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
>>
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>
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