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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D">I have tried running both gvn and newgvn but it did not helped in hoisting the alias checks:<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">Please check, maybe I have missed something.<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"><TestCase><o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D">void foo (char *A, char *B, char *C, int len) {<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D"> int i = 0;<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D"> for (i=0 ; i< len; i++)<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D"> A[i] = B[i] + C[i];<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"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D"><Command><o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D"> $ opt –O3 –gvn test.ll –o test.opt.ll<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D"> $ opt –O3 –newgvn test.ll –o test.opt.ll<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">“test.ll” is attached, it got already vectorized by the approach running vectorizer twice by annotate the remainder loop with metadata to limit the vectorization
factor for epilog vector loop.<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:#1F497D">Regards,<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D">Ashutosh<o:p></o:p></span></p>
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<p class="MsoNormal" style="margin-left:.5in"><b><span style="font-size:11.0pt;font-family:"Calibri",sans-serif">From:</span></b><span style="font-size:11.0pt;font-family:"Calibri",sans-serif"> anemet@apple.com [mailto:anemet@apple.com]
<br>
<b>Sent:</b> Tuesday, February 28, 2017 1:33 AM<br>
<b>To:</b> Hal Finkel <hfinkel@anl.gov><br>
<b>Cc:</b> Daniel Berlin <dberlin@dberlin.org>; Nema, Ashutosh <Ashutosh.Nema@amd.com>; Zaks, Ayal <ayal.zaks@intel.com>; Renato Golin <renato.golin@linaro.org>; mkuper@google.com; Mehdi Amini <mehdi.amini@apple.com>; llvm-dev <llvm-dev@lists.llvm.org><br>
<b>Subject:</b> Re: [llvm-dev] [Proposal][RFC] Epilog loop vectorization<o:p></o:p></span></p>
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<p class="MsoNormal" style="margin-left:.5in">On Feb 27, 2017, at 12:01 PM, Hal Finkel <<a href="mailto:hfinkel@anl.gov">hfinkel@anl.gov</a>> wrote:<o:p></o:p></p>
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<p class="MsoNormal" style="margin-left:.5in">On 02/27/2017 01:47 PM, Daniel Berlin wrote:<o:p></o:p></p>
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<p class="MsoNormal" style="margin-left:.5in">On Mon, Feb 27, 2017 at 11:29 AM, Adam Nemet <<a href="mailto:anemet@apple.com" target="_blank">anemet@apple.com</a>> wrote:<o:p></o:p></p>
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<p class="MsoNormal" style="margin-left:.5in">On Feb 27, 2017, at 10:11 AM, Hal Finkel <<a href="mailto:hfinkel@anl.gov" target="_blank">hfinkel@anl.gov</a>> wrote:<o:p></o:p></p>
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<p class="MsoNormal" style="margin-left:.5in">On 02/27/2017 11:47 AM, Adam Nemet wrote:<o:p></o:p></p>
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<p class="MsoNormal" style="margin-left:.5in">On Feb 27, 2017, at 9:39 AM, Daniel Berlin <<a href="mailto:dberlin@dberlin.org" target="_blank">dberlin@dberlin.org</a>> wrote:<o:p></o:p></p>
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<p class="MsoNormal" style="margin-left:.5in">On Mon, Feb 27, 2017 at 9:29 AM, Adam Nemet <<a href="mailto:anemet@apple.com" target="_blank">anemet@apple.com</a>> wrote:<o:p></o:p></p>
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<p class="MsoNormal" style="margin-left:.5in">On Feb 27, 2017, at 7:27 AM, Hal Finkel <<a href="mailto:hfinkel@anl.gov" target="_blank">hfinkel@anl.gov</a>> wrote:<o:p></o:p></p>
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On 02/27/2017 06:29 AM, Nema, Ashutosh wrote:<o:p></o:p></span></p>
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<p class="MsoNormal" style="margin-left:.5in;background:white"><span style="font-size:7.5pt;font-family:"Helvetica",sans-serif">Thanks for looking into this.<o:p></o:p></span></p>
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<p class="MsoNormal" style="margin-left:.5in;background:white"><span style="font-size:7.5pt;font-family:"Helvetica",sans-serif"> <o:p></o:p></span></p>
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<p class="MsoNormal" style="margin-left:.5in;background:white"><span style="font-size:7.5pt;font-family:"Helvetica",sans-serif">1) Issues with re running vectorizer:<o:p></o:p></span></p>
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<p class="MsoNormal" style="margin-left:.5in;background:white"><span style="font-size:7.5pt;font-family:"Helvetica",sans-serif">Vectorizer might generate redundant alias checks while vectorizing epilog loop.<o:p></o:p></span></p>
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<p class="MsoNormal" style="margin-left:.5in;background:white"><span style="font-size:7.5pt;font-family:"Helvetica",sans-serif">Redundant alias checks are expensive, we like to reuse the results of already computed alias checks.<o:p></o:p></span></p>
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<p class="MsoNormal" style="margin-left:.5in;background:white"><span style="font-size:7.5pt;font-family:"Helvetica",sans-serif">With metadata we can limit the width of epilog loop, but not sure about reusing alias check result.<o:p></o:p></span></p>
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<p class="MsoNormal" style="margin-left:.5in;background:white"><span style="font-size:7.5pt;font-family:"Helvetica",sans-serif">Any thoughts on rerunning vectorizer with reusing the alias check result ?<o:p></o:p></span></p>
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<span style="background:white">One way of looking at this is: Reusing the alias-check result is really just a conditional propagation problem; if we don't already have an optimization that can combine these after the fact, then we should.</span></span><o:p></o:p></p>
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<p class="MsoNormal" style="margin-left:.5in"><o:p> </o:p></p>
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<p class="MsoNormal" style="margin-left:.5in">+Danny<o:p></o:p></p>
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<p class="MsoNormal" style="margin-left:.5in">Isn’t Extended SSA supposed to help with this?<o:p></o:p></p>
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<p class="MsoNormal" style="margin-left:.5in">Yes, it will solve this with no issue already. GVN probably does already too.<o:p></o:p></p>
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<p class="MsoNormal" style="margin-left:.5in">even if if you have<o:p></o:p></p>
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<p class="MsoNormal" style="margin-left:.5in"><o:p> </o:p></p>
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<p class="MsoNormal" style="margin-left:.5in">if (a == b)<o:p></o:p></p>
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<p class="MsoNormal" style="margin-left:.5in">if (a == c)<o:p></o:p></p>
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<p class="MsoNormal" style="margin-left:.5in"> if (a == d)<o:p></o:p></p>
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<p class="MsoNormal" style="margin-left:.5in"> if (a == e)<o:p></o:p></p>
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<p class="MsoNormal" style="margin-left:.5in"> if (a == g)<o:p></o:p></p>
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<p class="MsoNormal" style="margin-left:.5in">and we can prove a ... g equivalent, newgvn will eliminate them all and set all the branches true.<o:p></o:p></p>
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<p class="MsoNormal" style="margin-left:.5in">If you need a simpler clean up pass, we could run it on sub-graphs.<o:p></o:p></p>
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<p class="MsoNormal" style="margin-left:.5in">Yes we probably don’t want to run a full GVN after the “loop-scheduling” passes.<o:p></o:p></p>
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FWIW, we could, just without the memory-dependence analysis enabled (i.e. set the NoLoads constructor parameter to true). GVN is pretty fast in that mode.<o:p></o:p></p>
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<p class="MsoNormal" style="margin-left:.5in">OK. Another data point is that I’ve seen cases in the past where the alias checks required for the loop passes could enable GVN to remove redundant loads/stores. Currently we can only pick these up with LTO when
GVN is rerun.<o:p></o:p></p>
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<p class="MsoNormal" style="margin-left:.5in">This is just GVN brokenness, newgvn should not have this problem.<o:p></o:p></p>
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<p class="MsoNormal" style="margin-left:.5in">If it does, i'd love to see it.<o:p></o:p></p>
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I thought that the problem is that we just don't run GVN after that point in the pipeline.<o:p></o:p></p>
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<p class="MsoNormal" style="margin-left:.5in">Yeah, that is the problem but I think Danny misunderstood what I was trying to say.<o:p></o:p></p>
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<p class="MsoNormal" style="margin-left:.5in">This was a datapoint to possibly rerun GVN with memory-awareness.<o:p></o:p></p>
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-Hal<br>
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<p class="MsoNormal" style="margin-left:.5in">(I'm working on the last few parts of turning it on by default, but it requires a new getModRefInfo interface to be able to get the last few testcases)<o:p></o:p></p>
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<o:p></o:p></p>
<pre style="margin-left:.5in">-- <o:p></o:p></pre>
<pre style="margin-left:.5in">Hal Finkel<o:p></o:p></pre>
<pre style="margin-left:.5in">Lead, Compiler Technology and Programming Languages<o:p></o:p></pre>
<pre style="margin-left:.5in">Leadership Computing Facility<o:p></o:p></pre>
<pre style="margin-left:.5in">Argonne National Laboratory<o:p></o:p></pre>
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