[llvm-commits] [PATCH] Update peephole optimization for ARM: modified OptimizeCompare

[Stephen Canon]

Assuming that I understand what is really happening here[1], this does not suffer from the problem that caused <rdar://problem/11081762> and is safe.  That said, a little more clarity about exactly what you're optimizing would be helpful for those playing along at home, Manman.

- Steve

1) based on a conversation with Evan, my understanding is that we're looking at a peephole transformation to replace something like:

	sub  r0,  r1,  r3
	cmp r1,  r3

with

	subs r0, r1, r3

-- is that correct, manman?

On May 9, 2012, at 2:04 PM, Jim Grosbach <grosbach at apple.com> wrote:

> Hi Manman,
> 
> There are subtleties here. See <rdar://problem/11081762>.
> 
> -Jim
> 
> On May 8, 2012, at 6:05 PM, Manman Ren wrote:
> 
>> 
>> The purpose is to optimize the following cases:
>> //       sub r1, r3
>> //       cmp r3, r1 or cmp r1, r3
>> //       bge L1
>> 
>> //      sub r1, imm
>> //      cmp r1, imm
>> //      bge L1
>> 
>> //     If the branch instruction can use flag from "sub", then we can eliminate
>> //     the "cmp" instruction.
>> 
>> 
>> Comments are welcome.
>> 
>> Thanks,
>> Manman
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