For Open/NetBSD, it might make sense, as they run even is 13bit, nobody-heard-of, CPUs. We don't run on anything which is not GCC, *and* 64bit, so caring about Arduino being possibly slower on that code does not make much sense.
On Mon, Nov 30, 2015 at 7:50 PM, Dan Cross <[email protected]> wrote: > 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. > 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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