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. 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