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Date: Mon, 7 Jun 2021 01:39:02 +0200
From: Rasmus Villemoes <linux@...musvillemoes.dk>
To: Linus Torvalds <torvalds@...ux-foundation.org>,
Alexander Monakov <amonakov@...ras.ru>
Cc: Jakub Jelinek <jakub@...hat.com>,
Alan Stern <stern@...land.harvard.edu>,
Segher Boessenkool <segher@...nel.crashing.org>,
"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 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
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