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

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Date:   Tue, 8 Jun 2021 12:48:40 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Peter Zijlstra' <peterz@...radead.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        "will@...nel.org" <will@...nel.org>,
        "paulmck@...nel.org" <paulmck@...nel.org>,
        "stern@...land.harvard.edu" <stern@...land.harvard.edu>,
        "parri.andrea@...il.com" <parri.andrea@...il.com>,
        "boqun.feng@...il.com" <boqun.feng@...il.com>,
        "npiggin@...il.com" <npiggin@...il.com>,
        "dhowells@...hat.com" <dhowells@...hat.com>,
        "j.alglave@....ac.uk" <j.alglave@....ac.uk>,
        "luc.maranget@...ia.fr" <luc.maranget@...ia.fr>,
        "akiyks@...il.com" <akiyks@...il.com>
CC:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-toolchains@...r.kernel.org" <linux-toolchains@...r.kernel.org>,
        "linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>
Subject: RE: [RFC] LKMM: Add volatile_if()

From: Peter Zijlstra
> Sent: 04 June 2021 11:12
> 
> Hi!
> 
> With optimizing compilers becoming more and more agressive and C so far
> refusing to acknowledge the concept of control-dependencies even while
> we keep growing the amount of reliance on them, things will eventually
> come apart.
> 
> There have been talks with toolchain people on how to resolve this; one
> suggestion was allowing the volatile qualifier on branch statements like
> 'if', but so far no actual compiler has made any progress on this.
> 
> Rather than waiting any longer, provide our own construct based on that
> suggestion. The idea is by Alan Stern and refined by Paul and myself.
> 
> Code generation is sub-optimal (for the weak architectures) since we're
> forced to convert the condition into another and use a fixed conditional
> branch instruction, but shouldn't be too bad.

What happens on mips-like architectures (I think includes riscv)
that have 'compare two registers and branch' instructions rather
than a more traditional 'flags register'?

The generated code it likely to be somewhat different.

	David

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