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<p style="margin-top:0;margin-bottom:0">Hello llvm-dev,</p>
<p style="margin-top:0;margin-bottom:0">my name is Emanuele and I am an intern in ARM. As part of the project I am doing here, I would like to manually replicate the optimizations that LLVM applies when I type -O3. In other words, I would like to know what
are the compilation flags/passes that -O3 triggers. </p>
<p style="margin-top:0;margin-bottom:0">I noticed that GCC reports, on its website, all the flags that are enforced by -O3 (<a href="https://gcc.gnu.org/onlinedocs/gcc/Optimize-Options.html" class="OWAAutoLink">https://gcc.gnu.org/onlinedocs/gcc/Optimize-Options.html</a>),
but I wasn't able to find something similar within LLVM documentation. On the other hand, I found that this command displays all the optimization passes applied by opt when -O3 flag is on:</p>
<p style="margin-top:0;margin-bottom:0">llvm-as < /dev/null | opt -O3 -disable-output -debug-pass=Arguments</p>
<p style="margin-top:0;margin-bottom:0">I tried to apply the same optimization passes through opt, but, even though the performance are similar, the resulting binary is slower than the one generated using -O3 (also the binaries differ, of course).</p>
<p style="margin-top:0;margin-bottom:0">Again, I found this other command that does something similar (it lists the sequence of optimization passes applied):</p>
<p style="margin-top:0;margin-bottom:0">clang -O3 -mllvm -debug-pass=Arguments file.c </p>
<p style="margin-top:0;margin-bottom:0">In this case, the performance are still different and some of the optimization passes listed in the last block of passes (e.g. -machinemoduleinfo, -stack-protector, etc.) are unknown to opt.</p>
<p style="margin-top:0;margin-bottom:0"><br>
</p>
<p style="margin-top:0;margin-bottom:0">Said that, my question is: how can I find out what optimization passes/flags -O3 enforces in order to manually apply the same optimizations and have, hopefully, the same binary and performance?</p>
<p style="margin-top:0;margin-bottom:0"><br>
</p>
<p style="margin-top:0;margin-bottom:0">I am currently using <span style="font-size: 12pt;">LLVM version 5.0.2.</span></p>
<p style="margin-top:0;margin-bottom:0"><span style="font-size: 12pt;"><br>
</span></p>
<p style="margin-top:0;margin-bottom:0"><span style="font-size: 12pt;">Thank you for both your help and your time!</span></p>
<p style="margin-top:0;margin-bottom:0"><span style="font-size: 12pt;"><br>
</span></p>
<p style="margin-top:0;margin-bottom:0"><span style="font-size: 12pt;">Best regards</span></p>
<p style="margin-top:0;margin-bottom:0"><span style="font-size: 12pt;">Emanuele</span></p>
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