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Improve performance of bitscanning intrinsics

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This change helps spectre more intelligently plan execution, by working
around false output dependencies, impacting ops like popcnt bsr and bsf
This commit is contained in:
Justine Tunney 2021-06-15 06:24:46 -07:00
parent 29cb53881e
commit 87d7010495
13 changed files with 100 additions and 224 deletions
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18
libc/bits/popcnt.h
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@ -7,15 +7,15 @@ COSMOPOLITAN_C_START_
unsigned long popcnt(unsigned long) pureconst;
#if defined(__GNUC__) && !defined(__STRICT_ANSI__)
#define popcnt(X) \
(__builtin_constant_p(X) ? __builtin_popcountll(X) : ({ \
unsigned long Res, Pop = (X); \
if (X86_HAVE(POPCNT)) { \
asm("popcnt\t%1,%0" : "=r"(Res) : "r"(Pop) : "cc"); \
} else { \
Res = (popcnt)(Pop); \
} \
Res; \
#define popcnt(X) \
(__builtin_constant_p(X) ? __builtin_popcountll(X) : ({ \
unsigned long PoP = (X); \
if (X86_HAVE(POPCNT)) { \
asm("popcnt\t%0,%0" : "+r"(PoP) : /* no inputs */ : "cc"); \
} else { \
PoP = (popcnt)(PoP); \
} \
PoP; \
}))
#endif /* GNUC && !ANSI */