linux-kernel - Re: static_branch/jump

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Message-ID: <20210422170820.GB27473@gate.crashing.org>
Date:   Thu, 22 Apr 2021 12:08:20 -0500
From:   Segher Boessenkool <segher@...nel.crashing.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     David Malcolm <dmalcolm@...hat.com>,
        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 Thu, Apr 22, 2021 at 01:48:39PM +0200, Peter Zijlstra wrote:
> Can we please have a __pure__ attribute that is prescriptive and not a
> hint the compiler is free to ignore for $raisins ?

What does that mean?  What actual semantics do you want it to have?

The "pure" attribute means the compiler can assume this function does
not have side effects.  But in general (and in practice in many cases)
there is no way the compiler can actually check that, if that is what
you were asking for.

And any such checking will depend on optimisation level and related
flags, making that a no-go anyway.

Segher