Messages in this thread | | | From | Linus Torvalds <> | Date | Wed, 18 Oct 2023 09:26:38 -0700 | Subject | Re: [PATCH v2 -tip] x86/percpu: Use C for arch_raw_cpu_ptr() |
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On Wed, 18 Oct 2023 at 09:03, Nadav Amit <namit@vmware.com> wrote: > > Having said that, I am not sure what other usages you have in mind. > “current” is a pretty obvious straight forward case with considerable > impact on code generation. There may be additional variables, but it is > likely that there would be more functions/TU in which they would not be > constant and would require more refined techniques to avoid mistakes > such as the use of stale cached values.
Yeah, I don't think there really are other cases.
We do have things that could be considered stable (like "smp_processor_id()" which is stable as long as preemption or migration is disabled (or it's in an irq-off section).
And it might be lovely to optimize those too, *BUT* that would require that there be a barrier against that optimization that works.
And if there is anything that this thread has made clear, it's that the whole 'load from a constant section' doesn't seem to have any sane barriers.
So while the CSE for inline asm statements is a bit too weak with that whole "only CSE within a basic block" thing, the CSE of "load a constant value from memory" is too *strong*, in that we don't seem to have _any_ sane way to say "now you need to reload".
The traditional way we've done that is with our "barrier()" macro, which does the whole inline asm with a memory clobber, but even that doesn't act as a barrier for gcc optimizing the constant load.
Which means that while we'd probably love for the compiere to optimize smp_processor_id() a bit more, we can't use the 'stable memory location' trick for it.
Because I can't think of anything but 'current' that would be _that_ stable as far as C code is concerned.
Linus
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