Add -O4 flag for link-time optimization

[alexchandel]

Currently we have -Z lto which will "Perform LLVM link-time optimizations". Clang has the -O4 flag for that purpose, which is convenient and makes sense.

[thestinger]

The -O4 flag in clang no longer enables link-time optimization, it's the same as -O3.

[alexchandel]

I still like it...

[huonw]

The -Z flags represent experimental/"internals" features. LTO is something that is still a work-in-progress with many bugs etc, and so it is under -Z for this reason.

[errordeveloper]

This flag would be very misleading and limited, it would have to imply one of the optimisations levels and LTO, e.g. optimisation level 3 and LTO. What if I want to optimise for size and still do LTO?

I am voting to close this 👎

[alexchandel]

@errordeveloper
I don't think rustc supports size optimization currently (except perhaps through --llvm-args="..."). Combining flags isn't necessarily limiting. For example, gcc's -Os option implies -O2 plus the size-for-speed tradeoff optimizations.

And it hopefully won't be -Z lto forever. I admit link time optimizations are somewhat orthogonal to earlier optimizations. What about an additional argument to opt-level like --opt-level 3l, or --lto?

[thestinger]

I think -C lto would make the most sense, and it could just be moved there now.

@alexchandel: -Os in clang is different than just optsize on every function, it runs slightly different passes and uses a lower inline threshold

[thestinger]

Closing in favour of #12443.