linux-kernel - Re: static_branch/jump

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Message-ID: <YIFilwwzWiSDZU6b@hirez.programming.kicks-ass.net>
Date:   Thu, 22 Apr 2021 13:48:39 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     David Malcolm <dmalcolm@...hat.com>
Cc:     Ard Biesheuvel <ardb@...nel.org>, 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>,
        yuanzhaoxiong@...du.com
Subject: Re: static_branch/jump_label vs branch merging

On Fri, Apr 09, 2021 at 11:39:49PM +0200, Peter Zijlstra wrote:
> On Fri, Apr 09, 2021 at 05:07:15PM -0400, David Malcolm wrote:
> 
> > You've built a very specific thing out of asm-goto to fulfil the tough
> > requirements you outlined above - as well as the nops, there's a thing
> > in another section to contend with.
> > 
> > How to merge these asm-goto constructs?
> 
> By calling the function less, you emit less of them. Which then brings
> us back to the whole pure/const thing.
> 
> > Doing so feels very special-case to the kernel and not something that
> > other GCC users would find useful.
> 
> Doesn't it boil down to 'fixing' the pure/const vs asm-goto interaction?
> I could imagine that having that interaction fixed could allow other
> creative uses.

Here is another variant:

  https://lore.kernel.org/lkml/830177B0-45E0-4768-80AB-A99B85D3A52F@baidu.com/

Can we please have a __pure__ attribute that is prescriptive and not a
hint the compiler is free to ignore for $raisins ?