Since GCC 10 and llvm.org Clang 11, -fno-common is the default. However Apple's Xcode Clang hasn't followed suit yet, and still defaults to -fcommon. Compiling with -fcommon causes uninitialized global variables to be treated as "common" (which allows multiple object files to have similar definitions). Common variables seem to have the issue that their intended alignment isn't signaled, so the linker assumes that they may need alignment according to their full size. With large global tables, this can lead to linker warnings like this, with Xcode 16.3: ld: warning: reducing alignment of section __DATA,__common from 0x8000 to 0x4000 because it exceeds segment maximum alignment This can be reproduced with a small snippet like this: char table[16385]; int main(int argc, char* argv[]) { return 0; } Compiling with -fno-common avoids this issue and warning, and matches the default behaviour of other compilers. (Compiling with -fno-common also avoids the risk of accidentally accepting duplicate definitions of global variables, as long as they are uninitialized.)
Unified · Split
| @@ -5851,6 +5851,13 @@ case $target_os in · 5850 unmodified lines | ||
| 5851 | 5851 | clang_version=$($cc -dumpversion) |
| 5852 | 5852 | test ${clang_version%%.*} -eq 11 && add_cflags -fno-stack-check |
| 5853 | 5853 | fi |
| 5854 | + | |
| 5855 | + # Xcode Clang doesn't default to -fno-common while upstream llvm.org | |
| 5856 | + # Clang (and GCC) do. This avoids linker warnings on Xcode 16.3 about | |
| 5857 | + # "reducing alignment of section __DATA,__common from 0x8000 to 0x4000 | |
| 5858 | + # because it exceeds segment maximum alignment". | |
| 5859 | + check_cflags -fno-common | |
| 5860 | + | |
| 5854 | 5861 | ;; |
| 5855 | 5862 | msys*) |
| 5856 | 5863 | die "Native MSYS builds are discouraged, please use the MINGW environment." |