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Use "table look-up" method for reversing byte

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This method takes constant time and is less prone to (CPU) pipeline stalling due to less
computation. Also it is slightly faster than the previous method.
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Chocobo1 2022-02-09 18:30:05 +08:00
commit 4cb075b168
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1 changed files with 12 additions and 2 deletions
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14
src/base/bittorrent/ltqbitarray.cpp
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@ -37,8 +37,18 @@ namespace
{
unsigned char reverseByte(const unsigned char byte)
{
// https://graphics.stanford.edu/~seander/bithacks.html#ReverseByteWith64Bits
return (((byte * 0x80200802ULL) & 0x0884422110ULL) * 0x0101010101ULL) >> 32;
// https://graphics.stanford.edu/~seander/bithacks.html#BitReverseTable
static const unsigned char table[] =
{
#define R2(n) n, (n + (2 * 64)), (n + 64), (n + (3 * 64))
#define R4(n) R2(n), R2(n + (2 * 16)), R2(n + 16), R2(n + (3 * 16))
#define R6(n) R4(n), R4(n + (2 * 4)), R4(n + 4), R4(n + (3 * 4))
R6(0), R6(2), R6(1), R6(3)
#undef R6
#undef R4
#undef R2
};
return table[byte];
}
}