Hello. On Fri, Apr 30, 2021 at 03:06:30PM -0700, Linus Torvalds wrote: > On Fri, Apr 30, 2021 at 1:46 PM Vineet Gupta <Vineet.Gupta1@xxxxxxxxxxxx> wrote: > > > > I've hit a mainline gcc 10.2 (also gcc 9.3) bug which triggers at -O3 > > causing wrong codegen. > > I'd be more than happy to just disable CC_OPTIMIZE_FOR_PERFORMANCE_O3 entirely. FWIW, we used to find real bugs using -O3 in the past [1]. > The advantages are very questionable - with a lot of the optimizations > at O3 being about loops, something which the kernel to a close > approximation doesn't have. > > Most kernel loops are "count on one hand" iterations, and loop > optimizations generally just make things worse. > > And we've had problems with -O3 before, because not only are the > optimizations a bit esoteric, they are often relatively untested. If > you look around at various projects (outside the kernel), -O2 is > generally the "default". > > And that's entirely ignoring the gcc history - where -O3 has often > been very buggy indeed. It's gotten much better, but I just don't see > the upside of using -O3. > > In fact, it looks like we already have that > > depends on ARC > > for -O3, exactly because nobody really wants to use this. > > So this bug seems to be entirely ARC-specific, in that only ARC can > use -O3 for the kernel already. > > Linus [1] https://lore.kernel.org/lkml/673b885183fb64f1cbb3ed2387524077@xxxxxxxxxxxxxx/ -- Oleksandr Natalenko (post-factum)
