[llvm-dev] How to produce the same result of clang++ -Oz through opt -Oz (original) (raw)
Jiayu Zhao via llvm-dev llvm-dev at lists.llvm.org
Sat Mar 20 21:05:30 PDT 2021
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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<mailto: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.
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
Then I use clang -O3 directly: clang++ -O3 raytracer.cpp -o raytracer.out
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
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)
Then I use clang -Oz directly: clang++ -Oz -c raytracer.cpp -o raytracer.o
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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