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<p class="MsoNormal">Without actually trying it myself, I would say that the -O1 command line runs optimization passes where the -O0 command line does not. Thus, your “baseline” IR is already somewhat optimized in the -O1 case. If you want to see IR with
no optimizations run at all, you want to add `-Xclang -disable-llvm-passes` to your command line for producing unoptimized IR. I think this would produce the same results for -O0 and -O1.<o:p></o:p></p>
<p class="MsoNormal">--paulr<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal"><b>From:</b> llvm-dev <llvm-dev-bounces@lists.llvm.org> <b>On Behalf Of
</b>hameeza ahmed via llvm-dev<br>
<b>Sent:</b> Wednesday, November 13, 2019 4:02 AM<br>
<b>To:</b> llvm-dev <llvm-dev@lists.llvm.org><br>
<b>Subject:</b> [llvm-dev] Difference between clang -O1 -Xclang -disable-O0-optnone and clang -O0 -Xclang -disable-O0-optnone in LLVM 9<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">Hello,<o:p></o:p></p>
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<p class="MsoNormal">I m trying to test individual O3 optimizations/ replicating O3 behavior on IR. I took unoptimized IR (O0), used disable-o0-optnone via (<b>clang -O0 -Xclang -disable-O0-optnone</b>). I read somewhere about
<b>clang -O1 -Xclang -disable-O0-optnone,</b> so I also tested on this initial IR.<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">I have observed by using individual optimizations, the performance (i.e time) is better when the base/initial IR is generated via
<b>clang -O1 -Xclang -disable-O0-optnone</b>. In case of <b>clang -O0 -Xclang -disable-O0-optnone</b> initial IR, the performance is reduced.<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">What is the possible reason for this?<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">What is the right way?<o:p></o:p></p>
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<p class="MsoNormal">Please guide.<o:p></o:p></p>
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<p class="MsoNormal">Thank You <o:p></o:p></p>
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