[Bug target/104315] [AArch64] Failure to optimize 8-bit bitreverse pattern

[gabravier at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104315

--- Comment #1 from Gabriel Ravier <gabravier at gmail dot com> ---
PS: I've just stumbled upon the more generic case, which would be this code:

unsigned int stb_bitreverse(unsigned int n)
{
  n = ((n & 0xAAAAAAAA) >>  1) | ((n & 0x55555555) << 1);
  n = ((n & 0xCCCCCCCC) >>  2) | ((n & 0x33333333) << 2);
  n = ((n & 0xF0F0F0F0) >>  4) | ((n & 0x0F0F0F0F) << 4);
  n = ((n & 0xFF00FF00) >>  8) | ((n & 0x00FF00FF) << 8);
  return (n >> 16) | (n << 16);
}

which GCC optimizes to this:

stb_bitreverse(unsigned int):
  lsl w2, w0, 1
  lsr w1, w0, 1
  and w0, w1, 1431655765
  and w1, w2, -1431655766
  orr w0, w0, w1
  lsr w1, w0, 2
  lsl w0, w0, 2
  and w0, w0, -858993460
  and w1, w1, 858993459
  orr w1, w1, w0
  lsr w0, w1, 4
  lsl w1, w1, 4
  and w1, w1, -252645136
  and w0, w0, 252645135
  orr w0, w0, w1
  rev w0, w0
  ret

and LLVM to this:

stb_bitreverse(unsigned int):
  rbit w0, w0
  ret