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<p class="MsoNormal">Hi all,<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">The DAGCombiner pass actually runs even if the optimize level is set to None. This can result in incorrect debug information or unexpected stepping/debugging experience. Not to mention that having good stepping/debugging experience is the
major reason to compile at /O0.<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">I recently suggested a patch to disable one specific DAG combine at /O0 that broke stepping on a particular case (<a href="http://reviews.llvm.org/D19268">http://reviews.llvm.org/D19268</a>), but other similar cases could appear. In the
same way, another patch was submitted last year for a similar reason (<a href="http://reviews.llvm.org/D7181">http://reviews.llvm.org/D7181</a>).<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">So, since the DAGCombiner is in fact an optimization pass, could it be disabled completely (or partially?) in /O0? And how should it be done?<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">For example, would this patch be too aggressive?<o:p></o:p></p>
<p class="MsoNormal" style="margin-left:1.0in"><br>
<span style="font-size:10.0pt">Index: DAGCombiner.cpp<o:p></o:p></span></p>
<p class="MsoNormal" style="margin-left:1.0in"><span style="font-size:10.0pt">===================================================================<o:p></o:p></span></p>
<p class="MsoNormal" style="margin-left:1.0in"><span style="font-size:10.0pt">--- DAGCombiner.cpp (revision 269301)<o:p></o:p></span></p>
<p class="MsoNormal" style="margin-left:1.0in"><span style="font-size:10.0pt">+++ DAGCombiner.cpp (working copy)<o:p></o:p></span></p>
<p class="MsoNormal" style="margin-left:1.0in"><span style="font-size:10.0pt">@@ -1251,6 +1251,10 @@<o:p></o:p></span></p>
<p class="MsoNormal" style="margin-left:1.0in"><span style="font-size:10.0pt">//===----------------------------------------------------------------------===//<o:p></o:p></span></p>
<p class="MsoNormal" style="margin-left:1.0in"><span style="font-size:10.0pt"><o:p> </o:p></span></p>
<p class="MsoNormal" style="margin-left:1.0in"><span style="font-size:10.0pt"> void DAGCombiner::Run(CombineLevel AtLevel) {<o:p></o:p></span></p>
<p class="MsoNormal" style="margin-left:1.0in"><span style="font-size:10.0pt">+<o:p></o:p></span></p>
<p class="MsoNormal" style="margin-left:1.0in"><span style="font-size:10.0pt">+ if (OptLevel == CodeGenOpt::None)<o:p></o:p></span></p>
<p class="MsoNormal" style="margin-left:1.0in"><span style="font-size:10.0pt">+ return;<o:p></o:p></span></p>
<p class="MsoNormal" style="margin-left:1.0in"><span style="font-size:10.0pt">+<o:p></o:p></span></p>
<p class="MsoNormal" style="margin-left:1.0in"><span style="font-size:10.0pt"> // set the instance variables, so that the various visit routines may use it.<o:p></o:p></span></p>
<p class="MsoNormal" style="margin-left:1.0in"><span style="font-size:10.0pt"> Level = AtLevel;<o:p></o:p></span></p>
<p class="MsoNormal" style="margin-left:1.0in"><span style="font-size:10.0pt"> LegalOperations = Level >= AfterLegalizeVectorOps;<o:p></o:p></span></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">It would most likely break some CodeGen tests since it would have an impact on the code produced at /O0. In fact, I tried to run the CodeGen lit tests with this patch and got 25 new test failures on different targets. These tests would
probably need to be updated.<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">Thanks,<o:p></o:p></p>
<p class="MsoNormal">Marianne<o:p></o:p></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif""><o:p> </o:p></span></p>
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