Messages in this thread | | | Date | Tue, 24 Mar 2020 18:03:06 +0100 | From | Peter Zijlstra <> | Subject | Re: [RESEND][PATCH v3 14/17] static_call: Add static_cond_call() |
| |
On Tue, Mar 24, 2020 at 09:33:21AM -0700, Linus Torvalds wrote: > On Tue, Mar 24, 2020 at 9:22 AM Andy Lutomirski <luto@amacapital.net> wrote: > > > > I haven’t 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 isn’t 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?
I actually (abuse) do that in the last patch... the reason being that DEFINE_STATIC_COND_CALL() ends up only needing a type expression for the second argument, while DEFINE_STATIC_CALL() needs an actual function.
> 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.
Correct. The only problem is that our text patching machinery can't replace multiple instructions :/
| |