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<div>Hello Emanuele,</div><div><br></div><div>When you provide the optimization level -O3 to the clang driver, it does not simply schedule a sequence of passes to be run on the intermediate representation.</div><div>Indeed, it schedules target-independent and target-dependent passes.</div><div>Moreover, IIRC, the optimization level is also used in the later stages of the code generation to apply target-dependent optimizations (i.e. vectorizer).</div><div><br></div><div>The most common use case when someone wants to test its own pass/work within the LLVM toolchain is the following<br></div><div>- use clang to generate a LLVM-IR file</div><div>- use opt to run your desired pass / pass sequence and output another LLVM-IR file</div><div>- use clang -O3 to compile to executable machine code</div><div><br></div><div>However, with this approach you will run the passes on the LLVM-IR twice.</div><div>There are use cases when this could invalidate your results.</div><div>A<span></span>s opt stops at LLVM-IR level, I would suggest you to use also other LLVM tools to run individually the backend stages / sequence of passes which cannot be run by opt (such as llc / llvm-mc).</div><div>An extensive list of tools/commands you can use is available at [0].</div><div>For your specific case, I would suggest you to have a look at this restricted schema [1].</div><div><br></div><div>Yet there is another way to get into even fine grain detail.</div><div>You can check which are the clang DriverActions you are running with a given command line. See [2].</div><div>>From that point you can rebuild the exact whole sequence of commands that the clang driver triggers.</div><div><br></div><div>If you can provide more details about what is your use case (measure performance, pass development and testing, flag selection, phase ordering), we can suggest the most suitable approach.</div><div><br></div><div>Kind regards,</div><div><br></div><div>Stefano Cherubin</div><div><br></div><div>[0] http://llvm.org/docs/CommandGuide/</div><div>[1] https://github.com/skeru/LLVM-intro/blob/master/img/03/toolchain.pdf</div><div>[2] https://clang.llvm.org/docs/DriverInternals.html#driver-stages<br></div><div><br></div><div><br></div><div><br></div><div><br></div>
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On Thursday, 16 August 2018, 12:46:04 CEST, Emanuele Del Sozzo via llvm-dev <llvm-dev@lists.llvm.org> wrote:
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Hello llvm-dev,</p>
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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
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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="ydp8cb328fayiv4757402819x_OWAAutoLink" rel="nofollow" target="_blank">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>
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llvm-as < /dev/null | opt -O3 -disable-output -debug-pass=Arguments</p>
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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>
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Again, I found this other command that does something similar (it lists the sequence of optimization passes applied):</p>
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clang -O3 -mllvm -debug-pass=Arguments file.c </p>
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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>
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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>
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I am currently using <span style="font-size:12pt;">LLVM version 5.0.2.</span></p>
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<span style="font-size:12pt;">Thank you for both your help and your time!</span></p>
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<span style="font-size:12pt;">Best regards</span></p>
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<span style="font-size:12pt;">Emanuele</span></p>
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