From: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
To: Segher Boessenkool <segher@kernel.crashing.org>
Cc: Florian Weimer <fweimer@redhat.com>,
Will Deacon <will@kernel.org>, paulmck <paulmck@kernel.org>,
Peter Zijlstra <peterz@infradead.org>,
linux-kernel <linux-kernel@vger.kernel.org>,
Linus Torvalds <torvalds@linux-foundation.org>,
Alan Stern <stern@rowland.harvard.edu>,
Andrea Parri <parri.andrea@gmail.com>,
Boqun Feng <boqun.feng@gmail.com>,
Nicholas Piggin <npiggin@gmail.com>,
David Howells <dhowells@redhat.com>,
j alglave <j.alglave@ucl.ac.uk>,
luc maranget <luc.maranget@inria.fr>, akiyks <akiyks@gmail.com>,
linux-toolchains <linux-toolchains@vger.kernel.org>,
linux-arch <linux-arch@vger.kernel.org>
Subject: Re: [RFC PATCH] LKMM: Add ctrl_dep() macro for control dependency
Date: Fri, 1 Oct 2021 12:13:28 -0400 (EDT) [thread overview]
Message-ID: <2088260319.47978.1633104808220.JavaMail.zimbra@efficios.com> (raw)
In-Reply-To: <20210929174146.GF22689@gate.crashing.org>
----- On Sep 29, 2021, at 1:41 PM, Segher Boessenkool segher@kernel.crashing.org wrote:
> 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 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.
Hi Segher,
Let me try a slightly improved attempt at describing what I am looking
for in terms of language semantics.
First, let's suppose we define two new compiler builtins, e.g.
__sync_ctrl_dep_rw() and __sync_ctrl_dep_acquire().
Their task would be to ensure that a R->W or R->RW (acquire) dependency between the
volatile loads used as input of the evaluated expression and following volatile
stores, volatile loads for R->RW, volatile asm, memory clobbers, is present in the
following situations:
When the builtin is used around evaluation of the left operand of the && (logical
AND) and || (logical OR) expression, the R->W or R->RW dependency should be
present before evaluating the right operand.
When the builtin is used around evaluation of the first operand of the ternary
"question-mark" operator, the R->W or R->RW dependency should be present before
evaluating the second or third operands.
When the builtin is used around evaluation of the controlling expressions of
if, switch, while, and do-while statements, as well as of the second operand of
the for statement, the R->W or R->RW dependency should be present before the
next sequence point is evaluated.
One cheap way to achieve said R->W dependency (as well as R->RW on architectures which
to not reorder R->R) is to ensure that the generated assembly contains a conditional
branch. Other ways to ensure this include more heavy-weight approaches such as explicit
barriers.
Hopefully my description above is slightly closer to the expected language
semantics.
Thanks,
Mathieu
--
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com
next prev parent reply other threads:[~2021-10-01 16:13 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
2021-10-01 16:13 ` Mathieu Desnoyers [this message]
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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