[clang][fat-lto-objects] Make module flags match non-FatLTO pipelines (llvm#83159)

In addition to being rather hard to follow, there isn't a good reason
why FatLTO shouldn't just share the same code for setting module flags
for (Thin)LTO. This patch simplifies the logic and makes sure we use set
these flags in a consistent way, independent of FatLTO.

Additionally, we now test that output in the .llvm.lto section actually
matches the output from Full and Thin LTO compilation.

(cherry picked from commit 7d8b50a)

Author: ilovepi

clang/lib/CodeGen/BackendUtil.cpp

@@ -186,6 +186,14 @@ class EmitAssemblyHelper {
            TargetTriple.getVendor() != llvm::Triple::Apple;
   }
 
+  /// Check whether we should emit a flag for UnifiedLTO.
+  /// The UnifiedLTO module flag should be set when UnifiedLTO is enabled for
+  /// ThinLTO or Full LTO with module summaries.
+  bool shouldEmitUnifiedLTOModueFlag() const {
+    return CodeGenOpts.UnifiedLTO &&
+           (CodeGenOpts.PrepareForThinLTO || shouldEmitRegularLTOSummary());
+  }
+
 public:
   EmitAssemblyHelper(DiagnosticsEngine &_Diags,
                      const HeaderSearchOptions &HeaderSearchOpts,
@@ -1029,7 +1037,8 @@ void EmitAssemblyHelper::RunOptimizationPipeline(
   if (!actionRequiresCodeGen(Action) && CodeGenOpts.VerifyModule)
     MPM.addPass(VerifierPass());
 
-  if (Action == Backend_EmitBC || Action == Backend_EmitLL) {
+  if (Action == Backend_EmitBC || Action == Backend_EmitLL ||
+      CodeGenOpts.FatLTO) {
     if (CodeGenOpts.PrepareForThinLTO && !CodeGenOpts.DisableLLVMPasses) {
       if (!TheModule->getModuleFlag("EnableSplitLTOUnit"))
         TheModule->addModuleFlag(llvm::Module::Error, "EnableSplitLTOUnit",
@@ -1040,11 +1049,9 @@ void EmitAssemblyHelper::RunOptimizationPipeline(
           if (!ThinLinkOS)
             return;
         }
-        if (CodeGenOpts.UnifiedLTO)
-          TheModule->addModuleFlag(llvm::Module::Error, "UnifiedLTO", uint32_t(1));
         MPM.addPass(ThinLTOBitcodeWriterPass(
             *OS, ThinLinkOS ? &ThinLinkOS->os() : nullptr));
-      } else {
+      } else if (Action == Backend_EmitLL) {
         MPM.addPass(PrintModulePass(*OS, "", CodeGenOpts.EmitLLVMUseLists,
                                     /*EmitLTOSummary=*/true));
       }
@@ -1058,24 +1065,17 @@ void EmitAssemblyHelper::RunOptimizationPipeline(
         if (!TheModule->getModuleFlag("EnableSplitLTOUnit"))
           TheModule->addModuleFlag(llvm::Module::Error, "EnableSplitLTOUnit",
                                    uint32_t(1));
-        if (CodeGenOpts.UnifiedLTO)
-          TheModule->addModuleFlag(llvm::Module::Error, "UnifiedLTO", uint32_t(1));
       }
-      if (Action == Backend_EmitBC)
+      if (Action == Backend_EmitBC) {
         MPM.addPass(BitcodeWriterPass(*OS, CodeGenOpts.EmitLLVMUseLists,
                                       EmitLTOSummary));
-      else
+      } else if (Action == Backend_EmitLL) {
         MPM.addPass(PrintModulePass(*OS, "", CodeGenOpts.EmitLLVMUseLists,
                                     EmitLTOSummary));
+      }
     }
-  }
-  if (CodeGenOpts.FatLTO) {
-    // Set the EnableSplitLTOUnit and UnifiedLTO module flags, since FatLTO
-    // uses a different action than Backend_EmitBC or Backend_EmitLL.
-    if (!TheModule->getModuleFlag("EnableSplitLTOUnit"))
-      TheModule->addModuleFlag(llvm::Module::Error, "EnableSplitLTOUnit",
-                               uint32_t(CodeGenOpts.EnableSplitLTOUnit));
-    if (CodeGenOpts.UnifiedLTO && !TheModule->getModuleFlag("UnifiedLTO"))
+
+    if (shouldEmitUnifiedLTOModueFlag())
       TheModule->addModuleFlag(llvm::Module::Error, "UnifiedLTO", uint32_t(1));
   }
 

clang/test/CodeGen/fat-lto-objects.c

@@ -11,10 +11,11 @@
 // RUN: llvm-objcopy --dump-section=.llvm.lto=%t.full.split.bc %t.full.split.o
 // RUN: llvm-dis %t.full.split.bc -o - | FileCheck %s --check-prefixes=FULL,SPLIT,NOUNIFIED
 
+/// Full LTO always sets EnableSplitLTOUnit when the summary is used.
 // RUN: %clang -cc1 -triple x86_64-unknown-linux-gnu -flto=full -ffat-lto-objects -emit-obj < %s -o %t.full.nosplit.o
 // RUN: llvm-readelf -S %t.full.nosplit.o | FileCheck %s --check-prefixes=ELF
 // RUN: llvm-objcopy --dump-section=.llvm.lto=%t.full.nosplit.bc %t.full.nosplit.o
-// RUN: llvm-dis %t.full.nosplit.bc -o - | FileCheck %s --check-prefixes=FULL,NOSPLIT,NOUNIFIED
+// RUN: llvm-dis %t.full.nosplit.bc -o - | FileCheck %s --check-prefixes=FULL,SPLIT,NOUNIFIED
 
 // RUN: %clang -cc1 -triple x86_64-unknown-linux-gnu -flto=thin -fsplit-lto-unit -ffat-lto-objects -emit-obj < %s -o %t.thin.split.o
 // RUN: llvm-readelf -S %t.thin.split.o | FileCheck %s --check-prefixes=ELF
@@ -34,6 +35,21 @@
 // RUN: %clang -cc1 -triple x86_64-unknown-linux-gnu -flto=full -ffat-lto-objects -fsplit-lto-unit -S < %s -o - \
 // RUN: | FileCheck %s --check-prefixes=ASM
 
+/// Make sure that FatLTO generates .llvm.lto sections that are the same as the output from normal LTO compilations
+// RUN: %clang -O2 --target=x86_64-unknown-linux-gnu -fPIE -flto=full -ffat-lto-objects -c %s -o %t.fatlto.full.o
+// RUN: llvm-objcopy --dump-section=.llvm.lto=%t.fatlto.full.bc %t.fatlto.full.o
+// RUN: llvm-dis < %t.fatlto.full.bc -o %t.fatlto.full.ll
+// RUN: %clang -O2 --target=x86_64-unknown-linux-gnu -fPIE -flto=full -c %s -o %t.nofat.full.bc
+// RUN: llvm-dis < %t.nofat.full.bc -o %t.nofat.full.ll
+// RUN: diff %t.fatlto.full.ll %t.nofat.full.ll
+
+// RUN: %clang -O2 --target=x86_64-unknown-linux-gnu -fPIE -flto=thin -ffat-lto-objects -c %s -o %t.fatlto.thin.o
+// RUN: llvm-objcopy --dump-section=.llvm.lto=%t.fatlto.thin.bc %t.fatlto.thin.o
+// RUN: llvm-dis < %t.fatlto.thin.bc -o %t.fatlto.thin.ll
+// RUN: %clang -O2 --target=x86_64-unknown-linux-gnu -fPIE -flto=thin -c %s -o %t.nofat.thin.bc
+// RUN: llvm-dis < %t.nofat.thin.bc -o %t.nofat.thin.ll
+// RUN: diff %t.fatlto.thin.ll %t.nofat.thin.ll
+
 /// Be sure we enable split LTO units correctly under -ffat-lto-objects.
 //   SPLIT: ![[#]] = !{i32 1, !"EnableSplitLTOUnit", i32 1}
 // NOSPLIT: ![[#]] = !{i32 1, !"EnableSplitLTOUnit", i32 0}
@@ -51,6 +67,9 @@
 // ASM-NEXT:        .asciz  "BC
 // ASM-NEXT: .size   .Lllvm.embedded.object
 
+const char* foo = "foo";
+
 int test(void) {
+  const char* bar = "bar";
   return 0xabcd;
 }