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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">Thanks for the reply, they were very helpful.<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">Is it enough to prevent BBVectorize from packing together double precision instructions? If a non-clang frontend is used, such as ISPC, is it possible that
the IR may contain packed double instruction?<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">Tyler<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"><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""> Cameron McInally [mailto:cameron.mcinally@nyu.edu]
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<b>Sent:</b> Thursday, February 21, 2013 6:39 PM<br>
<b>To:</b> Nowicki, Tyler<br>
<b>Cc:</b> Nadav Rotem; LLVM Developers Mailing List<br>
<b>Subject:</b> Re: [LLVMdev] Generate scalar SSE instructions instead of packed instructions<o:p></o:p></span></p>
<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">On Thu, Feb 21, 2013 at 12:14 PM, Nadav Rotem <<a href="mailto:nrotem@apple.com" target="_blank">nrotem@apple.com</a>> wrote:<o:p></o:p></p>
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<p class="MsoNormal">You can change the input LLVM-IR. <o:p></o:p></p>
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<p class="MsoNormal">On Feb 21, 2013, at 7:16 AM, "Nowicki, Tyler" <<a href="mailto:tyler.nowicki@intel.com" target="_blank">tyler.nowicki@intel.com</a>> wrote:<o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">Hi,<o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> <o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">I am interested in evaluating the performance of packed vs scalar double-precision floating point instructions on x86-atom and I was wondering if anyone knows more precisely where
to modify llvm to use one or the other. I know I probably need to change something in the type legalizer. Could anyone provide more details than that?<o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> <o:p></o:p></p>
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<p class="MsoNormal">Hey Tyler,<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">Nadav is correct. Un-vectorizing would best be done before the IR level.<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 one split the vectors at the ISel level, one would incur unnecessary extracts, which would skew the timing data.<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">To digress a bit, I've found that it's necessary to rewrite the scalar SSE patterns to accept true scalar operands; not fake vector operands like the GNU built-ins. This topic was discussed a while back and the popular belief is that partial
register updates would cause a performance hit when operating on true scalars. However, my empirical evidence suggests that the extra memory traffic of stuffing vectors is more of a performance hit than the partial register updates. Unfortunately, this is
counter-intuitive to the documentation available. And, this may only be true for the benchmarks that hold my interest.<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">For completeness, I'm mainly interested in Interlagos and Sandybridge, so this <span style="font-family:"Arial","sans-serif";color:#212121;background:white">conjecture</span> may not hold for other processors such as Atom.<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">Hope this helps,<o:p></o:p></p>
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<p class="MsoNormal">Cameron<o:p></o:p></p>
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<p class="MsoNormal"> <o:p></o:p></p>
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