On Mon, 11 May 2020, Linus Torvalds wrote: > On Mon, May 11, 2020 at 2:57 PM Jason A. Donenfeld <Jason@xxxxxxxxx> wrote: > > > > GCC 10 appears to have changed -O2 in order to make compilation time > > faster when using -flto, seemingly at the expense of performance, in > > particular with regards to how the inliner works. Since -O3 these days > > shouldn't have the same set of bugs as 10 years ago, this commit > > defaults new kernel compiles to -O3 when using gcc >= 10. > > I'm not convinced this is sensible. Note the real thing that changed for GCC 10 at -O2 is that -O2 now includes -finline-functions which means GCC considers inlining of functions not marked with 'inline' at -O2. To counter code-size growth and tune that back to previous levels the inlining limits in effect at -O2 have been lowered. Note this has been done based on analyzing larger C++ code and obviously not because the kernel would benefit (IIRC kernel folks like 'inline' to behave as written and thus rather may dislike the change to default to -finline-functions). > -O3 historically does bad things with gcc. Including bad things for > performance. It traditionally makes code larger and often SLOWER. > > And I don't mean slower to compile (although that's an issue). I mean > actually generating slower code. > > Things like trying to unroll loops etc makes very little sense in the > kernel, where we very seldom have high loop counts for pretty much > anything. > > There's a reason -O3 isn't even offered as an option. And I think that's completely sensible. I would not recommend to use -O3 for the kernel. Somehow feeding back profile data might help - though getting such data at all and with enough coverage is probably hard. As you said in the followup I wouldn't recommend tweaking GCCs defaults for the various --param affecting inlining. The behavior with this is not consistent across releases. Richard. > Maybe things have changed, and maybe they've improved. But I'd like to > see actual numbers for something like this. > > Not inlining as aggressively is not necessarily a bad thing. It can > be, of course. But I've actually also done gcc bugreports about gcc > inlining too much, and generating _worse_ code as a result (ie > inlinging things that were behind an "if (unlikely())" test, and > causing the likely path to grow a stack fram and stack spills as a > result). > > So just "O3 inlines more" is not a valid argument. > > Linus > -- Richard Biener <rguenther@xxxxxxx> SUSE Software Solutions Germany GmbH, Maxfeldstrasse 5, 90409 Nuernberg, Germany; GF: Felix Imendörffer; HRB 36809 (AG Nuernberg)
