Ah; yeah, perhaps. But bear in mind that this code is meant to be portable. 128-bit multiplication may be quite slow on RISC-V, for instance (though I don't know).
The OpenBSD people know what they're doing when it comes to this kind of thing.... On Mon, Nov 30, 2015 at 10:48 PM, 'Davide Libenzi' via Akaros < [email protected]> wrote: > No, the MUL checks are in place explicitly to try to avoid every time into > the DIV. > I feel my hair growing back, thinking at the times that code made sense ☺ > > > On Mon, Nov 30, 2015 at 7:46 PM, Dan Cross <[email protected]> wrote: > >> I don't think that's an optimization; it's an attempt to avoid UB. In >> that sense, it's explicitly an anti-optimization.... >> >> On Mon, Nov 30, 2015 at 10:43 PM, 'Davide Libenzi' via Akaros < >> [email protected]> wrote: >> >>> Oh, I would not have made any comment, had I not seen the silly >>> optimization in place. Likely 1993 era 😀 >>> No optimization in place, meant "we do not care at this point", which I >>> agree in this case. >>> But the code was suggesting "we do care", hence a much better way to do >>> it. >>> >>> >>> On Mon, Nov 30, 2015 at 7:41 PM, Dan Cross <[email protected]> wrote: >>> >>>> Also, bear in mind that this is right before we run the memory >>>> allocator. Shaving a few cycles off here is kind of like chipping a pebble >>>> off of an asteroid. >>>> >>>> On Mon, Nov 30, 2015 at 10:37 PM, Dan Cross <[email protected]> wrote: >>>> >>>>> Numbers of it didn't happen. >>>>> >>>>> On Mon, Nov 30, 2015 at 9:49 PM, 'Davide Libenzi' via Akaros < >>>>> [email protected]> wrote: >>>>> >>>>>> The MUL_NO_OVERFLOW thing makes little sense in the CPUs we are >>>>>> targeting. >>>>>> A couple of mis-predicted branches are more expensive than a DIV (~20 >>>>>> cycles in today Intel). >>>>>> Much faster: >>>>>> >>>>>> typedef unsigned __int128 uint128_t; >>>>>> >>>>>> { >>>>>> uint128_t n = (uint128_t) nmemb * (uint128_t) size; >>>>>> >>>>>> if (n > SIZE_MAX) foo(); >>>>>> >>>>>> } >>>>>> >>>>>> >>>>>> -------------------- >>>>>> typedef unsigned __int128 u128; >>>>>> >>>>>> static const unsigned long ulmax = ~0UL; >>>>>> >>>>>> unsigned long oc(unsigned long a, unsigned long b) >>>>>> { >>>>>> u128 m = (u128) a * (u128) b; >>>>>> >>>>>> if (m > ulmax) >>>>>> m = 0; >>>>>> >>>>>> return (unsigned long) m; >>>>>> } >>>>>> >>>>>> ---- >>>>>> oc: >>>>>> movq %rdi, %rax >>>>>> mulq %rsi >>>>>> movq %rax, %r9 >>>>>> xorl %eax, %eax >>>>>> testq %rdx, %rdx >>>>>> cmove %r9, %rax >>>>>> ret >>>>>> >>>>>> >>>>>> >>>>>> >>>>>> >>>>>> >>>>>> >>>>>> On Mon, Nov 30, 2015 at 1:18 PM, Barret Rhoden <[email protected]> >>>>>> wrote: >>>>>> >>>>>>> One thing to be careful of is that these functions can fail and >>>>>>> return 0, >>>>>>> regardless of your kmalloc flags. >>>>>>> >>>>>>> Signed-off-by: Barret Rhoden <[email protected]> >>>>>>> --- >>>>>>> kern/include/kmalloc.h | 5 +++-- >>>>>>> kern/src/kreallocarray.c | 9 +++++++++ >>>>>>> 2 files changed, 12 insertions(+), 2 deletions(-) >>>>>>> >>>>>>> diff --git a/kern/include/kmalloc.h b/kern/include/kmalloc.h >>>>>>> index 268cf2985ddb..26f23c1754c0 100644 >>>>>>> --- a/kern/include/kmalloc.h >>>>>>> +++ b/kern/include/kmalloc.h >>>>>>> @@ -16,8 +16,9 @@ >>>>>>> #define KMALLOC_LARGEST KMALLOC_SMALLEST << NUM_KMALLOC_CACHES >>>>>>> >>>>>>> void kmalloc_init(void); >>>>>>> -void* kmalloc(size_t size, int flags); >>>>>>> -void* kzmalloc(size_t size, int flags); >>>>>>> +void *kmalloc(size_t size, int flags); >>>>>>> +void *kmalloc_array(size_t nmemb, size_t size, int flags); >>>>>>> +void *kzmalloc(size_t size, int flags); >>>>>>> void *kmalloc_align(size_t size, int flags, size_t align); >>>>>>> void *kzmalloc_align(size_t size, int flags, size_t align); >>>>>>> void *krealloc(void *buf, size_t size, int flags); >>>>>>> diff --git a/kern/src/kreallocarray.c b/kern/src/kreallocarray.c >>>>>>> index d099721e0406..505159917317 100644 >>>>>>> --- a/kern/src/kreallocarray.c >>>>>>> +++ b/kern/src/kreallocarray.c >>>>>>> @@ -34,3 +34,12 @@ void *kreallocarray(void *optr, size_t nmemb, >>>>>>> size_t size, int flags) >>>>>>> } >>>>>>> return krealloc(optr, size * nmemb, flags); >>>>>>> } >>>>>>> + >>>>>>> +void *kmalloc_array(size_t nmemb, size_t size, int flags) >>>>>>> +{ >>>>>>> + if (((nmemb >= MUL_NO_OVERFLOW) || (size >= >>>>>>> MUL_NO_OVERFLOW)) && >>>>>>> + (nmemb > 0) && ((SIZE_MAX / nmemb) < size)) { >>>>>>> + return NULL; >>>>>>> + } >>>>>>> + return kmalloc(size * nmemb, flags); >>>>>>> +} >>>>>>> -- >>>>>>> 2.6.0.rc2.230.g3dd15c0 >>>>>>> >>>>>>> -- >>>>>>> You received this message because you are subscribed to the Google >>>>>>> Groups "Akaros" group. >>>>>>> To unsubscribe from this group and stop receiving emails from it, >>>>>>> send an email to [email protected]. >>>>>>> To post to this group, send email to [email protected]. >>>>>>> For more options, visit https://groups.google.com/d/optout. >>>>>>> >>>>>> >>>>>> -- >>>>>> You received this message because you are subscribed to the Google >>>>>> Groups "Akaros" group. >>>>>> To unsubscribe from this group and stop receiving emails from it, >>>>>> send an email to [email protected]. >>>>>> To post to this group, send email to [email protected]. >>>>>> For more options, visit https://groups.google.com/d/optout. >>>>>> >>>>> >>>>> >>>> -- >>>> You received this message because you are subscribed to the Google >>>> Groups "Akaros" group. >>>> To unsubscribe from this group and stop receiving emails from it, send >>>> an email to [email protected]. >>>> To post to this group, send email to [email protected]. >>>> For more options, visit https://groups.google.com/d/optout. >>>> >>> >>> -- >>> You received this message because you are subscribed to the Google >>> Groups "Akaros" group. >>> To unsubscribe from this group and stop receiving emails from it, send >>> an email to [email protected]. >>> To post to this group, send email to [email protected]. >>> For more options, visit https://groups.google.com/d/optout. >>> >> >> -- >> You received this message because you are subscribed to the Google Groups >> "Akaros" group. >> To unsubscribe from this group and stop receiving emails from it, send an >> email to [email protected]. >> To post to this group, send email to [email protected]. >> For more options, visit https://groups.google.com/d/optout. >> > > -- > You received this message because you are subscribed to the Google Groups > "Akaros" group. > To unsubscribe from this group and stop receiving emails from it, send an > email to [email protected]. > To post to this group, send email to [email protected]. > For more options, visit https://groups.google.com/d/optout. > -- You received this message because you are subscribed to the Google Groups "Akaros" group. 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