Re: [PATCH RFC 0/3] Static calls

[Rasmus Villemoes]

On 09/11/2018 16.16, Andy Lutomirski wrote:
> On Thu, Nov 8, 2018 at 11:28 PM Ingo Molnar <mingo@xxxxxxxxxx> wrote:
>>
>>
>> All other usecases are bonus, but it would certainly be interesting to
>> investigate the impact of using these APIs for tracing: that too is a
>> feature enabled everywhere but utilized only by a small fraction of Linux
>> users - so literally every single cycle or instruction saved or hot-path
>> shortened is a major win.
> 
> For tracing, we'd want static_call_set_to_nop() or something like that, right?
> 

Hm. IIUC, when gcc sees static_call(key)(...), it has to generate code
to put the right values in %rdi, %rsi etc.. Even if the function is void
(*)(void), gcc would still need to shuffle things around (either spill
and reload, or move %rdi to some callee saved register). So if the
static_call is noop'ed out most of the time, that seems like a net loss?
With an unlikely static_key, gcc can do all the parameter setup and
reloading in an out-of-line chunk of code.

static calls seems like a quite useful concept, but only/mostly if
_some_ function needs to be called at that spot.

Aside: there should be some compile-time check that
static_call_set_to_nop can only be used if the return type is void.

Rasmus