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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