Messages in this thread | | | From | Linus Torvalds <> | Date | Fri, 30 Apr 2021 15:06:30 -0700 | Subject | Re: Heads up: gcc miscompiling initramfs zlib decompression code at -O3 |
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On Fri, Apr 30, 2021 at 1:46 PM Vineet Gupta <Vineet.Gupta1@synopsys.com> 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.
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
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