Options that control optimization

Hi Jeani

I'll give you two answers because it is not clear to me which one is most applicable. If you are able to share a bit more about what you are trying to achieve then I may be able to help you further.

You can see which flags are being passed down to clang -cc1 using the -### option + a bit of scripting. There are a number of articles on this (e.g. https://gist.github.com/lolo32/fd8ce29b218ac2d93a9e) You can then use upstream documentation to decipher these. Sadly, it looks like the best source is going to be the code (see https://github.com/llvm/llvm-project/blob/release/13.x/clang/include/clang/Driver/Options.td) and even then you need to use some imagination to interpret them.

In terms of which mid-end passes are enabled/disabled by default at each optimization level, you can find this out using the option -Xclang -fdebug-pass-manager which gives you a list of pass names in the order that they ran. You can then get a description of what these are at https://llvm.org/docs/Passes.html. I believe there is no general way to turn on/off optimization passes via clang, but I could be wrong there.

Hopefully those were useful pointers. If you can share more about what you are trying to achieve then I may be able to help further.

Hi Jeani

I'll give you two answers because it is not clear to me which one is most applicable. If you are able to share a bit more about what you are trying to achieve then I may be able to help you further.

You can see which flags are being passed down to clang -cc1 using the -### option + a bit of scripting. There are a number of articles on this (e.g. https://gist.github.com/lolo32/fd8ce29b218ac2d93a9e) You can then use upstream documentation to decipher these. Sadly, it looks like the best source is going to be the code (see https://github.com/llvm/llvm-project/blob/release/13.x/clang/include/clang/Driver/Options.td) and even then you need to use some imagination to interpret them.

In terms of which mid-end passes are enabled/disabled by default at each optimization level, you can find this out using the option -Xclang -fdebug-pass-manager which gives you a list of pass names in the order that they ran. You can then get a description of what these are at https://llvm.org/docs/Passes.html. I believe there is no general way to turn on/off optimization passes via clang, but I could be wrong there.

Hopefully those were useful pointers. If you can share more about what you are trying to achieve then I may be able to help further.