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

[Tzu-Ching Yang]

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.

Thanks,
Tzu-Ching