...but without taking a profile, how do you know its *actually* slower? Looking at assembly output can be misleading. On Nov 30, 2015 11:53 PM, "'Davide Libenzi' via Akaros" < [email protected]> wrote:
> 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. > 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.
