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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