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:

quoted

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.

quoted

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