[LLVMdev] Shuffle combine

[Nicolas Capens]

Hi Stefanus,

 

Thanks for verifying this. Could you patch this or should I open a new bug
report and find a generic solution first?

 

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

 

 

 

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