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From:   Florian Weimer <fweimer@redhat.com>
To:     Segher Boessenkool <segher@kernel.crashing.org>
Cc:     Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
        will@kernel.org, paulmck@kernel.org,
        Peter Zijlstra <peterz@infradead.org>,
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        j.alglave@ucl.ac.uk, luc.maranget@inria.fr, akiyks@gmail.com,
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Subject: Re: [RFC PATCH] LKMM: Add ctrl_dep() macro for control dependency
References: <20210928211507.20335-1-mathieu.desnoyers@efficios.com>
        <87lf3f7eh6.fsf@oldenburg.str.redhat.com>
        <20210929174146.GF22689@gate.crashing.org>
Date:   Wed, 29 Sep 2021 21:46:47 +0200
In-Reply-To: <20210929174146.GF22689@gate.crashing.org> (Segher Boessenkool's
        message of "Wed, 29 Sep 2021 12:41:46 -0500")
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* 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