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Subject: Re: [RFC] LKMM: Add volatile_if()
To:     Linus Torvalds <torvalds@linux-foundation.org>,
        Alexander Monakov <amonakov@ispras.ru>
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        Boqun Feng <boqun.feng@gmail.com>,
        Nick Piggin <npiggin@gmail.com>,
        David Howells <dhowells@redhat.com>,
        Jade Alglave <j.alglave@ucl.ac.uk>,
        Luc Maranget <luc.maranget@inria.fr>,
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From:   Rasmus Villemoes <linux@rasmusvillemoes.dk>
Message-ID: <e11bc0fd-f231-d015-f8b1-6de71fa403f6@rasmusvillemoes.dk>
Date:   Mon, 7 Jun 2021 01:39:02 +0200
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On 07/06/2021 00.38, Linus Torvalds wrote:

> Example: variable_test_bit(), which generates a "bt" instruction, does
> 
>                      : "m" (*(unsigned long *)addr), "Ir" (nr) : "memory");
> 
> and the memory clobber is obviously wrong: 'bt' only *reads* memory,
> but since the whole reason we use it is that it's not just that word
> at address 'addr', in order to make sure that any previous writes are
> actually stable in memory, we use that "memory" clobber.
> 
> It would be much nicer to have a "memory read" marker instead, to let
> the compiler know "I need to have done all pending writes to memory,
> but I can still cache read values over this op because it doesn't
> _change_ memory".
> 
> Anybody have ideas or suggestions for something like that?

The obvious thing is to try and mark the function as pure. But when
applied to a static inline, gcc seems to read the contents and say "nah,
you have something here that declares itself to possibly write to
memory". Replacing with a call to an extern function marked pure does
indeed cause gcc to cache the value of y*z, so in theory this should be
possible, if one could convince gcc to "trust me, this really is a pure
function".

https://godbolt.org/z/s4546K6Pj

Rasmus