[LLVMdev] Generating movq2dq using IRBuilder

[Evan Cheng]

On Jul 31, 2008, at 10:46 PM, Nicolas Capens wrote:

> Hi Dan,
>
> Yes, they could be represented with insertelement and  
> extractelement, but I
> don't think they actually generate optimal code using movq2dq and  
> such. Else
> both bugs 2584 and 2585 would be fixed.
>
> Anyway, I'm actually already encouraged to get involved myself. I'm  
> quite
> experienced with MMX and SSE but I'm still trying to learn more  
> about how
> LLVM does instruction selection and such.

Nice. Perhaps Dan's talk at the Developer Meeting was helpful. :-)

>
>
> By the way, I noticed that movq2dq and such are missing from the  
> intrinsics
> as well. Maybe I could make myself useful by starting to add them?  
> Do you
> know whether http://llvm.org/docs/ExtendingLLVM.html#intrinsic is  
> still a
> good description on how to get started?

No. We don't need intrinsics for most of the vector instructions. They  
can and should be lowered into the right combination of  
vector_shuffle, extractelement, etc. The right way to go about this is  
figuring out why x86 isn't doing what's expected. That probably  
involved fixing 2584 / 2585 and perhaps more.

We usually handle SSE selections very well. Unfortunately MMX has not  
received much love. Perhaps you can take that lead? :-)

Evan

>
>
> Thank you,
>
> Nicolas
>
>
> -----Original Message-----
> From: llvmdev-bounces at cs.uiuc.edu [mailto:llvmdev- 
> bounces at cs.uiuc.edu] On
> Behalf Of Dan Gohman
> Sent: Thursday, 31 July, 2008 23:39
> To: LLVM Developers Mailing List
> Subject: Re: [LLVMdev] Generating movq2dq using IRBuilder
>
>
>
> On Jul 31, 2008, at 7:22 AM, Nicolas Capens wrote:
>
>> In the same breath I'd also like to kindly ask if someone could have
>> a look at the reverse operations, namely trunk from 128 to 64 bit
>> using movdq2q, and 128 to 32 and 64 to 32 using movd. This also
>> seems related to Bug 2585. Thanks again.
>
> The operations you're describing can be represented as insertelement
> and extractelement in LLVM IR.
>
> I don't know of anyone actively working on MMX tuning for LLVM, so
> if you'd like to see it improve, consider yourself encouraged to
> get involved directly :-).
>
> Dan
>
>
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