[clang][cmake] Don't pass -fno-strict-aliasing for GCC

[thesamesam]

This was added a long time ago..

• to the Makefiles in 40fee63;
• first to CMake in b3ce035;
• then moved to only apply when building Clang with GCC in c5635a6.

This shouldn't be needed these days. If an issue does arise, it really ought to be documented better and the cause will certainly be different than it was back then.

The two GCC bugs cited in 40fee63 were:

• https://gcc.gnu.org/PR41874
• https://gcc.gnu.org/PR41838 and both are long-fixed. Not only that, if those issues did come up again, we'd be better off doing -Wno-strict-aliasing where appropriate if there weren't a real code issue or some suppression that was tighter in scope wasn't appropriate.

[llvmbot]

@llvm/pr-subscribers-clang

Author: Sam James (thesamesam)

Changes

This was added a long time ago..

• to the Makefiles in 40fee63;
• first to CMake in b3ce035;
• then moved to only apply when building Clang with GCC in c5635a6.

This shouldn't be needed these days. If an issue does arise, it really ought to be documented better and the cause will certainly be different than it was back then.

The two GCC bugs cited in 40fee63 were:

• https://gcc.gnu.org/PR41874
• https://gcc.gnu.org/PR41838 and both are long-fixed. Not only that, if those issues did come up again, we'd be better off doing -Wno-strict-aliasing where appropriate if there weren't a real code issue or some suppression that was tighter in scope wasn't appropriate.

---

Full diff: https://github.com/llvm/llvm-project/pull/144222.diff

1 Files Affected:

• (modified) clang/CMakeLists.txt (-3)

diff --git a/clang/CMakeLists.txt b/clang/CMakeLists.txt
index ab2ac9bc6b9ad..94607a8e8473c 100644
--- a/clang/CMakeLists.txt
+++ b/clang/CMakeLists.txt
@@ -345,9 +345,6 @@ configure_file(
 # Add appropriate flags for GCC
 if (LLVM_COMPILER_IS_GCC_COMPATIBLE)
   set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -fno-common -Woverloaded-virtual")
-  if (NOT "${CMAKE_CXX_COMPILER_ID}" MATCHES "Clang")
-    set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -fno-strict-aliasing")
-  endif ()
 
   # Enable -pedantic for Clang even if it's not enabled for LLVM.
   if (NOT LLVM_ENABLE_PEDANTIC)

[thesamesam]

FWIW, there seems to be similar code in Flang which was likely copied from Clang, but I'm not in a position to test removing that right now.

[nikic]

To confirm, you do not get any -Wstrict-aliasing warnings with modern GCC after this change?

[MaskRay]

LGTM. Thanks for removing this tech debt.

[thesamesam]

Huh, I thought I'd written a reply to nikic but I don't see it here? Anyway, rewriting it..

To confirm, you do not get any -Wstrict-aliasing warnings with modern GCC after this change?

Yes. Sorry, it's obvious in hindsight I should've stated this explicitly.