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

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Date:   Sun, 6 Jun 2021 06:53:36 -0500
From:   Segher Boessenkool <segher@...nel.crashing.org>
To:     Alan Stern <stern@...land.harvard.edu>
Cc:     "Paul E. McKenney" <paulmck@...nel.org>,
        Linus Torvalds <torvalds@...ux-foundation.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 Sat, Jun 05, 2021 at 09:29:03PM -0400, Alan Stern wrote:
> Interesting.  And changing one of the branches from barrier() to __asm__ 
> __volatile__("nop": : :"memory") also causes a branch to be emitted.  So 
> even though the compiler doesn't "look inside" assembly code, it does 
> compare two pieces at least textually and apparently assumes if they are 
> identical then they do the same thing.

And that is a simple fact, since the same assembler code (at the same
spot in the program) will do the same thing no matter how that ended up
there.

And the compiler always is allowed to duplicate, join, delete, you name
it, inline assembler code.  The only thing that it cares about is
semantics of the code, just like for any other code.


Segher