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

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Date:   Fri, 4 Jun 2021 12:24:07 -0500
From:   Segher Boessenkool <segher@...nel.crashing.org>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Will Deacon <will@...nel.org>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Alan Stern <stern@...land.harvard.edu>,
        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 Fri, Jun 04, 2021 at 10:10:29AM -0700, Linus Torvalds wrote:
> The compiler *cannot* just say "oh, I'll do that 'volatile asm
> barrier' whether the condition is true or not". That would be a
> fundamental compiler bug.

Yes.

> Of course, we might want to make sure that the compiler doesn't go
> "oh, empty asm, I can ignore it",

It isn't allowed to do that.  GCC has this arguable misfeature where it
doesn't show empty asm in the assembler output, but that has no bearing
on anything but how human-readable the output is.


Segher