[kazutakahirata]

This patch updates LLVM_ATTRIBUTE_WARN_UNUSED in Compiler.h to use the
C++11 attribute syntax [[gnu::warn_unused]] via LLVM_HAS_CPP_ATTRIBUTE
instead of attribute((warn_unused)).

GCC 12 and earlier reject:

class __attribute__((warn_unused)) [[nodiscard]] Foo { ... };
class [[nodiscard]] __attribute__((warn_unused)) Bar { ... };

This is exactly what happened in #237802. Specifically, it added
LLVM_ATTRIBUTE_WARN_UNUSED to APInt, which already had [[nodiscard]],
and broke GCC 12 buildbots, leading to a revert in #239152:
https://lab.llvm.org/buildbot/#/builders/181/builds/53859

This patch drops the attribute((warn_unused)) fallback because an
older compiler like Clang 5.0 would select it and fail when a class
also has [[nodiscard]]. It's always safe to expand this optional
warning macro to empty.

Assisted-by: Antigravity

[llvmorg-github-actions]

@llvm/pr-subscribers-llvm-support

Author: Kazu Hirata (kazutakahirata)

Changes

This patch updates LLVM_ATTRIBUTE_WARN_UNUSED in Compiler.h to use the
C++11 attribute syntax [[gnu::warn_unused]] via LLVM_HAS_CPP_ATTRIBUTE
instead of attribute((warn_unused)).

GCC 12 and earlier reject:

class __attribute__((warn_unused)) [[nodiscard]] Foo { ... };
class [[nodiscard]] __attribute__((warn_unused)) Bar { ... };

This patch drops the attribute((warn_unused)) fallback because an
older compiler like Clang 5.0 would select it and fail when a class
also has [[nodiscard]]. It's always safe to expand this optional
warning macro to empty.

Assisted-by: Antigravity

---

Full diff: https://github.com/llvm/llvm-project/pull/239486.diff

1 Files Affected:

• (modified) llvm/include/llvm/Support/Compiler.h (+2-2)

diff --git a/llvm/include/llvm/Support/Compiler.h b/llvm/include/llvm/Support/Compiler.h
index af06dbb441751..b3afc96842ed0 100644
--- a/llvm/include/llvm/Support/Compiler.h
+++ b/llvm/include/llvm/Support/Compiler.h
@@ -234,8 +234,8 @@
 #define LLVM_ATTRIBUTE_USED
 #endif
 
-#if __has_attribute(warn_unused)
-#define LLVM_ATTRIBUTE_WARN_UNUSED __attribute__((warn_unused))
+#if LLVM_HAS_CPP_ATTRIBUTE(gnu::warn_unused)
+#define LLVM_ATTRIBUTE_WARN_UNUSED [[gnu::warn_unused]]
 #else
 #define LLVM_ATTRIBUTE_WARN_UNUSED
 #endif

[boomanaiden154]

Seems reasonable enough to me.

Is there a buildbot failure link or something that this is fixing? If so, please put it in the PR description.

[kazutakahirata]

Seems reasonable enough to me.

Is there a buildbot failure link or something that this is fixing? If so, please put it in the PR description.

Thanks for the review! I've added the link just below the "GCC 12" story.