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fix mi_bsr to not use lzcnt directly

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Daan 2025-02-18 06:33:55 -08:00
parent d56d18f45f
commit f8857a5189
1 changed files with 3 additions and 9 deletions
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12
include/mimalloc/bits.h
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@ -231,9 +231,8 @@ static inline size_t mi_clz(size_t x) {
#elif mi_has_builtinz(clz)
return (x!=0 ? (size_t)mi_builtinz(clz)(x) : MI_SIZE_BITS);
#elif defined(__GNUC__) && (MI_ARCH_X64 || MI_ARCH_X86)
if (x==0) return MI_SIZE_BITS;
size_t r;
__asm ("bsr\t%1, %0" : "=r"(r) : "r"(x) : "cc");
size_t r = MI_SIZE_BITS; // bsr leaves destination unmodified if the argument is 0 (see <https://github.com/llvm/llvm-project/pull/102885>)
__asm ("bsr\t%1, %0" : "+r"(r) : "r"(x) : "cc");
return (MI_SIZE_BITS - 1 - r);
#else
#define MI_HAS_FAST_BITSCAN 0
@ -270,12 +269,7 @@ static inline bool mi_bsf(size_t x, size_t* idx) {
// return false if `x==0` (with `*idx` undefined) and true otherwise,
// with the `idx` is set to the bit index (`0 <= *idx < MI_BFIELD_BITS`).
static inline bool mi_bsr(size_t x, size_t* idx) {
#if defined(__GNUC__) && MI_ARCH_X64 && defined(__BMI1__) && (!defined(__clang_major__) || __clang_major__ >= 9)
// on x64 the carry flag is set on zero which gives better codegen
bool is_zero;
__asm ("lzcnt\t%2, %1" : "=@ccc"(is_zero), "=r"(*idx) : "r"(x) : "cc");
return !is_zero;
#elif 0 && defined(_MSC_VER) && (MI_ARCH_X64 || MI_ARCH_X86 || MI_ARCH_ARM64 || MI_ARCH_ARM32)
#if 0 && defined(_MSC_VER) && (MI_ARCH_X64 || MI_ARCH_X86 || MI_ARCH_ARM64 || MI_ARCH_ARM32)
unsigned long i;
return (mi_msc_builtinz(_BitScanReverse)(&i, x) ? (*idx = (size_t)i, true) : false);
#else