objects.c
+++ b/clang/test/CodeGen/fat-lto-objects.c
@@ -35,8 +35,7 @@
 //   SPLIT: ![[#]] = !{i32 1, !"EnableSplitLTOUnit", i32 1}
 // NOSPLIT: ![[#]] = !{i32 1, !"EnableSplitLTOUnit", i32 0}
 
-/// Check that the ThinLTO metadata is set true for both full and thin LTO, since FatLTO is based on UnifiedLTO.
-//     FULL: ![[#]] = !{i32 1, !"ThinLTO", i32 1}
+// FULL-NOT: ![[#]] = !{i32 1, !"ThinLTO", i32 0}
 // THIN-NOT: ![[#]] = !{i32 1, !"ThinLTO", i32 0}
 
 /// FatLTO always uses UnifiedLTO. It's an error if they aren't set together