Re: [PATCH] csky: bitops: Use the compiler builtin for __fls()

[Yury Norov <ynorov@nvidia.com>]

On Fri, Oct 23, 2026 at 10:08:06PM +0800, Tzu-Ching Yang wrote:
> On Thu, Oct 22, 2026 at 03:41:07PM -0400, Yury Norov wrote:
> > So I didn't understand if you have an access to the hardware or not. I
> > don't know the story behind using the non-generic version of ffl(),
> > but it may be for performance reasons. Can you / someone else please
> > run the lib/find_bit_benchmark.c on real hardware before we move this?
> 
> No, I have no csky hardware; all run-time tests were under XuanTie's
> QEMU.
> 
> The ff1 asm is in fls(), which the patch keeps. __fls() has been a C
> wrapper around it, fls(x) - 1, since the 2018 port (735ee005c371), with
> no reason recorded. The builtin compiles to the same ff1: 4
> instructions before and after with every GCC 9 to 16 and CPU tested,
> so __fls() itself is neither faster nor slower. The gain is elsewhere:
> the builtin folds constant arguments and merges repeated calls, which
> saves 128 to 320 bytes of text. It is the csky part of the conversions
> Arnd suggested in the ffs()/fls() thread (the Link: in the patch).
> 
> In lib/find_bit_benchmark.c only find_last_bit() reaches __fls(); the
> other six tests use the unchanged __ffs() asm. _find_last_bit() is 27
> instructions with one ff1 before and after with GCC 10 to 16 (28 -> 27
> with GCC 9), and under QEMU with -icount it executes the same number of
> instructions per call before and after with GCC 12 and 16, one fewer
> with GCC 9. QEMU has no pipeline model, so a real core could still
> differ.
> 
> Guo Ren, or anyone with a CK8xx board: could you run the benchmark
> (CONFIG_FIND_BIT_BENCHMARK=y) before and after? I'm happy to wait.

While waiting happy, can you print the bloat-o-meter results, and a
couple listings of the code generation, including compile- and runtime
cases.

Thanks,
Yury