https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110334
--- Comment #7 from Richard Biener <rguenth at gcc dot gnu.org> --- (In reply to Jan Hubicka from comment #6) > Comdats are really in conflict with the fact that we have command line > options. I blame C++ standard for that and I don't think there is fully > satisfactory solution to this problem. > > I was playing with the idea of warning when at lto time when comdats have > different command line options, but this triggers way too often in practice. Really? :/ > We would need to determine "dangerous" one i.e. when -fno-avx2 is replaced > by -favx2 code. > There are many ways one can stubly change semantics of the IL which makes > merging possibly dangerous which is done often in larger projects, like > firefox. I think it would be desirable to diagnose these, maybe with an option to selectively disable this specific diagnostic. Because while it is not always a correctness issue it can be a performance issue as well. > With syntactic aliases it is possible to keep multiple copies of comdat > function through merging process so inlining will chose corresponding one, > but it does make other things harder. One important anoyance is that it > makes it a lot harder to estimate overall size of the translation unit and > how inlining affects it. We currently assume that every function will need > offline unless all calls to it disappears. We will need to understand that > this is true across all syntacit aliases. > > Also that conditional that disables early inliner for all always_inlines is > probably bit harmful these days as libstdc++ indeed has quite interesting > set of always_inlines that call normal inlines. I noticed that just recently > while looking into the push_back implementation how complex maze it got. > > I will fix early inliner to allow safe inlining to always_inlines. Beware of new always-inline calls then appearing after greedy inlining (though that's exactly the case that we try to avoid here). I suppose you could disable inlining of a function which contains always-inline calls or simply functions that did not yet have the early inliner run on them (so keep the current behavior in cycles). Beware of indirect always-inline calls then. Btw, for Skia the issue is really that some auto-generated CTOR isn't marked always-inline but everything else is. Maybe they should use flatten instead of always-inline ...
