[llvm-commits] [llvm] r48356 - /llvm/trunk/lib/Target/X86/README.txt

[Christopher Lamb]

On Mar 17, 2008, at 11:44 AM, Evan Cheng wrote:

>
> On Mar 16, 2008, at 7:24 PM, Chris Lattner wrote:
>
>>
>> On Mar 15, 2008, at 2:01 PM, Christopher Lamb wrote:
>>
>>> Here's a proposed solution. It seems to work OK for me. It'll be
>>> even better when the undef and insert_subreg coalescing support gets
>>> done.
>>
>> Thanks for tackling this!  Could this be done as a target independent
>> dag combine by selecting to "sext_in_reg(shrl)" when sext_in_reg is
>> available (or before legalize)?  This avoids the apparent size  
>> changes
>> of the register.  On PPC, for example, this would codegen to:
>
> I am not sure about utilizing sext_in_reg, is that correct?

I looked at this and settled on sext, rather than sext_in_reg.

> However, this can definitely be target independent. There is a
> TLI.isTruncateFree() that you can check to ensure using truncate is
> profitable.
>
> +      if (CN->getValue() == ValueSize - 8) {
> +        TruncVT  = MVT::i8;
> +      } else if (CN->getValue() == ValueSize - 16) {
> +        TruncVT  = MVT::i16;
> +      } else if (CN->getValue() == ValueSize - 32) {
> +        TruncVT  = MVT::i32;
> +      }
> +
> +      if (TruncVT != MVT::isVoid) {
>
> Please just use early exit:
>
> } else {
>    return SDOperand();
> }

You are correct, for now. I was thinking of the future when there may  
be other xforms that the function would need to handle! But we can  
cross that bridge when we come to it.

> It's just a matter of style, but checking TruncVT is not MVT::isVoid
> seems weird to me.

It is. What would you suggest if I needed to not exit early, say  
because of other xforms that follow?
--
Chris

>> _test:
>> 	srwi r2, r3, 16
>> 	extsb r3, r2
>> 	blr
>>
>> instead of:
>>
>> _test2:
>> 	slwi r2, r3, 8
>> 	srawi r3, r2, 24
>> 	blr
>>
>> On X86, this is a win because sextinreg (aka movsbl and friends) are
>> 3-
>> address, but sar is 2-address, giving:
>>
>> _test:
>> 	shrl	$16, %edi
>> 	movsbl	%dil, %eax
>> 	ret
>>
>> instead of:
>>
>> _test2:
>> 	shll	$8, %edi
>> 	movl	%edi, %eax
>> 	sarl	$24, %eax
>> 	ret
>>
>> What do you think?
>>
>> -Chris
>>
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--
Christopher Lamb



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