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consolidate bit scan operations

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This commit is contained in:
daan 2020-09-08 09:27:57 -07:00
parent c7272afa9a
commit 30b993ecf3
4 changed files with 113 additions and 96 deletions
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46
src/page-queue.c
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@ -49,50 +49,6 @@ static inline bool mi_page_queue_is_special(const mi_page_queue_t* pq) {
Bins
----------------------------------------------------------- */
// Bit scan reverse: return the index of the highest bit.
static inline uint8_t mi_bsr32(uint32_t x);
#if defined(_MSC_VER)
#include <intrin.h>
static inline uint8_t mi_bsr32(uint32_t x) {
uint32_t idx;
_BitScanReverse((DWORD*)&idx, x);
return (uint8_t)idx;
}
#elif defined(__GNUC__) || defined(__clang__)
static inline uint8_t mi_bsr32(uint32_t x) {
return (31 - __builtin_clz(x));
}
#else
static inline uint8_t mi_bsr32(uint32_t x) {
// de Bruijn multiplication, see <http://supertech.csail.mit.edu/papers/debruijn.pdf>
static const uint8_t debruijn[32] = {
31, 0, 22, 1, 28, 23, 18, 2, 29, 26, 24, 10, 19, 7, 3, 12,
30, 21, 27, 17, 25, 9, 6, 11, 20, 16, 8, 5, 15, 4, 14, 13,
};
x |= x >> 1;
x |= x >> 2;
x |= x >> 4;
x |= x >> 8;
x |= x >> 16;
x++;
return debruijn[(x*0x076be629) >> 27];
}
#endif
// Bit scan reverse: return the index of the highest bit.
uint8_t _mi_bsr(uintptr_t x) {
if (x == 0) return 0;
#if MI_INTPTR_SIZE==8
uint32_t hi = (x >> 32);
return (hi == 0 ? mi_bsr32((uint32_t)x) : 32 + mi_bsr32(hi));
#elif MI_INTPTR_SIZE==4
return mi_bsr32(x);
#else
# error "define bsr for non-32 or 64-bit platforms"
#endif
}
// Return the bin for a given field size.
// Returns MI_BIN_HUGE if the size is too large.
// We use `wsize` for the size in "machine word sizes",
@ -125,7 +81,7 @@ extern inline uint8_t _mi_bin(size_t size) {
#endif
wsize--;
// find the highest bit
uint8_t b = mi_bsr32((uint32_t)wsize);
uint8_t b = (uint8_t)mi_bsr(wsize); // note: wsize != 0
// and use the top 3 bits to determine the bin (~12.5% worst internal fragmentation).
// - adjust with 3 because we use do not round the first 8 sizes
// which each get an exact bin