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<p class="MsoNormal">I find that clang is very slow compiling a C function (generated by a tool). The function contains a very large switch statement within two nested loops. From opt –time-passes, I can see that almost all the time (95%) is being spent in
GVN. I cannot afford to reduce the optimization level to –O1 (which will shut off GVN) as it causes the run-time to degrade by an unacceptable factor (>2.5X). However, I would like the compile time to be better than what it is. And I suspect that it can be
better. gcc 4.7.2 is able to perform much better on this code (both compile-time and run-time).<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">The same file compiled by gcc 4.7.2 (-O3) is about 6 times faster in compile time than clang (-O2). The run-time performance is the same. gcc –O2 compile time is even faster (10X) but is significantly worse in run-time performance than
clang –O2. <o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">One thing I noticed was that if I converted the switch from:<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal"> switch (x) { … } <o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">to:<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal"> extern int black_box(int); // opaque identity function<o:p></o:p></p>
<p class="MsoNormal"> switch (black_box(x)) { … }<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">the compile time improves significantly. <o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">I am using clang version 3.3.<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">I also noticed that there have been similar reports about GVN performance in the past. See this link:
<a href="http://lists.cs.uiuc.edu/pipermail/llvmbugs/2009-June/008477.html">http://lists.cs.uiuc.edu/pipermail/llvmbugs/2009-June/008477.html</a><o:p></o:p></p>
<p class="MsoNormal">That was 4.5 years ago. I could not find any follow up on that thread and so I don’t know if it is still an open problem.
<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">I am unable to share the source code containing the switch statement but I am willing to spend the time to diagnose this performance problem myself. However, I would appreciate any suggestions on how to proceed.<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoListParagraph" style="text-indent:-.25in;mso-list:l0 level1 lfo1"><![if !supportLists]><span style="font-family:Wingdings"><span style="mso-list:Ignore">n<span style="font:7.0pt "Times New Roman"">
</span></span></span><![endif]>Riyaz<o:p></o:p></p>
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