[PATCH] D53214: [X86][SSE] Change CTTZ vector lowering to cttz(x) = ctpop(~x & (x - 1))

[Simon Pilgrim via Phabricator via llvm-commits]

RKSimon created this revision.
RKSimon added reviewers: craig.topper, spatel, lebedev.ri, andreadb.

This patch changes the vector CTTZ lowering from:

cttz(x) = ctpop((x & -x) - 1)

to:

cttz(x) = ctpop(~x & (x - 1))

Not only does this make better use of the PANDN instruction, but it also matches the LegalizeDAG method which should allow us to remove the x86 specific code at some point in the future (we need to fit some issues with the bitcasted logic ops and CTPOP lowering first).


Repository:
  rL LLVM

https://reviews.llvm.org/D53214

Files:
  lib/Target/X86/X86ISelLowering.cpp
  test/CodeGen/X86/vec_ctbits.ll
  test/CodeGen/X86/vector-tzcnt-128.ll
  test/CodeGen/X86/vector-tzcnt-256.ll
  test/CodeGen/X86/vector-tzcnt-512.ll

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