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Polyfill the 32-bit _bsr() function

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Justine Tunney 2023-08-13 21:13:32 -07:00
parent c776a32f75
commit ef6387ee5e
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GPG key ID: BE714B4575D6E328
3 changed files with 47 additions and 68 deletions
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1
libc/intrin/bsf.h
View file

@ -6,7 +6,6 @@ COSMOPOLITAN_C_START_
int _bsf(int) pureconst; int _bsf(int) pureconst;
int _bsfl(long) pureconst; int _bsfl(long) pureconst;
int _bsfll(long long) pureconst; int _bsfll(long long) pureconst;
int _bsf128(uintmax_t) pureconst;
#if defined(__GNUC__) && !defined(__STRICT_ANSI__) #if defined(__GNUC__) && !defined(__STRICT_ANSI__)
#define _bsf(x) __builtin_ctz(x) #define _bsf(x) __builtin_ctz(x)

45
libc/intrin/bsr.c
View file

@ -17,40 +17,7 @@
│ PERFORMANCE OF THIS SOFTWARE. │ │ PERFORMANCE OF THIS SOFTWARE. │
╚─────────────────────────────────────────────────────────────────────────────*/ ╚─────────────────────────────────────────────────────────────────────────────*/
#include "libc/intrin/bsr.h" #include "libc/intrin/bsr.h"
// clang-format off
static const char kDebruijn[64] = {
0, 47, 1, 56, 48, 27, 2, 60, 57, 49, 41, 37, 28, 16, 3, 61,
54, 58, 35, 52, 50, 42, 21, 44, 38, 32, 29, 23, 17, 11, 4, 62,
46, 55, 26, 59, 40, 36, 15, 53, 34, 51, 20, 43, 31, 22, 10, 45,
25, 39, 14, 33, 19, 30, 9, 24, 13, 18, 8, 12, 7, 6, 5, 63,
};
/**
* Returns binary logarithm of 𝑥.
*
* ctz(𝑥) 31^clz(𝑥) clz(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 x is a 64-bit integer
* @return number in range 0..63 or undefined if 𝑥 is 0
*/
int(_bsrl)(long x) {
x |= x >> 1;
x |= x >> 2;
x |= x >> 4;
x |= x >> 8;
x |= x >> 16;
x |= x >> 32;
return kDebruijn[(x * 0x03f79d71b4cb0a89) >> 58];
}
/** /**
* Returns binary logarithm of 𝑥. * Returns binary logarithm of 𝑥.
@ -70,7 +37,11 @@ int(_bsrl)(long x) {
* @return number in range 0..31 or undefined if 𝑥 is 0 * @return number in range 0..31 or undefined if 𝑥 is 0
*/ */
int(_bsr)(int x) { int(_bsr)(int x) {
return _bsrl((unsigned)x); int r = 0;
if(x & 0xFFFF0000u) { x >>= 16; r |= 16; }
if(x & 0xFF00) { x >>= 8; r |= 8; }
if(x & 0xF0) { x >>= 4; r |= 4; }
if(x & 0xC) { x >>= 2; r |= 2; }
if(x & 0x2) { r |= 1; }
return r;
} }
__weak_reference(_bsrl, _bsrll);

69
libc/intrin/bsr128.S → libc/intrin/bsrl.c
View file

@ -1,7 +1,7 @@
/*-*- mode:unix-assembly; indent-tabs-mode:t; tab-width:8; coding:utf-8 -*-│ /*-*- mode:c;indent-tabs-mode:nil;c-basic-offset:2;tab-width:8;coding:utf-8 -*-│
│vi: set et ft=asm ts=8 tw=8 fenc=utf-8 :vi│ │vi: set net ft=c ts=2 sts=2 sw=2 fenc=utf-8 :vi│
╞══════════════════════════════════════════════════════════════════════════════╡ ╞══════════════════════════════════════════════════════════════════════════════╡
│ Copyright 2020 Justine Alexandra Roberts Tunney │ │ Copyright 2023 Justine Alexandra Roberts Tunney │
│ │ │ │
│ Permission to use, copy, modify, and/or distribute this software for │ │ Permission to use, copy, modify, and/or distribute this software for │
│ any purpose with or without fee is hereby granted, provided that the │ │ any purpose with or without fee is hereby granted, provided that the │
@ -16,31 +16,40 @@
│ TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION WITH THE USE OR │ │ TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION WITH THE USE OR │
│ PERFORMANCE OF THIS SOFTWARE. │ │ PERFORMANCE OF THIS SOFTWARE. │
╚─────────────────────────────────────────────────────────────────────────────*/ ╚─────────────────────────────────────────────────────────────────────────────*/
#include "libc/macros.internal.h" #include "libc/intrin/bsr.h"
// Returns binary logarithm of integer 𝑥. static const char kDebruijn[64] = {
// 0, 47, 1, 56, 48, 27, 2, 60, 57, 49, 41, 37, 28, 16, 3, 61,
// uint32 𝑥 _bsf(𝑥) tzcnt(𝑥) ffs(𝑥) bsr(𝑥) lzcnt(𝑥) 54, 58, 35, 52, 50, 42, 21, 44, 38, 32, 29, 23, 17, 11, 4, 62,
// 0x00000000 wut 32 0 wut 32 46, 55, 26, 59, 40, 36, 15, 53, 34, 51, 20, 43, 31, 22, 10, 45,
// 0x00000001 0 0 1 0 31 25, 39, 14, 33, 19, 30, 9, 24, 13, 18, 8, 12, 7, 6, 5, 63,
// 0x80000001 0 0 1 31 0 };
// 0x80000000 31 31 32 31 0
// 0x00000010 4 4 5 4 27 /**
// 0x08000010 4 4 5 27 4 * Returns binary logarithm of 𝑥.
// 0x08000000 27 27 28 27 4 *
// 0xffffffff 0 0 1 31 0 * ctz(𝑥) 31^clz(𝑥) clz(𝑥)
// * uint32 𝑥 _bsf(𝑥) tzcnt(𝑥) ffs(𝑥) _bsr(𝑥) lzcnt(𝑥)
// @param rsi:rdi is 128-bit unsigned 𝑥 value * 0x00000000 wut 32 0 wut 32
// @return eax number in range [0,128) or undef if 𝑥 is 0 * 0x00000001 0 0 1 0 31
// @see also treasure trove of nearly identical functions * 0x80000001 0 0 1 31 0
.ftrace1 * 0x80000000 31 31 32 31 0
_bsr128: * 0x00000010 4 4 5 4 27
.ftrace2 * 0x08000010 4 4 5 27 4
.leafprologue * 0x08000000 27 27 28 27 4
bsr %rsi,%rax * 0xffffffff 0 0 1 31 0
jnz 2f *
bsr %rdi,%rax * @param x is a 64-bit integer
1: .leafepilogue * @return number in range 0..63 or undefined if 𝑥 is 0
2: add $64,%eax */
jmp 1b int(_bsrl)(long x) {
.endfn _bsr128,globl x |= x >> 1;
x |= x >> 2;
x |= x >> 4;
x |= x >> 8;
x |= x >> 16;
x |= x >> 32;
return kDebruijn[(x * 0x03f79d71b4cb0a89) >> 58];
}
__weak_reference(_bsrl, _bsrll);