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

[Mehdi AMINI via llvm-dev]

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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