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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">Hi Arnold,<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">Wow, that’s pretty nasty! I’ll keep my eye on Hal’s scoped noalias stuff.<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">James<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 lang="EN-US" style="font-size:10.0pt;font-family:"Tahoma","sans-serif"">From:</span></b><span lang="EN-US" style="font-size:10.0pt;font-family:"Tahoma","sans-serif""> llvm-commits-bounces@cs.uiuc.edu [mailto:llvm-commits-bounces@cs.uiuc.edu]
<b>On Behalf Of </b>Arnold Schwaighofer<br>
<b>Sent:</b> 25 July 2014 17:10<br>
<b>To:</b> Tobias Grosser<br>
<b>Cc:</b> llvm-commits<br>
<b>Subject:</b> Re: SLP/Loop vectorizer pass ordering<o:p></o:p></span></p>
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<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">On Jul 25, 2014, at 8:48 AM, Tobias Grosser <<a href="mailto:tobias@grosser.es">tobias@grosser.es</a>> wrote:<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"">On 25/07/2014 17:41, James Molloy wrote:<br style="orphans: auto;text-align:start;widows: auto;-webkit-text-stroke-width: 0px;word-spacing:0px">
<br>
</span><o:p></o:p></p>
<p class="MsoNormal"><span style="font-size:9.0pt;font-family:"Helvetica","sans-serif"">Hi Nadav, Arnold,<br>
<br>
<br>
<br>
I've come across an interesting optimization problem in one of the SPEC<br>
benchmarks. There is a loop that can be optimized by both the SLP vectorizer<br>
and the loop vectorizer (when I patch the loop vectorizer to deal with fsub<br>
reductions).<br>
<br>
<br>
<br>
The SLP vectorizer actually makes the performance worse - I think this is<br>
due to a lack of loop unrolling afterwards. The Loop vectorizer can improve<br>
the performance.<br>
<br>
<br>
<br>
However, the loop vectorizer runs after the SLP vectorizer, so it never gets<br>
a chance. I'd have thought the ideal order would be Loop Vectorizer -> SLP<br>
vectorizer -> BB vectorizer, given that the loop vectorizer if it can run<br>
will probably give greater speedup than SLP.<br>
<br>
<br>
<br>
The current sequence is SLP vectorizer -> BB vectorizer -> Loop vectorizer.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:9.0pt;font-family:"Helvetica","sans-serif""><br>
Even though I was not directly addressed. I still reply.<br>
<br>
The proposed new order is what I think makes most sense. I wonder what was the reason to go for the current order. Nadav, Arnold did you choose this order?</span><o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">See my answer to James. We loose noalias information during inlining (until recently - thanks Hal for moving forward with the scoped noalias feature!).<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">We should absolutely evaluate moving the SLPVectorizer out of the inliner PM once we have working scoped noalias information.<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">Inlining could obscure patterns of parallelism the SLPVectorizer can recognize so their might be some losses - Erik’s work on making the SLPVectorizer more robust with respect to recognizing what schedules prevent vectorization plus work
on recognizing more (possible mid-tree) patterns should enable us to deal with some of the fallout I think.<o:p></o:p></p>
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