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

[Manman Ren]

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.

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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