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From: Florian Weimer <fweimer@redhat.com>
To: Segher Boessenkool <segher@kernel.crashing.org>
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
	will@kernel.org, paulmck@kernel.org,
	Peter Zijlstra <peterz@infradead.org>,
	linux-kernel@vger.kernel.org,
	Linus Torvalds <torvalds@linux-foundation.org>,
	stern@rowland.harvard.edu, parri.andrea@gmail.com,
	boqun.feng@gmail.com, npiggin@gmail.com, dhowells@redhat.com,
	j.alglave@ucl.ac.uk, luc.maranget@inria.fr, akiyks@gmail.com,
	linux-toolchains@vger.kernel.org, linux-arch@vger.kernel.org
Subject: Re: [RFC PATCH] LKMM: Add ctrl_dep() macro for control dependency
Date: Wed, 29 Sep 2021 21:46:47 +0200	[thread overview]
Message-ID: <877dez5fmg.fsf@oldenburg.str.redhat.com> (raw)
In-Reply-To: <20210929174146.GF22689@gate.crashing.org> (Segher Boessenkool's message of "Wed, 29 Sep 2021 12:41:46 -0500")

* Segher Boessenkool:

> Hi!
>
> On Wed, Sep 29, 2021 at 02:28:37PM +0200, Florian Weimer wrote:
>> If you need a specific instruction emitted, you need a compiler
>> intrinsic or inline assembly.
>
> Not an intrinsic.  Builtins (like almost all other code) do not say
> "generate this particular machine code", they say "generate code that
> does <this>".  That is one reason why builtins are more powerful than
> inline assembler (another related reason is that they tell the compiler
> exactly what behaviour is expected).

I meant that if the object code has to contain a specific instruction
sequence involving a conditional, it needs some form of compiler
support.  Adding some volatile here and some form of a compiler barrier
there is very brittle.

>> I don't think it's possible to piggy-back this on something else.
>
> Unless we get a description of what this does in term of language
> semantics (instead of generated machine code), there is no hope, even.

True.  For example, if the argument contains a sequence point, what does
that even mean?

Thanks,
Florian


  reply	other threads:[~2021-09-29 19:46 UTC|newest]

Thread overview: 35+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-09-28 21:15 [RFC PATCH] LKMM: Add ctrl_dep() macro for control dependency Mathieu Desnoyers
2021-09-29 12:06 ` Marco Elver
2021-10-01 15:45   ` Mathieu Desnoyers
2021-10-01 16:20     ` Linus Torvalds
2021-10-01 17:28       ` Mathieu Desnoyers
2021-10-01 18:18         ` Linus Torvalds
2021-09-29 12:28 ` Florian Weimer
2021-09-29 17:41   ` Segher Boessenkool
2021-09-29 19:46     ` Florian Weimer [this message]
2021-10-01 16:13     ` Mathieu Desnoyers
2021-10-01 16:26       ` Florian Weimer
2021-10-01 16:35         ` Linus Torvalds
2021-10-10 14:02           ` Florian Weimer
2021-10-14  0:01             ` Paul E. McKenney
2021-10-14  2:14               ` Alan Stern
2021-10-14 16:14                 ` Paul E. McKenney
2021-10-14 15:58               ` Florian Weimer
2021-10-14 16:23                 ` Paul E. McKenney
2021-10-14 18:19                   ` Florian Weimer
2021-10-14 21:09                     ` Paul E. McKenney
2021-10-14 22:36                       ` Linus Torvalds
2021-09-30 13:28   ` Mathieu Desnoyers
2021-09-29 14:47 ` Linus Torvalds
2021-09-29 14:54   ` Linus Torvalds
2021-09-29 19:50     ` Mathieu Desnoyers
2021-09-29 20:13       ` Mathieu Desnoyers
2021-09-29 19:27   ` Mathieu Desnoyers
2021-09-29 22:14     ` Linus Torvalds
2021-09-29 21:47 ` Segher Boessenkool
2021-09-29 23:57   ` Paul E. McKenney
2021-10-01 15:28     ` Mathieu Desnoyers
2021-10-01 22:53       ` Paul E. McKenney
2021-10-01 19:10     ` Segher Boessenkool
2021-10-01 22:50       ` Paul E. McKenney
2021-10-02 14:29       ` Alan Stern

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