[llvm-dev] [CMake][ELF] Add -fno-semantic-interposition and -Bsymbolic-functions

[Fāng-ruì Sòng via llvm-dev]

On Fri, May 7, 2021 at 1:28 PM Fangrui Song <maskray at google.com> wrote:
>
> Motivated by a recent rant on too slow clang (linking against libLLVM.so and libclang-cpp.so)
> when building the Linux kernel, I just created a CMake patch https://reviews.llvm.org/D102090
> which will add -fno-semantic-interposition and -Wl,-Bsymbolic-functions to our build.
> (My x86 kernel defconfig build is at least 15% faster)
>
> I have added some distribution packagers as reviewers. Sending this message to
> attract more users.

Circling back on this. Two patches have landed and will benefit Clang
13 onwards.

https://reviews.llvm.org/D102090  [CMake][ELF] Link libLLVM.so and
libclang-cpp.so with -Bsymbolic-functions
(my Linux kernel build with the libLLVM.so and libclang-cpp.so clang
is more than 15%+ faster, presumably due to much fewer symbol lookups.
~90% fewer JUMP_SLOT dynamic relocations)

https://reviews.llvm.org/D102453 [CMake][ELF] Add
-fno-semantic-interposition for GCC and Clang>=13
(https://llvm-compile-time-tracker.com/compare.php?from=73c3530f5e057333eeac300b9d9925a13c55c9a6&to=a96f875fe98de5eb5f44b0148baa442c998364ed&stat=instructions
measures 3~4% improvement when compiled with GCC!
the binary size is 2% smaller)