[LLVMdev] Shuffle combine

[Stefanus Du Toit]

Hi Nicolas,

On 2-Apr-09, at 6:04 PM, Nicolas Capens wrote:
> Thanks for verifying this. Could you patch this or should I open a  
> new bug report and find a generic solution first?

I don't have write access so the best I could do would be to submit a  
patch, and I'm crazy busy at the moment.

I actually think the check I described below is fine and would fix  
this bug (but haven't tested it), but the function could be improved  
to operate on more cases. I'd actually be happy to work on a patch for  
this in my spare time since I haven't hacked on LLVM in a while, but  
if you're interested in doing it let me know. In any case, the one- 
line patch (plus a test case, and probably an assert to do the bounds  
check in debug mode) would probably be worthwhile to do right away.

Stefanus

>
> Cheers,
>
> Nicolas
>
> From: llvmdev-bounces at cs.uiuc.edu [mailto:llvmdev- 
> bounces at cs.uiuc.edu] On Behalf Of Stefanus Du Toit
> Sent: woensdag 1 april 2009 18:59
> To: LLVM Developers Mailing List
> Subject: Re: [LLVMdev] Shuffle combine
>
> On 1-Apr-09, at 12:42 PM, Nicolas Capens wrote:
> Hi Stefanus,
>
> Thanks for the info. I still think it’s a bug though. Take for  
> example a case where the vectors each have four elements. The values  
> in Mask[] can range from 0 to 7, while HLSMask only has 4 elements.  
> So LHSMask[Mask[i]] can go out of bounds, no?
>
> Good point! One easy way to fix this would be to use:
>
> if (Mask[i] >= e) {
>
> instead. The code could also be improved to consider the new mask  
> equal to lhsmask/mask in places where they both refer to undefined  
> elements.
>
> Stefanus
>
> Cheers,
>
> Nicolas
>
> From: llvmdev-bounces at cs.uiuc.edu [mailto:llvmdev- 
> bounces at cs.uiuc.edu] On Behalf Of Stefanus Du Toit
> Sent: woensdag 1 april 2009 16:31
> To: LLVM Developers Mailing List
> Subject: Re: [LLVMdev] Shuffle combine
>
> Hi Nicolas,
>
> On 1-Apr-09, at 7:34 AM, Nicolas Capens wrote:
> I’m having some trouble understanding the following lines in  
> InstructionCombining.cpp, which possibly contain a bug:
>
> if (Mask[i] >= 2*e)
>     NewMask.push_back(2*e);
> else
>     NewMask.push_back(LHSMask[Mask[i]]);
>
> When Mask[i] is bigger than the size of LHSMask it reads out of  
> bounds on that last line. I believe the first line is there to try  
> to prevent that but then it should be comparing to LHSMask.size()  
> not 2*e (e being Mask.size()).
>
> Upon quick inspection of the code, I don't think this is necessarily  
> a bug right now, but the function could be improved.
>
> I believe this stems (in part at least) from the fact that  
> shufflevector used to require that the sources and the mask have the  
> same size, which was changed a few months ago to allow an  
> arbitrarily-sized mask.
>
> This would be a bug all throughout this function (which generally  
> assumes this is still the case), if the function didn't do the  
> following check early:
>
>   unsigned VWidth = cast<VectorType>(SVI.getType())->getNumElements();
>
>   if (VWidth != cast<VectorType>(LHS->getType())->getNumElements())
>     return 0;
>
> In other words, if the mask size is not equal to the number of  
> elements in the vectors, it skips this transformation.
>
> Because the LHS is a shufflevector in the part of the code you are  
> mentioning, and the result of a shufflevector has the same number of  
> elements as its mask, you are actually guaranteed that  
> LHSMask.size() == Mask.size(), because LHSMask.size() == LHS- 
> >getNumElements() == Mask.size().
>
> It shouldn't be too hard to relax the constraint that this  
> optimization requires the number of elements being shuffle to be  
> equal to the mask size, but it'll probably take some careful testing!
>
> Stefanus
>
> --
> Stefanus Du Toit <stefanus.dutoit at rapidmind.com>
>   RapidMind Inc.
>   phone: +1 519 885 5455 x116 -- fax: +1 519 885 1463
>
>
>
> <ATT00001.txt>
>
> --
> Stefanus Du Toit <stefanus.dutoit at rapidmind.com>
>   RapidMind Inc.
>   phone: +1 519 885 5455 x116 -- fax: +1 519 885 1463
>
>
>
> <ATT00001.txt>

--
Stefanus Du Toit <stefanus.dutoit at rapidmind.com>
   RapidMind Inc.
   phone: +1 519 885 5455 x116 -- fax: +1 519 885 1463



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