[PATCH] [AArch64] Generate CMN when comparing a short int with minus

[Xu Jiangwei]

Hi t.p.northover, Jiangning, kevin.qin, HaoLiu,

This patch generates cmn instruction when comparing a short int with minus

ldrh	 w8, [x0]
orr	w9, wzr, #0xffff
cmp	 w8, w9

to

ldrsh	 w8, [x0]
cmn	 w8, #1

This patch fixes the issue by signed extending the operand.The performance test shows there are some improvements:
spec.cpu2000.ref.253_perlbmk -2.15%,
spec.cpu2006.ref.482_sphinx3 -1.59%.

As to why this patch just deals with short int, please look at the comments in the patch.

Please have a look.

David

http://reviews.llvm.org/D4966

Files:
  lib/Target/AArch64/AArch64ISelLowering.cpp
  test/CodeGen/AArch64/cmpwithshort.ll
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