[PATCH] D45173: [InstCombine] Recognize idioms for ctpop and ctlz

[Craig Topper via Phabricator via llvm-commits]

craig.topper added a comment.

I think we need to evaluate what popcount sequences we want to handle. The code you're handling isn't the most optimal version

For example compiler-rt uses this

  su_int x = (su_int)a;
  x = x - ((x >> 1) & 0x55555555);
  /* Every 2 bits holds the sum of every pair of bits */
  x = ((x >> 2) & 0x33333333) + (x & 0x33333333);
  /* Every 4 bits holds the sum of every 4-set of bits (3 significant bits) */
  x = (x + (x >> 4)) & 0x0F0F0F0F;
  /* Every 8 bits holds the sum of every 8-set of bits (4 significant bits) */
  x = (x + (x >> 16));
  /* The lower 16 bits hold two 8 bit sums (5 significant bits).*/
  /*    Upper 16 bits are garbage */
  return (x + (x >> 8)) & 0x0000003F;  /* (6 significant bits) */

Then there is another form here that uses a multiply in the last step.

https://graphics.stanford.edu/~seander/bithacks.html#CountBitsSetParallel


Repository:
  rL LLVM

https://reviews.llvm.org/D45173