Re: Deduplicate io_*_prep calls?

[Andres Freund <andres@anarazel.de>]

Hi,

On 2020-02-24 19:18:26 +0300, Pavel Begunkov wrote:
> On 24/02/2020 19:02, Jens Axboe wrote:
> >> Usually doesn't work because of such possible "hackier assignments".
> >> Ok, I have to go and experiment a bit. Anyway, it probably generates a lot of
> >> useless stuff, e.g. for req->ctx
> > 
> > Tried this, and it generates the same code...
> 
> Maybe it wasn't able to optimise in the first place
> 
> E.g. for the following code any compiler generates 2 reads (thanks godbolt).
> 
> extern void foo(int);
> int bar(const int *v)
> {
>     foo(*v);
>     return *v;
> }

Yea, the compiler really can't assume anything for this kind of
thing.
a) It's valid C to cast away the const here, as long as it's guaranteed
   that v isn't pointing to to actually const memory.
b) foo() could actually have access to *v without the argument,
   e.g. through a global.
and even in the case of a const member of a struct, as far as I know
it's legal to change the values, as long as the allocation isn't const.

Greetings,

Andres Freund