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add the round_down_to_power_of_2 algoirthm 

This could probably be sped up by detecting LZCNT support, but it's
unlikely to be a bottleneck.
This commit is contained in:
Jack O'Connor 2020-01-21 13:20:45 -05:00
parent 087d72e08f
commit de1cf0038e
1 changed files with 13 additions and 0 deletions
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13
c/blake3_impl.h
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@ -57,6 +57,19 @@ INLINE uint8_t popcnt(uint64_t x) {
#endif
}
// Largest power of two less than or equal to x. As a special case, returns 1
// when x is 0. Based on
// https://graphics.stanford.edu/~seander/bithacks.html#RoundUpPowerOf2.
INLINE uint64_t round_down_to_power_of_2(uint64_t x) {
x |= x >> 1;
x |= x >> 2;
x |= x >> 4;
x |= x >> 8;
x |= x >> 16;
x |= x >> 32;
return (x >> 1) + 1;
}
INLINE uint32_t counter_low(uint64_t counter) { return (uint32_t)counter; }
INLINE uint32_t counter_high(uint64_t counter) {