[llvm-dev] clang optimizations

[Florian Hahn via llvm-dev]

Hi,

> On Jan 25, 2021, at 11:48, via llvm-dev <llvm-dev at lists.llvm.org> wrote:
> 
> Hello,
>  
> I am trying to apply level 3 of optimizations to a simple program. If I run:
>  
> clang -S -O3 -emit-llvm test.cpp -o testo.ll
>  
> all seems OK, the result testo.ll is highly optimized.
>  
> However if I attempt the same using:
>  
> clang -S -emit-llvm test.cpp -o a.ll
>               opt-10 -S --O3 a.ll -o test.ll
>  

Please take a look at the attributes added to the functions: https://godbolt.org/z/fs9f8n <https://godbolt.org/z/fs9f8n> . Without optimizations (-O0), Clang will add `optnone` to the functions, effectively disabling optimizations when you run `opt`. If you don’t want clang to run optimizations, but not add `optnone`, you can use `clang -O3 -emit-llvm -S -mllvm -disable-llvm-optzns`.

Cheers,
Florian 
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