From: Segher Boessenkool <segher@kernel.crashing.org>
To: Peter Zijlstra <peterz@infradead.org>
Cc: David Malcolm <dmalcolm@redhat.com>,
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>,
yuanzhaoxiong@baidu.com
Subject: Re: static_branch/jump_label vs branch merging
Date: Thu, 22 Apr 2021 13:31:51 -0500 [thread overview]
Message-ID: <20210422183151.GD27473@gate.crashing.org> (raw)
In-Reply-To: <YIG3I+ojW+bc+o4l@hirez.programming.kicks-ass.net>
On Thu, Apr 22, 2021 at 07:49:23PM +0200, Peter Zijlstra wrote:
> On Thu, Apr 22, 2021 at 12:08:20PM -0500, Segher Boessenkool wrote:
> > 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?
>
> I want a function marked as pure to be treated as such, unconditionally.
>
> > 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.
>
> Right, so currently the pure attribute gets ignored by the compiler
> because of various reasons, one of them being an asm volatile ("") being
> present somewhere inside it (AFAIU).
In general, the compiler only sees the *declaration* of the function, so
it cannot do such a thing.
> Does this mean we can have invalid code generation when we faultily
> mark things pure? Yes, but then it's our own damn fault for sticking on
> pure in the first place.
Nope, you have undefined behaviour in that case, and you get to keep
all N pieces, the compiler cannot do anything wrong in such a case :-)
> In short; I want pure (or really_pure if you want a second attribute) to
You cannot make the meaning of "pure" different from what it has been
historically, because existing programs will no longer build (or worse,
start behaving differently).
> be a do-what-I-tell-you-already and not a
> only-if-you-think-you-can-prove-I-didn't-make-a-mistake kinda knob. A
> little bit like inline vs always_inline.
It sounds like you want it to behave like attribute((pure)) already is
documented as doing. Please open a PR? https://gcc.gnu.org/bugs.html
(We need buildable stand-alone example code, with what flags to use, and
something like what should happen and what did happen).
> > And any such checking will depend on optimisation level and related
> > flags, making that a no-go anyway.
>
> Realistically I'm only bothered about -O2 and up since that's what we
> build the kernel with. Obviously one doesn't care about optimizations
> being lost when build with -O0.
GCC is used for other things as well, not just for building Linux ;-)
Segher
next prev parent reply other threads:[~2021-04-22 18:34 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
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 [this message]
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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