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Polyfill the 32-bit _bsr() function
This commit is contained in:
Justine Tunney
parent
c776a32f75
commit
ef6387ee5e
3 changed files with 47 additions and 68 deletions
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@ -6,7 +6,6 @@ COSMOPOLITAN_C_START_
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int _bsf(int) pureconst;
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int _bsfl(long) pureconst;
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int _bsfll(long long) pureconst;
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int _bsf128(uintmax_t) pureconst;
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#if defined(__GNUC__) && !defined(__STRICT_ANSI__)
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#define _bsf(x) __builtin_ctz(x)
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@ -17,40 +17,7 @@
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│ PERFORMANCE OF THIS SOFTWARE. │
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╚─────────────────────────────────────────────────────────────────────────────*/
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#include "libc/intrin/bsr.h"
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static const char kDebruijn[64] = {
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0, 47, 1, 56, 48, 27, 2, 60, 57, 49, 41, 37, 28, 16, 3, 61,
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54, 58, 35, 52, 50, 42, 21, 44, 38, 32, 29, 23, 17, 11, 4, 62,
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46, 55, 26, 59, 40, 36, 15, 53, 34, 51, 20, 43, 31, 22, 10, 45,
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25, 39, 14, 33, 19, 30, 9, 24, 13, 18, 8, 12, 7, 6, 5, 63,
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};
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/**
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* Returns binary logarithm of 𝑥.
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*
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* ctz(𝑥) 31^clz(𝑥) clz(𝑥)
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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 x is a 64-bit integer
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* @return number in range 0..63 or undefined if 𝑥 is 0
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*/
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int(_bsrl)(long x) {
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x |= x >> 1;
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x |= x >> 2;
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x |= x >> 4;
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x |= x >> 8;
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x |= x >> 16;
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x |= x >> 32;
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return kDebruijn[(x * 0x03f79d71b4cb0a89) >> 58];
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}
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// clang-format off
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/**
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* Returns binary logarithm of 𝑥.
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@ -70,7 +37,11 @@ int(_bsrl)(long x) {
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* @return number in range 0..31 or undefined if 𝑥 is 0
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*/
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int(_bsr)(int x) {
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return _bsrl((unsigned)x);
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int r = 0;
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if(x & 0xFFFF0000u) { x >>= 16; r |= 16; }
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if(x & 0xFF00) { x >>= 8; r |= 8; }
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if(x & 0xF0) { x >>= 4; r |= 4; }
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if(x & 0xC) { x >>= 2; r |= 2; }
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if(x & 0x2) { r |= 1; }
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return r;
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}
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__weak_reference(_bsrl, _bsrll);
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@ -1,7 +1,7 @@
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/*-*- mode:unix-assembly; indent-tabs-mode:t; tab-width:8; coding:utf-8 -*-│
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│vi: set et ft=asm ts=8 tw=8 fenc=utf-8 :vi│
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/*-*- mode:c;indent-tabs-mode:nil;c-basic-offset:2;tab-width:8;coding:utf-8 -*-│
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│vi: set net ft=c ts=2 sts=2 sw=2 fenc=utf-8 :vi│
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╞══════════════════════════════════════════════════════════════════════════════╡
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│ Copyright 2020 Justine Alexandra Roberts Tunney │
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│ Copyright 2023 Justine Alexandra Roberts Tunney │
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│ │
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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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@ -16,31 +16,40 @@
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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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╚─────────────────────────────────────────────────────────────────────────────*/
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#include "libc/macros.internal.h"
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#include "libc/intrin/bsr.h"
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// Returns binary logarithm of integer 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 rsi:rdi is 128-bit unsigned 𝑥 value
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// @return eax number in range [0,128) or undef if 𝑥 is 0
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// @see also treasure trove of nearly identical functions
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.ftrace1
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_bsr128:
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.ftrace2
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.leafprologue
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bsr %rsi,%rax
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jnz 2f
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bsr %rdi,%rax
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1: .leafepilogue
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2: add $64,%eax
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jmp 1b
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.endfn _bsr128,globl
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static const char kDebruijn[64] = {
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0, 47, 1, 56, 48, 27, 2, 60, 57, 49, 41, 37, 28, 16, 3, 61,
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54, 58, 35, 52, 50, 42, 21, 44, 38, 32, 29, 23, 17, 11, 4, 62,
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46, 55, 26, 59, 40, 36, 15, 53, 34, 51, 20, 43, 31, 22, 10, 45,
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25, 39, 14, 33, 19, 30, 9, 24, 13, 18, 8, 12, 7, 6, 5, 63,
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};
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/**
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* Returns binary logarithm of 𝑥.
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*
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* ctz(𝑥) 31^clz(𝑥) clz(𝑥)
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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 x is a 64-bit integer
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* @return number in range 0..63 or undefined if 𝑥 is 0
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*/
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int(_bsrl)(long x) {
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x |= x >> 1;
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x |= x >> 2;
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x |= x >> 4;
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x |= x >> 8;
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x |= x >> 16;
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x |= x >> 32;
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return kDebruijn[(x * 0x03f79d71b4cb0a89) >> 58];
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}
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__weak_reference(_bsrl, _bsrll);
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