[Bug middle-end/50481] builtin to reverse the bit order

[aleks at physik dot tu-berlin.de via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=50481

Alexander Kleinsorge <aleks at physik dot tu-berlin.de> changed:

           What    |Removed                     |Added
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                 CC|                            |aleks at physik dot 
tu-berlin.de

--- Comment #13 from Alexander Kleinsorge <aleks at physik dot tu-berlin.de> ---
for single bytes (uint8), there could be a faster way (x86 + x64).
there are only logical ops and shifts, nothing else.

static inline uint8 byte_rev(uint8 v) {
    const uint64 BREV64 = ~0x084c2a6e195d3b7fLLu; // verify this number (LUT
like)
    uint8 a = (BREV64) >> ((v % 16u) * 4u); // from low
    uint8 b = (BREV64) >> ((v / 16u) * 4u); // from high
    return (a * 16u) | (b % 16u);
}