From: Peter Zijlstra <peterz@infradead.org>
To: David Malcolm <dmalcolm@redhat.com>
Cc: Ard Biesheuvel <ardb@kernel.org>,
linux-toolchains@vger.kernel.org,
Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
Josh Poimboeuf <jpoimboe@redhat.com>,
Jason Baron <jbaron@akamai.com>,
"Steven Rostedt (VMware)" <rostedt@goodmis.org>
Subject: Re: static_branch/jump_label vs branch merging
Date: Fri, 9 Apr 2021 14:03:46 +0200 [thread overview]
Message-ID: <YHBCoijoopbsDn29@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <5f78b7e2f9ae937271ef52ee9e999a91c2719da9.camel@redhat.com>
On Fri, Apr 09, 2021 at 07:55:42AM -0400, David Malcolm wrote:
> 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.
I actually tried that, but it doesn't seem to work. Given the function
arguments are all compile time constants it should DTRT AFAICT, but
alas.
next prev parent reply other threads:[~2021-04-09 12:04 UTC|newest]
Thread overview: 25+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-04-08 16:52 static_branch/jump_label vs branch merging Peter Zijlstra
2021-04-09 9:57 ` Ard Biesheuvel
2021-04-09 10:55 ` Florian Weimer
2021-04-09 11:16 ` Peter Zijlstra
2021-04-09 19:33 ` Nick Desaulniers
2021-04-09 20:11 ` Peter Zijlstra
2021-04-10 17:02 ` Segher Boessenkool
2021-04-09 11:12 ` Peter Zijlstra
2021-04-09 11:55 ` David Malcolm
2021-04-09 12:03 ` Peter Zijlstra [this message]
2021-04-09 13:01 ` Peter Zijlstra
2021-04-09 13:13 ` Peter Zijlstra
2021-04-09 13:48 ` David Malcolm
2021-04-09 18:40 ` Peter Zijlstra
2021-04-09 19:21 ` David Malcolm
2021-04-09 20:09 ` Peter Zijlstra
2021-04-09 21:07 ` David Malcolm
2021-04-09 21:39 ` Peter Zijlstra
2021-04-22 11:48 ` Peter Zijlstra
2021-04-22 17:08 ` Segher Boessenkool
2021-04-22 17:49 ` Peter Zijlstra
2021-04-22 18:31 ` Segher Boessenkool
2021-04-26 17:13 ` Peter Zijlstra
2021-04-10 12:44 ` David Laight
2021-04-09 13:03 ` Segher Boessenkool
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