Messages in this thread | | | Date | Fri, 9 Apr 2021 22:11:01 +0200 | From | Peter Zijlstra <> | Subject | Re: static_branch/jump_label vs branch merging |
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On Fri, Apr 09, 2021 at 12:33:29PM -0700, Nick Desaulniers wrote: > On Fri, Apr 9, 2021 at 4:18 AM Peter Zijlstra <peterz@infradead.org> wrote: > > > > On Fri, Apr 09, 2021 at 12:55:18PM +0200, Florian Weimer wrote: > > > * Ard Biesheuvel: > > > > > > > Wouldn't that require the compiler to interpret the contents of the > > > > asm() block? > > > > > > Yes and no. It would require proper toolchain support, so in this case > > > a new ELF relocation type, with compiler, assembler, and linker support > > > to generate those relocations and process them. As far as I understand > > > it, the kernel doesn't do things this way. > > > > I don't think that all is needed. All we need is for the compiler to > > recognise that: > > > > if (cond) { > > stmt-A; > > } > > if (cond) { > > stmt-B; > > } > > > > both cond are equivalent and hence can merge the blocks like: > > > > if (cond) { > > stmt-A; > > stmt-B; > > } > > > > But because @cond is some super opaque asm crap, the compiler throws up > > it's imaginry hands and says it cannot possibly tell and leaves them as > > is. > > Right, because if `cond` has side effects (such as is implied by asm > statements that are volatile qualified), suddenly those side effects > would only occur once whereas previously they occurred twice. > > Since asm goto is implicitly volatile qualified, it sounds like > removing the implicit volatile qualifier from asm goto might help? > Then if there were side effects but you forgot to inform the compiler > that there were via an explicit volatile qualifier, and it performed > the suggested merge, oh well.
So, as mentioned elsewhere in this thread, it would be nice if either the pure or const function attribute could over-ride/constrain that volatile side effect.
I'm fine with things going side-ways if we get it wrong, that's more or less the game we're playing anyway ;-)
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