<div dir="ltr"><br><div class="gmail_extra"><br><div class="gmail_quote">On Fri, Oct 28, 2016 at 10:20 AM, Hal Finkel <span dir="ltr"><<a href="mailto:hfinkel@anl.gov" target="_blank">hfinkel@anl.gov</a>></span> wrote:<br><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div><div style="font-family:arial,helvetica,sans-serif;font-size:10pt;color:#000000"><br><hr id="m_-617916386397224861zwchr"><blockquote style="border-left:2px solid rgb(16,16,255);margin-left:5px;padding-left:5px;color:rgb(0,0,0);font-weight:normal;font-style:normal;text-decoration:none;font-family:Helvetica,Arial,sans-serif;font-size:12pt"><b>From: </b>"Xinliang David Li" <<a href="mailto:davidxl@google.com" target="_blank">davidxl@google.com</a>><br><b>To: </b><a href="mailto:reviews%2BD26037%2Bpublic%2B89d2f1349c56fe36@reviews.llvm.org" target="_blank">reviews+D26037+public+<wbr>89d2f1349c56fe36@reviews.llvm.<wbr>org</a><br><b>Cc: </b>"陈德颢" <<a href="mailto:danielcdh@gmail.com" target="_blank">danielcdh@gmail.com</a>>, "Hal Finkel" <<a href="mailto:hfinkel@anl.gov" target="_blank">hfinkel@anl.gov</a>>, "Junbum Lim" <<a href="mailto:junbuml@codeaurora.org" target="_blank">junbuml@codeaurora.org</a>>, "llvm-commits" <<a href="mailto:llvm-commits@lists.llvm.org" target="_blank">llvm-commits@lists.llvm.org</a>>, <a href="mailto:anna@azul.com" target="_blank">anna@azul.com</a><br><b>Sent: </b>Friday, October 28, 2016 12:11:04 PM<br><b>Subject: </b>Re: [PATCH] D26037: Add LoopSink pass for PGO.<span class=""><br><br><div dir="ltr"><br><div class="gmail_extra"><br><div class="gmail_quote">On Fri, Oct 28, 2016 at 9:40 AM, Dehao Chen <span dir="ltr"><<a href="mailto:danielcdh@gmail.com" target="_blank">danielcdh@gmail.com</a>></span> wrote:<br><blockquote class="gmail_quote" style="margin:0pt 0pt 0pt 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex">danielcdh added a comment.<br>
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We need this pass to remove LCSSA nodes because this will be the last loop pass.<br>
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Another question is: shall we pin this pass to PGO only? If not, we can simply add it to lib/Transforms/IPO/<wbr>PassManagerBuilder.cpp<br></blockquote><div><br></div><div id="m_-617916386397224861DWT5029">Static branch prediction is conservative in predicting loop iterations (loop scale). This means it is likely some blocks (if wrapped under __builtin_expect) inside the loop will be wrongly marked as colder than preheader which can trigger loop sinking wrongly. </div></div></div></div></span></blockquote>Can you please elaborate? If we statically predict a modest number of iterations, which seems like a reasonable trade off, and we have a block which is expected to not execute (because of __builtin_expect), then is it obviously wrong that it should be colder than the preheader? If the opposite of __builtin_expect is "almost never", </div></div></blockquote><div><br></div><div><br></div><div>My observation is that the interpretation and use of '__builtin_expect' varies a lot from user to user. Some consider 80% or even as low as ~70% probability as expected, while some considers >99%. Even with 99% expected probability, it still can get wrong. For instance in a loop with 1 million iterations, the cold block will execute 10k times -- still much hotter than the preheader.</div><div><br></div><div>David</div><div><br></div><div> </div><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div><div style="font-family:arial,helvetica,sans-serif;font-size:10pt;color:#000000">which is true for error paths and the like, then that seems not unreasonable. Why would loop sinking hurt there?<br><br></div></div></blockquote><div><br></div><div><br></div><div><br></div><div> </div><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div><div style="font-family:arial,helvetica,sans-serif;font-size:10pt;color:#000000">Thanks again,<br>Hal<span class=""><br><blockquote style="border-left:2px solid rgb(16,16,255);margin-left:5px;padding-left:5px;color:rgb(0,0,0);font-weight:normal;font-style:normal;text-decoration:none;font-family:Helvetica,Arial,sans-serif;font-size:12pt"><div dir="ltr"><div class="gmail_extra"><div class="gmail_quote"><div> Making this pass depending on PGO is a safer thing to do.<br></div><div><br></div><div>David</div><div><br></div><blockquote class="gmail_quote" style="margin:0pt 0pt 0pt 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex">
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<a href="https://reviews.llvm.org/D26037" rel="noreferrer" target="_blank">https://reviews.llvm.org/<wbr>D26037</a><br>
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</blockquote><br><br><br></span><span class="HOEnZb"><font color="#888888">-- <br><div><span name="x"></span>Hal Finkel<br>Lead, Compiler Technology and Programming Languages<br>Leadership Computing Facility<br>Argonne National Laboratory<span name="x"></span><br></div></font></span></div></div></blockquote></div><br></div></div>