linux-kernel - Re: [RFC PATCH] LKMM: Add ctrl_dep() macro for control dependency

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Message-ID: <20210929174146.GF22689@gate.crashing.org>
Date:   Wed, 29 Sep 2021 12:41:46 -0500
From:   Segher Boessenkool <segher@...nel.crashing.org>
To:     Florian Weimer <fweimer@...hat.com>
Cc:     Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        will@...nel.org, paulmck@...nel.org,
        Peter Zijlstra <peterz@...radead.org>,
        linux-kernel@...r.kernel.org,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        stern@...land.harvard.edu, parri.andrea@...il.com,
        boqun.feng@...il.com, npiggin@...il.com, dhowells@...hat.com,
        j.alglave@....ac.uk, luc.maranget@...ia.fr, akiyks@...il.com,
        linux-toolchains@...r.kernel.org, linux-arch@...r.kernel.org
Subject: Re: [RFC PATCH] LKMM: Add ctrl_dep() macro for control dependency

Hi!

On Wed, Sep 29, 2021 at 02:28:37PM +0200, Florian Weimer wrote:
> If you need a specific instruction emitted, you need a compiler
> intrinsic or inline assembly.

Not an intrinsic.  Builtins (like almost all other code) do not say
"generate this particular machine code", they say "generate code that
does <this>".  That is one reason why builtins are more powerful than
inline assembler (another related reason is that they tell the compiler
exactly what behaviour is expected).

> I don't think it's possible to piggy-back this on something else.

Unless we get a description of what this does in term of language
semantics (instead of generated machine code), there is no hope, even.

Segher