[Clang][LTO] Fix use of funified-lto and save-temps flags together

[bader]

clang -flto -funified-lto -save-temps test.c

Fails with the following error message:

module flag identifiers must be unique (or of 'require' type)
!"UnifiedLTO"
fatal error: error in backend: Broken module found, compilation aborted!
clang: error: clang frontend command failed with exit code 70 (use -v to see invocation)

Here is what the driver does when -save-temps flag is set:

clang -flto -funified-lto -save-temps test.c -ccc-print-phases
+- 0: input, "test.c", c
+- 1: preprocessor, {0}, cpp-output
+- 2: compiler, {1}, ir
+- 3: backend, {2}, lto-bc
4: linker, {3}, image

The IR output of "compiler" step has "UnifiedLTO" module flag. "backend"
step adds another module flag with "UnifiedLTO" identifier, which
invalidates the LLVM IR module.

[llvmbot]

@llvm/pr-subscribers-clang

@llvm/pr-subscribers-clang-codegen

Author: Alexey Bader (bader)

Changes

> clang -flto -funified-lto -save-temps test.c

Fails with the following error message:

module flag identifiers must be unique (or of 'require' type)
!"UnifiedLTO"
fatal error: error in backend: Broken module found, compilation aborted!
clang: error: clang frontend command failed with exit code 70 (use -v to see invocation)

Here is what the driver does when -save-temps flag is set:

> clang -flto -funified-lto -save-temps test.c -ccc-print-phases
> +- 0: input, "test.c", c
> +- 1: preprocessor, {0}, cpp-output
> +- 2: compiler, {1}, ir
> +- 3: backend, {2}, lto-bc
> 4: linker, {3}, image

The IR output of "compiler" step has "UnifiedLTO" module flag. "backend"
step adds another module flag with "UnifiedLTO" identifier, which
invalidates the LLVM IR module.

---

Full diff: https://github.com/llvm/llvm-project/pull/162763.diff

1 Files Affected:

• (modified) clang/lib/CodeGen/BackendUtil.cpp (+2-1)

diff --git a/clang/lib/CodeGen/BackendUtil.cpp b/clang/lib/CodeGen/BackendUtil.cpp
index 2d959827d6972..d28d886b3551e 100644
--- a/clang/lib/CodeGen/BackendUtil.cpp
+++ b/clang/lib/CodeGen/BackendUtil.cpp
@@ -1199,7 +1199,8 @@ void EmitAssemblyHelper::RunOptimizationPipeline(
       }
     }
 
-    if (shouldEmitUnifiedLTOModueFlag())
+    if (shouldEmitUnifiedLTOModueFlag() &&
+        !TheModule->getModuleFlag("UnifiedLTO"))
       TheModule->addModuleFlag(llvm::Module::Error, "UnifiedLTO", uint32_t(1));
   }
 

[bader]

The fix is similar to 53adfa8.

[bader]

@david-greene-cb, GitHub account seems to be inactive.
@teresajohnson, since you reviewed 53adfa8, could you take a look at this change, please?

[ilovepi]

LGTM.