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[dxvk] Avoid bsf instruction 

It's very slow compared to tzcnt on some CPUs, and we have a working fallback.
This commit is contained in:
Philip Rebohle 2022-07-24 15:20:03 +02:00
parent 9cd0473544
commit 41ec5d2c52
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GPG Key ID: C8CC613427A31C99
1 changed files with 17 additions and 31 deletions
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48
src/util/util_bit.h
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@ -56,11 +56,17 @@ namespace dxvk::bit {
#elif defined(__BMI__)
return __tzcnt_u32(n);
#elif defined(__GNUC__) || defined(__clang__)
// tzcnt is encoded as rep bsf, so we can use it on all
// processors, but the behaviour of zero inputs differs:
// - bsf: zf = 1, cf = ?, result = ?
// - tzcnt: zf = 0, cf = 1, result = 32
// We'll have to handle this case manually.
uint32_t res;
uint32_t tmp;
asm (
"tzcnt %2, %0;"
"mov $32, %1;"
"bsf %2, %0;"
"test %2, %2;"
"cmovz %1, %0;"
: "=&r" (res), "=&r" (tmp)
: "r" (n));
@ -86,41 +92,21 @@ namespace dxvk::bit {
uint64_t res;
uint64_t tmp;
asm (
"tzcnt %2, %0;"
"mov $64, %1;"
"bsf %2, %0;"
"test %2, %2;"
"cmovz %1, %0;"
: "=&r" (res), "=&r" (tmp)
: "r" (n));
return res;
#else
uint32_t r = 63;
n &= -n;
r -= (n & 0x00000000FFFFFFFFull) ? 32 : 0;
r -= (n & 0x0000FFFF0000FFFFull) ? 16 : 0;
r -= (n & 0x00FF00FF00FF00FFull) ? 8 : 0;
r -= (n & 0x0F0F0F0F0F0F0F0Full) ? 4 : 0;
r -= (n & 0x3333333333333333ull) ? 2 : 0;
r -= (n & 0x5555555555555555ull) ? 1 : 0;
return n != 0 ? r : 64;
#endif
}
inline uint32_t bsf(uint32_t n) {
#if defined(_MSC_VER) && !defined(__clang__)
unsigned long index;
_BitScanForward(&index, n);
return uint32_t(index);
#elif defined(__GNUC__) || defined(__clang__)
return __builtin_ctz(n);
#else
uint32_t r = 31;
n &= -n;
r -= (n & 0x0000FFFF) ? 16 : 0;
r -= (n & 0x00FF00FF) ? 8 : 0;
r -= (n & 0x0F0F0F0F) ? 4 : 0;
r -= (n & 0x33333333) ? 2 : 0;
r -= (n & 0x55555555) ? 1 : 0;
return r;
uint32_t lo = uint32_t(n);
if (lo) {
return tzcnt(lo);
} else {
uint32_t hi = uint32_t(n >> 32);
return tzcnt(hi) + 32;
}
#endif
}
@ -348,7 +334,7 @@ namespace dxvk::bit {
}
uint32_t operator * () const {
return bsf(m_mask);
return tzcnt(m_mask);
}
bool operator == (iterator other) const { return m_mask == other.m_mask; }