Additional memory handler features.

[Victor Shoup]

Thanks.  This would more-or-less solve all my problems,
if and when it all gets implemented.  I don't find such functions
right now in the GMP manual.

On Jan 4, 2015, at 6:08 PM, Niels Möller wrote:

> Victor Shoup <shoup at cs.nyu.edu> writes:
> 
>> But I see mention of itching and scratching: could somebody
>> describe what that is or provide a link? Sorry for my ignorance.
>> ....and sorry for the length of this post....
> 
> The idea is that instead of having gmp allocate temporary storage, it
> should have a function to tell how much temporary storage is needed for
> each operation, and then let the application allocate that anyway it
> please, and pass it as an additional argument. So instead of
> 
>  mpn_mul (rp, ap, an, bp, bn);
> 
> one would do something like
> 
>  mp_limb_t *scratch = xalloc (sizeof(mp_limb_t) * mpn_mul_itch (an, bn));
>  mpn_mul (rp, ap, bn, bp, bn, scratch);
>  free (scratch);
> 
> For convenience, higher level functions could allow a NULL scratch
> argument, and allocate temporary storage using registered gmp allocation
> functions.
> 
> All this applies to mpn functions only.
> 
> There are several motivations for this type of interface:
> 
> 1. For low-level mpn loops, this helps eliminate the frame pointer,
>   making one more register available. (Functions using alloca need a
>   frame pointer).
> 
> 2. For higher-level functions, it may help reuse temporary storage,
>   reducing the total storage need of GMP.
> 
> 3. It should make it possible for applications to allocate temporary
>   storage up front. E.g., when a cryptographic application initializes
>   a key, it may allocate up front all temporary storage needed for
>   operations using that key, so that later operations can never fail
>   for memory allocation reasons. Even static allocation may be possible
>   in some cases.
> 
> Of course there are also some drawbacks. It makes life more complicated
> for applications, and the implementation of functions like mpn_mul_itch,
> which interact with pretty complex algorithm choice machinery, is going
> to be a bit complex too.
> 
> Regards,
> /Niels
> 
> -- 
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