linux-kernel - Re: [RFC] LKMM: Add volatile

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Date:   Sun, 6 Jun 2021 12:07:02 -0700
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Segher Boessenkool <segher@...nel.crashing.org>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Alan Stern <stern@...land.harvard.edu>,
        Peter Zijlstra <peterz@...radead.org>,
        Will Deacon <will@...nel.org>,
        Andrea Parri <parri.andrea@...il.com>,
        Boqun Feng <boqun.feng@...il.com>,
        Nick Piggin <npiggin@...il.com>,
        David Howells <dhowells@...hat.com>,
        Jade Alglave <j.alglave@....ac.uk>,
        Luc Maranget <luc.maranget@...ia.fr>,
        Akira Yokosawa <akiyks@...il.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-toolchains@...r.kernel.org,
        linux-arch <linux-arch@...r.kernel.org>
Subject: Re: [RFC] LKMM: Add volatile_if()

On Sun, Jun 06, 2021 at 08:17:40AM -0500, Segher Boessenkool wrote:
> On Sat, Jun 05, 2021 at 09:43:33PM -0700, Paul E. McKenney wrote:
> > So gcc might some day note a do-nothing asm and duplicate it for
> > the sole purpose of collapsing the "then" and "else" clauses.  I
> > guess I need to keep my paranoia for the time being, then.  :-/
> 
> Or a "do-something" asm, even.  What it does is make sure it is executed
> on the real machine exactly like on the abstract machine.  That is how C
> is defined, what a compiler *does*.
> 
> The programmer does not have any direct control over the generated code.

I am not looking for direct control, simply sufficient influence.  ;-)

> > Of course, there is no guarantee that gcc won't learn about
> > assembler constants.  :-/
> 
> I am not sure what you call an "assembler constant" here.  But you can
> be sure that GCC will not start doing anything here.  GCC does not try
> to understand what you wrote in an inline asm, it just fills in the
> operands and that is all.  It can do all the same things to it that it
> can do to any other code of course: duplicate it, deduplicate it,
> frobnicate it, etc.

Apologies, that "assembler constants" should have been "assembler
comments".

							Thanx, Paul