2020-06-16 13:38:43 +00:00
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/*-*- mode:unix-assembly; indent-tabs-mode:t; tab-width:8; coding:utf-8 -*-│
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2020-06-15 14:18:57 +00:00
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│vi: set et ft=asm ts=8 tw=8 fenc=utf-8 :vi│
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╞══════════════════════════════════════════════════════════════════════════════╡
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│ Copyright 2020 Justine Alexandra Roberts Tunney │
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│ │
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2020-12-28 01:18:44 +00:00
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│ Permission to use, copy, modify, and/or distribute this software for │
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│ any purpose with or without fee is hereby granted, provided that the │
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│ above copyright notice and this permission notice appear in all copies. │
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2020-06-15 14:18:57 +00:00
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│ │
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2020-12-28 01:18:44 +00:00
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│ THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL │
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│ WARRANTIES WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED │
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│ WARRANTIES OF MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE │
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│ AUTHOR BE LIABLE FOR ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL │
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│ DAMAGES OR ANY DAMAGES WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR │
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│ PROFITS, WHETHER IN AN ACTION OF CONTRACT, NEGLIGENCE OR OTHER │
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│ TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION WITH THE USE OR │
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│ PERFORMANCE OF THIS SOFTWARE. │
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2020-06-15 14:18:57 +00:00
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╚─────────────────────────────────────────────────────────────────────────────*/
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2021-03-01 07:42:35 +00:00
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#include "libc/macros.internal.h"
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2020-06-15 14:18:57 +00:00
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2021-02-08 17:19:00 +00:00
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// Finds lowest set bit in 𝑥.
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//
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// uint32 𝑥 bsf(𝑥) tzcnt(𝑥) ffs(𝑥) bsr(𝑥) lzcnt(𝑥)
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// 0x00000000 wut 32 0 wut 32
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// 0x00000001 0 0 1 0 31
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// 0x80000001 0 0 1 31 0
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// 0x80000000 31 31 32 31 0
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// 0x00000010 4 4 5 4 27
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// 0x08000010 4 4 5 27 4
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// 0x08000000 27 27 28 27 4
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// 0xffffffff 0 0 1 31 0
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//
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// @param rdi is 64-bit unsigned 𝑥 value
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// @return rax number in range [0,64) or 64 if 𝑥 is 0
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// @see also treasure trove of nearly identical functions
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2020-06-15 14:18:57 +00:00
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tzcntl: .leafprologue
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.profilable
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mov $64,%esi
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bsf %rdi,%rax
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cmovz %esi,%eax
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.leafepilogue
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.endfn tzcntl,globl
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.alias tzcntl,tzcntll
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2020-06-16 02:01:28 +00:00
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.source __FILE__
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