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

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Message-ID: <877dez5fmg.fsf@oldenburg.str.redhat.com>
Date:   Wed, 29 Sep 2021 21:46:47 +0200
From:   Florian Weimer <fweimer@...hat.com>
To:     Segher Boessenkool <segher@...nel.crashing.org>
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

* Segher Boessenkool:

> 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 meant that if the object code has to contain a specific instruction
sequence involving a conditional, it needs some form of compiler
support.  Adding some volatile here and some form of a compiler barrier
there is very brittle.

>> 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.

True.  For example, if the argument contains a sequence point, what does
that even mean?

Thanks,
Florian