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

[Evan Cheng]

On May 9, 2012, at 2:57 PM, Manman Ren wrote:

> 
> Hi Steve,
> 
> Thanks for reviewing this.
> Yes, we are removing cmp instruction and replacing sub with subs.
> I will put more details in the commit log.
> 
> I don't have access to rdar:://problem/11081762 at this point and should have it soon.
>> From my discussion with Jim, it may not be a problem but I should add more testing cases related to rdar://problem/11081762.

It's not really the same issue as rdar://11081762 (it's confusing!). The transformation you are proposing is correct.

Evan

> 
> Thanks,
> Manman
> 
> On May 9, 2012, at 2:41 PM, Stephen Canon wrote:
> 
>> 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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