cosmopolitan/libc/nexgen32e/ffs.S

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/*-*- mode:unix-assembly; indent-tabs-mode:t; tab-width:8; coding:utf-8 -*-│
vi: set et ft=asm ts=8 tw=8 fenc=utf-8 :vi
Copyright 2020 Justine Alexandra Roberts Tunney
Permission to use, copy, modify, and/or distribute this software for
any purpose with or without fee is hereby granted, provided that the
above copyright notice and this permission notice appear in all copies.
THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL
WARRANTIES WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED
WARRANTIES OF MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE
AUTHOR BE LIABLE FOR ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL
DAMAGES OR ANY DAMAGES WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR
PROFITS, WHETHER IN AN ACTION OF CONTRACT, NEGLIGENCE OR OTHER
TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION WITH THE USE OR
PERFORMANCE OF THIS SOFTWARE.
*/
#include "libc/macros.h"
/ Finds lowest set bit in word.
/
/ uint32 𝑥 bsf(𝑥) tzcnt(𝑥) ffs(𝑥) bsr(𝑥) lzcnt(𝑥)
/ 0x00000000 wut 32 0 wut 32
/ 0x00000001 0 0 1 0 31
/ 0x80000001 0 0 1 31 0
/ 0x80000000 31 31 32 31 0
/ 0x00000010 4 4 5 4 27
/ 0x08000010 4 4 5 27 4
/ 0x08000000 27 27 28 27 4
/ 0xffffffff 0 0 1 31 0
/
/ @param edi is the input number
/ @return number in range [1,32] or 0 if no bits set
/ @see also treasure trove of nearly identical functions
/ @asyncsignalsafe
ffs: .leafprologue
.profilable
or $-1,%edx
bsf %edi,%eax
cmovz %edx,%eax
inc %eax
.leafepilogue
.endfn ffs,globl
.source __FILE__