[PATCH] D33572: [PPC] Implement fast bit reverse in PPCDAGToDAGISel

[Guozhi Wei via Phabricator via llvm-commits]

Carrot created this revision.
Herald added a subscriber: nemanjai.

This patch fixes pr33093.

Current PPCDAGToDAGISel doesn't handle bit reverse efficiently, it generates bit by bit moves for it, even if we give it the following fast algorithm.

unsigned int ReverseBits(unsigned int n) {

  n = ((n >> 1) & 0x55555555) | ((n & 0x55555555) << 1);
  n = ((n >> 2) & 0x33333333) | ((n & 0x33333333) << 2);
  n = ((n >> 4) & 0x0F0F0F0F) | ((n & 0x0F0F0F0F) << 4);
  return ((n & 0xff000000u) >> 24) | ((n & 0x00ff0000u) >> 8) | ((n & 0x0000ff00u) << 8) | ((n & 0x000000ffu) << 24);

}

This patch recognizes bit reverse in BitPermutationSelector, and generates the fast code shown above.


https://reviews.llvm.org/D33572

Files:
  lib/Target/PowerPC/PPCISelDAGToDAG.cpp
  test/CodeGen/PowerPC/pr33093.ll

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