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[ARM] add overrides for isCheapToSpeculateCttz() and isCheapToSpeculateCtlz()
authorSanjay Patel <spatel@rotateright.com>
Tue, 10 Nov 2015 19:24:31 +0000 (19:24 +0000)
committerSanjay Patel <spatel@rotateright.com>
Tue, 10 Nov 2015 19:24:31 +0000 (19:24 +0000)
commitda103fa05e4483d465bb0c6991fae5ce35f617d9
tree8c6fd66d12f0cf1e502937ff184bb16888f7ee94
parenta987cc630dcb1d10fa6069ccf9b46f2131a0cd03
[ARM] add overrides for isCheapToSpeculateCttz() and isCheapToSpeculateCtlz()

ARM V6T2 has instructions for efficient count-leading/trailing-zeros, so this should be
considered a cheap operation (and therefore fair game for speculation) for any ARM V6T2
implementation.

The net result of allowing this speculation for the regression tests in this patch is
that we get this code:

ctlz:
  clz  r0, r0
  bx  lr
cttz:
  rbit  r0, r0
  clz  r0, r0
  bx  lr

Instead of:

ctlz:
  cmp  r0, #0
  moveq  r0, #32
  clzne  r0, r0
  bx  lr
cttz:
  cmp   r0, #0
  moveq  r0, #32
  rbitne  r0, r0
  clzne  r0, r0
  bx  lr

This will help solve a general speculation/despeculation problem noted in PR24818:
https://llvm.org/bugs/show_bug.cgi?id=24818

Differential Revision: http://reviews.llvm.org/D14469

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@252639 91177308-0d34-0410-b5e6-96231b3b80d8
lib/Target/ARM/ARMISelLowering.cpp
lib/Target/ARM/ARMISelLowering.h
test/Transforms/SimplifyCFG/ARM/cttz-ctlz.ll [new file with mode: 0644]
test/Transforms/SimplifyCFG/ARM/lit.local.cfg [new file with mode: 0644]