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

[Florian Weimer]

* 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