linux-kernel - Re: static_branch/jump

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Message-ID: <5f78b7e2f9ae937271ef52ee9e999a91c2719da9.camel@redhat.com>
Date:   Fri, 09 Apr 2021 07:55:42 -0400
From:   David Malcolm <dmalcolm@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>,
        Ard Biesheuvel <ardb@...nel.org>
Cc:     linux-toolchains@...r.kernel.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Jason Baron <jbaron@...mai.com>,
        "Steven Rostedt (VMware)" <rostedt@...dmis.org>
Subject: Re: static_branch/jump_label vs branch merging

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@...radead.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