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.
