cosmopolitan/libc/nexgen32e/bsrmax.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
This program is free software; you can redistribute it and/or modify │
it under the terms of the GNU General Public License as published by
the Free Software Foundation; version 2 of the License. │
This program is distributed in the hope that it will be useful, but
WITHOUT ANY WARRANTY; without even the implied warranty of │
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
General Public License for more details.
You should have received a copy of the GNU General Public License
along with this program; if not, write to the Free Software │
Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA
02110-1301 USA
*/
#include "libc/macros.h"
/ Returns binary logarithm of integer 𝑥.
/
/ 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 rsi:rdi is 128-bit unsigned 𝑥 value
/ @return eax number in range [0,128) or undef if 𝑥 is 0
/ @see also treasure trove of nearly identical functions
bsrmax: .leafprologue
.profilable
bsr %rsi,%rax
jnz 2f
bsr %rdi,%rax
1: .leafepilogue
2: add $64,%eax
jmp 1b
.endfn bsrmax,globl
.source __FILE__