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

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Date:   Sun, 6 Jun 2021 13:11:53 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Segher Boessenkool <segher@...nel.crashing.org>
Cc:     Alan Stern <stern@...land.harvard.edu>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        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 6, 2021 at 12:56 PM Segher Boessenkool
<segher@...nel.crashing.org> wrote:
>
> Yes, I know.  But it is literally the *only* way to *always* get a
> conditional branch: by writing one.

The thing is, I don't actually believe you.

The barrier() thing can work - all we need to do is to simply make it
impossible for gcc to validly create anything but a conditional
branch.

If either side of the thing have an asm that cannot be combined, gcc
simply doesn't have any choice in the matter. There's no other valid
model than a conditional branch around it (of some sort - doing an
indirect branch that has a data dependency isn't wrong either, it just
wouldn't be something that a sane compiler would generate because it's
obviously much slower and more complicated).

We are very used to just making the compiler generate the code we
need. That is, fundamentally, what any use of inline asm is all about.
We want the compiler to generate all the common cases and all the
regular instructions.

The conditional branch itself - and the instructions leading up to it
- are exactly those "common regular instructions" that we'd want the
compiler to generate. That is in fact more true here than for most
inline asm, exactly because there are so many different possible
combinations of conditional branches (equal, not equal, less than,..)
and so many ways to generate the code that generates the condition.

So we are much better off letting the compiler do all that for us -
it's very much what the compiler is good at.

               Linus