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

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