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SubjectRe: static_branch/jump_label vs branch merging
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On Fri, 2021-04-09 at 13:12 +0200, Peter Zijlstra wrote:
> On Fri, Apr 09, 2021 at 11:57:22AM +0200, Ard Biesheuvel wrote:
> > On Thu, 8 Apr 2021 at 18:53, Peter Zijlstra <peterz@infradead.org>
> > wrote:
>
> > > Is there *any* way in which we can have the compiler recognise
> > > that the
> > > asm_goto only depends on its arguments and have it merge the
> > > branches
> > > itself?
> > >
> > > I do realize that asm-goto being volatile this is a fairly huge
> > > ask, but
> > > I figured I should at least raise the issue, if only to raise
> > > awareness.
> > >
> >
> > Wouldn't that require the compiler to interpret the contents of the
> > asm() block?
>
> Yeah, this is more or less asking for ponies :-) One option would be
> some annotation that conveys the desired semantics without it having
> to
> untangle the mess in the asm block.
>
> The thing the compiler needs to know is that the branch is constant
> for
> any @key, and hence allow the obvious optimizations. I'm not sure if
> this is something compiler folks would be even willing to consider,
> but
> I figured asking never hurts.
>

Sorry if this is a dumb question, but does the function attribute:
__attribute__ ((pure))
help here? It's meant to allow multiple calls to a predicate to be
merged - though I'd be nervous of using it here, the predicate isn't
100% pure, since AIUI the whole point of what you've built is for
predicates that very rarely change - but can change occasionally.

Hope this is constructive
Dave


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