llvm dev - Mar 2021 - Experimenting with opt -Os/-Oz

Dear LLVMers,

     I would like to fiddle a bit with two standard optimization
levels (Os/Oz) used by opt. So, could someone tell me where, in the
implementation of LLVM, is the order of the passes within those levels
defined? Ideally, I would like to recompile LLVM and change the
default levels in my experiments.

Kind regards,

Fernando

> On Mar 17, 2021, at 13:55, Fernando Magno Quintao Pereira via llvm-dev
<llvm-dev at lists.llvm.org> wrote:
> 
> Dear LLVMers,
> 
>     I would like to fiddle a bit with two standard optimization
> levels (Os/Oz) used by opt. So, could someone tell me where, in the
> implementation of LLVM, is the order of the passes within those levels
> defined? Ideally, I would like to recompile LLVM and change the
> default levels in my experiments.

I think a good place to start would be PassBuilder.cpp. Some passes are only
added for certain optimization levels (e.g. see
https://github.com/llvm/llvm-project/blob/main/llvm/lib/Passes/PassBuilder.cpp#L703)
or take optimization level dependent options. You can also get the executed
passes by using the `-debug-pass-manager` option (e.g. see
https://github.com/llvm/llvm-project/blob/main/llvm/test/Other/new-pm-defaults.ll#L19)

Another thing to note is that some passes check for the `optsize`/`minsize`
attributes on functions as well, so the result could be different even if the
executed passes are the same. Frontends have to add those attributes.

Cheers,
Florian