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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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45
libc/intrin/bsr.c
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@ -17,40 +17,7 @@
│ PERFORMANCE OF THIS SOFTWARE. │
╚─────────────────────────────────────────────────────────────────────────────*/
#include "libc/intrin/bsr.h"
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];
}
// clang-format off
/**
* 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
*/
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);