[libc++] Annotate generic_category/system_category as const

[d0k]

This lets the compiler eliminate unused/duplicate calls to it.

[llvmbot]

@llvm/pr-subscribers-libcxx

Author: Benjamin Kramer (d0k)

Changes

This lets the compiler eliminate unused/duplicate calls to it.

---

Full diff: https://github.com/llvm/llvm-project/pull/78052.diff

1 Files Affected:

• (modified) libcxx/include/__system_error/error_category.h (+2-2)

diff --git a/libcxx/include/__system_error/error_category.h b/libcxx/include/__system_error/error_category.h
index 11c920e32657c2..7233e221104471 100644
--- a/libcxx/include/__system_error/error_category.h
+++ b/libcxx/include/__system_error/error_category.h
@@ -67,8 +67,8 @@ class _LIBCPP_HIDDEN __do_message : public error_category {
   string message(int __ev) const override;
 };
 
-_LIBCPP_EXPORTED_FROM_ABI const error_category& generic_category() _NOEXCEPT;
-_LIBCPP_EXPORTED_FROM_ABI const error_category& system_category() _NOEXCEPT;
+[[__gnu__::__const__]] _LIBCPP_EXPORTED_FROM_ABI const error_category& generic_category() _NOEXCEPT;
+[[__gnu__::__const__]] _LIBCPP_EXPORTED_FROM_ABI const error_category& system_category() _NOEXCEPT;
 
 _LIBCPP_END_NAMESPACE_STD
 

[ldionne]

CI doesn't seem happy: https://buildkite.com/llvm-project/libcxx-ci/builds/32831#018d03dd-6965-4ebd-a7b6-7d6b9ec5d619

I wonder if this is just the AppleClang bots using a slightly older compiler?

[d0k]

CI doesn't seem happy: https://buildkite.com/llvm-project/libcxx-ci/builds/32831#018d03dd-6965-4ebd-a7b6-7d6b9ec5d619

I wonder if this is just the AppleClang bots using a slightly older compiler?

Looks like it's compiling with -std=c++03 and doesn't like [[attribute]] syntax. Should it be compiling system_error.h in C++03 mode? Seens to be coming from <ios>.

[ldionne]

I see two options:

1. Use an alternative attribute syntax that works in C++03 mode
2. Don't provide <system_error> in C++03 mode, since it is technically a C++11 addition to the library

(2) is a lot harder and would be breaking for users. We could investigate doing that, but (1) would be significantly easier especially given the simplicity of the patch that motivates it.

[d0k]

Changed it to use GNU __attribute__(__const__).