[llvm-commits] [Patch] Optimize integer ABS on X86

[Manman Ren]

Hi Evan,

PerformXorCombine was enabled only when Subtarget has BMI.
Since now I have to check Xor for abs pattern, I enabled it for all X86 Subtargets.

First, we check whether it is abs, if it is, RV will not be NULL.
We will continue optimizations for BMI if RV is NULL and Subtarget has BMI.

I will add more comments in the code.

Thanks for reviewing.
Manman 

On Jun 6, 2012, at 6:15 PM, Evan Cheng wrote:

> This part looks weird to me:
> 
> +  SDValue RV = performIntegerAbsCombine(N, DAG);
> +  if (RV.getNode() || !Subtarget->hasBMI())
> +    return RV;
> +
> 
> Why is the code returning RV if its Node is null just because Subtarget->hasBMI() is false? Should the subtarget feature check happen earlier? The rest of the patch looks fine to me. Please commit after you have fixed the hasBMI issue.
> 
> Thanks,
> 
> Evan
> 
> On Jun 6, 2012, at 4:01 PM, Manman Ren <mren at apple.com> wrote:
> 
>> 
>> Optimize generated code for integer ABS on X86:
>>       movl    %edi, %ecx
>>       sarl    $31, %ecx
>>       leal    (%rdi,%rcx), %eax
>>       xorl    %ecx, %eax
>>       ret
>> TO
>>       movl	%edi, %eax
>> 	negl	%eax
>> 	cmovll	%edi, %eax
>> 	ret
>> 
>> This reduces code size and has better performance on sandy bridge.
>> There exists a target-independent DAG combine for integer ABS, which converts abs to sar+add+xor.
>> For X86, I tried to match this pattern back to neg+cmov. This is implemented in PerformXorCombine.
>> 
>> Thanks,
>> Manman
>> 
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