[PATCH] D21578: [SelectionDAG] Optimization of BITREVERSE legalization for power-of-2 integer scalar/vector types

[Simon Pilgrim via llvm-commits]

RKSimon created this revision.
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An extension of D19978, this patch replaces the default BITREVERSE evaluation of individual bit masks+shifts with block mask+shifts when we have integer elements of power-of-2 bits in size.

After calling BSWAP to reverse the order of the constituent bytes (which typically follows a similar approach), every neighbouring 4-bits, 2-bits and finally 1-bit pairs are masked off and swapped over with shifts.

In doing so we can significantly reduce the number of operations required.

Repository:
  rL LLVM

http://reviews.llvm.org/D21578

Files:
  lib/CodeGen/SelectionDAG/LegalizeDAG.cpp
  test/CodeGen/AArch64/bitreverse.ll
  test/CodeGen/X86/vector-bitreverse.ll

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