Should we have a llvm.loop.optsize/minsize attribute

[lukel97]

This stems from the discussion in #159522. That PR aims to use PGOForceFunctionAttrsPass (added in #69030) to prevent SimpleLoopUnswitch from increasing the code size of cold functions.

Is it worthwhile to have a loop-level equivalent, something like PGOForceLoopAttrsPass that annotates cold loops with a llvm.loop.optsize attribute or equivalent?

We have the llvm.loop.unroll.disable attribute for LoopUnroll, but this would be more general. Just from a quick grep in llvm/lib/Transforms/Scalar it looks like LoopUnroll, SimpleLoopUnswitch, LoopBoundSplit, LoopLoadElimination and LoopIdiomRecognize could take advantage of this to get more granular behaviour.

cc) @aeubanks @nikic @fhahn @teresajohnson

[aeubanks]

Makes sense to me.

If we do add these new loop metadata, we could reuse PGOForceFunctionAttrsPass to add the metadata.