Re: [RESEND][PATCH v3 14/17] static_call: Add static_cond_call()

[Linus Torvalds]

On Tue, Mar 24, 2020 at 9:22 AM Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote:
>
> I havenât checked if static calls currently support return values, but
> the conditional case only makes sense for functions that return void.
>
> Aside from that, it might be nice for passing NULL in to warn or bug
> when the NULL pointer is stored instead of silently NOPping out the
> call in cases where having a real implementation isnât optional.

Both good points. I take back my question.

And it aside from warning about passing in NULL then it doesn't work,
I wonder if we could warn - at build time - when then using the COND
version with a function that doesn't return void?

Of course, one alternative is to just say "instead of using NOP, use
'xorl %eax,%eax'", and then we'd have the rule that a NULL conditional
function returns zero (or NULL).

I _think_ a "xorl %eax,%eax ; retq" is just three bytes and would fit
in the tailcall slot too.

                Linus