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<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'>Hi Stefanus,<o:p></o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'><o:p> </o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'>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?<o:p></o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'><o:p> </o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'>Cheers,<o:p></o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'><o:p> </o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'>Nicolas<o:p></o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'><o:p> </o:p></span></p>
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<p class=MsoNormal><b><span style='font-size:10.0pt;font-family:"Tahoma","sans-serif"'>From:</span></b><span
style='font-size:10.0pt;font-family:"Tahoma","sans-serif"'>
llvmdev-bounces@cs.uiuc.edu [mailto:llvmdev-bounces@cs.uiuc.edu] <b>On Behalf
Of </b>Stefanus Du Toit<br>
<b>Sent:</b> woensdag 1 april 2009 16:31<br>
<b>To:</b> LLVM Developers Mailing List<br>
<b>Subject:</b> Re: [LLVMdev] Shuffle combine<o:p></o:p></span></p>
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<p class=MsoNormal><o:p> </o:p></p>
<p class=MsoNormal>Hi Nicolas,<o:p></o:p></p>
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<p class=MsoNormal><o:p> </o:p></p>
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<p class=MsoNormal>On 1-Apr-09, at 7:34 AM, Nicolas Capens wrote:<o:p></o:p></p>
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<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:black'>I’m having some trouble understanding the following lines in
InstructionCombining.cpp, which possibly contain a bug:<o:p></o:p></span></p>
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<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:black'> <o:p></o:p></span></p>
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<p class=MsoNormal><span style='font-size:10.0pt;font-family:"Courier New";
color:blue'>if</span><span class=apple-converted-space><span style='font-size:
10.0pt;font-family:"Courier New";color:black'> </span></span><span
style='font-size:10.0pt;font-family:"Courier New";color:black'>(Mask[i] >=
2*e)</span><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:black'><o:p></o:p></span></p>
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<p class=MsoNormal><span style='font-size:10.0pt;font-family:"Courier New";
color:black'> NewMask.push_back(2*e);</span><span
style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:black'><o:p></o:p></span></p>
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<p class=MsoNormal><span style='font-size:10.0pt;font-family:"Courier New";
color:blue'>else</span><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:black'><o:p></o:p></span></p>
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<p class=MsoNormal><span style='font-size:10.0pt;font-family:"Courier New";
color:black'> NewMask.push_back(LHSMask[Mask[i]]);</span><span
style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:black'><o:p></o:p></span></p>
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<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:black'> <o:p></o:p></span></p>
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<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:black'>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()). <o:p></o:p></span></p>
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<p class=MsoNormal><o:p> </o:p></p>
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<p class=MsoNormal>Upon quick inspection of the code, I don't think this is
necessarily a bug right now, but the function could be improved.<o:p></o:p></p>
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<p class=MsoNormal><o:p> </o:p></p>
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<p class=MsoNormal>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.<o:p></o:p></p>
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<p class=MsoNormal><o:p> </o:p></p>
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<p class=MsoNormal>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:<o:p></o:p></p>
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<p class=MsoNormal><o:p> </o:p></p>
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<p class=MsoNormal> unsigned VWidth =
cast<VectorType>(SVI.getType())->getNumElements();<o:p></o:p></p>
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<p class=MsoNormal><o:p> </o:p></p>
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<p class=MsoNormal> if (VWidth !=
cast<VectorType>(LHS->getType())->getNumElements())<o:p></o:p></p>
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<p class=MsoNormal> return 0;<o:p></o:p></p>
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<p class=MsoNormal><o:p> </o:p></p>
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<p class=MsoNormal>In other words, if the mask size is not equal to the number
of elements in the vectors, it skips this transformation.<o:p></o:p></p>
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<p class=MsoNormal><o:p> </o:p></p>
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<p class=MsoNormal>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().<o:p></o:p></p>
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<p class=MsoNormal><o:p> </o:p></p>
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<p class=MsoNormal>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!<o:p></o:p></p>
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<p class=MsoNormal><o:p> </o:p></p>
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<p class=MsoNormal>Stefanus<o:p></o:p></p>
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<p class=MsoNormal><o:p> </o:p></p>
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<p class=MsoNormal><span style='font-size:9.0pt;font-family:"Helvetica","sans-serif";
color:black'>--<o:p></o:p></span></p>
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<p class=MsoNormal><span style='font-size:9.0pt;font-family:"Helvetica","sans-serif";
color:black'>Stefanus Du Toit <<a href="mailto:stefanus.dutoit@rapidmind.com">stefanus.dutoit@rapidmind.com</a>><o:p></o:p></span></p>
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<p class=MsoNormal><span style='font-size:9.0pt;font-family:"Helvetica","sans-serif";
color:black'> RapidMind Inc.<o:p></o:p></span></p>
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<p class=MsoNormal><span style='font-size:9.0pt;font-family:"Helvetica","sans-serif";
color:black'> phone: +1 519 885 5455 x116 -- fax: +1 519 885 1463<o:p></o:p></span></p>
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<p class=MsoNormal><span style='font-size:9.0pt;font-family:"Helvetica","sans-serif";
color:black'><o:p> </o:p></span></p>
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<p class=MsoNormal><o:p> </o:p></p>
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<p class=MsoNormal><o:p> </o:p></p>
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