[llvm-dev] How to produce the same result of clang++ -Oz through opt -Oz (original) (raw)

Mehdi AMINI via llvm-dev llvm-dev at lists.llvm.org
Sat Mar 20 21:13:10 PDT 2021

In general you'll find many cases where O3 does not reproduce either.

Have you tried llc -O2 in your case? Clang sets the backend optimization level that way for Oz/Os/O2: https://github.com/llvm/llvm-project/blob/main/clang/lib/CodeGen/BackendUtil.cpp#L430

-- Mehdi

On Sat, Mar 20, 2021 at 9:06 PM Jiayu Zhao <scjzh at leeds.ac.uk> wrote:

Hi,

I can reproduce the results of clang++ -O3 by opt -O3. But I cannot reproduce the results of clang++ -Oz by opt -Oz. Just see the previous commands I used to produce the result of opt -O3. ------------------------------ From: Mehdi AMINI <joker.eph at gmail.com> Sent: 21 March 2021 3:23 To: Jiayu Zhao <scjzh at leeds.ac.uk> Cc: llvm-dev at lists.llvm.org <llvm-dev at lists.llvm.org> Subject: Re: [llvm-dev] How to produce the same result of clang++ -Oz through opt -Oz Hi, This is a known issue: clang -O3 is slightly different from opt -O3 and it is hard to reproduce exactly. It'd be great to refactor it all so that LLVM exposes a common way for frontend to run the exact same thing. -- Mehdi

On Sat, Mar 20, 2021 at 9:17 AM Jiayu Zhao via llvm-dev <_ _llvm-dev at lists.llvm.org> wrote: Dear developers, I am trying to use 'opt' command with different Optlevels -O3 and -Oz to an IR file to reproduce the results of directly applying clang++ -O3 and -Oz to a source c++ file. 1. Firstly I use the following commands to produce the result of opt -O3: *clang++ -O3 -Xclang -disable-llvm-optzns -emit-llvm -c raytracer.cpp -o raytracer.bc* *opt -O3 raytracer.bc -o tmp.bc * llc -O3 tmp.bc -o tmp.s clang++ tmp.s -o tmp.out 2. Then I use clang -O3 directly: clang++ -O3 raytracer.cpp -o raytracer.out 3. Finally I compare the two files tmp.o and raytracer.o: diff tmp.out raytracer.out We can find the two files are exactly the same. However, things are different in -Oz level 1. I use the following commands to produce the result of opt -Oz: *clang++ -Oz -Xclang -disable-llvm-optzns -emit-llvm -c raytracer.cpp -o raytracer.bc* *opt -Oz raytracer.bc -o tmp.bc * llc -filetype=obj tmp.bc -o tmp.o (there is no -Oz option for llc) 2. Then I use clang -Oz directly: clang++ -Oz -c raytracer.cpp -o raytracer.o 3. Finally I compare the two files tmp.o and raytracer.o: diff tmp.o raytracer.o It shows 'Binary files tmp.o and raytracer.o differ' why 'opt -Oz' cannot produce the same result as 'clang++ -Oz' and how to solve it ? I use LLVM 10.0.1 and CentOS 7.6. I'd very much appreciate if you could help me with this. Thank you. Kind Regards, Jiayu Zhao

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