On Wed, Jun 6, 2018 at 12:52 AM, Gleb Smirnoff <gleb...@freebsd.org> wrote:
> On Tue, Jun 05, 2018 at 06:35:37PM +0200, Mateusz Guzik wrote:
> M> > On 06/02/2018 17:20, Mateusz Guzik wrote:
> M> > > +#ifdef _KERNEL
> M> > > +#define malloc(size, type, flags) ({
> \
> M> > > + void *_malloc_item;
> \
> M> > > + size_t _size = (size);
> \
> M> > > + if (__builtin_constant_p(size) && __builtin_constant_p(flags)
> &&\
> M> > > + ((flags) & M_ZERO)) {
> \
> M> > > + _malloc_item = malloc(_size, type, (flags) &~
> M_ZERO); \
> M> > > + if (((flags) & M_WAITOK) || _malloc_item != NULL)
> \
> M> > > + bzero(_malloc_item, _size);
> \
> M> > > + } else {
> \
> M> > > + _malloc_item = malloc(_size, type, flags);
> \
> M> > > + }
> \
> M> > > + _malloc_item;
> \
> M> > > +})
> M> > > +#endif
> M> >
> M> > Mateusz,
> M> >
> M> > Thank you for this and for all of your performance work. It is all
> very
> M> > interesting stuff.
> M> >
> M> >
> M> Thank you for the kind words. It is positive feedback like this which
> M> keeps me going!
>
> Btw, what was the point of checking flags || result? Most places in kernel
> ignore flags and just test result regerdless of M_WAITOK/M_NOWAIT.
>
> The result is already in a register, why do you think checking for absense
> of M_WAITOK is faster that checking for !NULL _malloc_item?
>
This part is only reachable if flags are known at compilation time. If they
contain
M_WAITOK, the flag check will get elided along (we know for a fact it
passes)
and subsequently the NULL check will be short circuited, iow for known
M_WAITOK|M_ZERO flags this is:
_malloc_item = malloc(_size, type, flags & ~ M_ZERO);
bzero(_malloc_item, _size);
--
Mateusz Guzik <mjguzik gmail.com>
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