How large are arbitrarily large integers? (Was Re: Bitfield subtlety)

[Marc Glisse]

On Thu, 29 Mar 2018, Richard Biener wrote:

> On Thu, 29 Mar 2018, Torbjörn Granlund wrote:
>
>> Richard Biener <rguenther at suse.de> writes:
>>
>>   You can also steal low-order bits from the pointer depending on
>>   alignment...  and depending on the host also some high-order bits.
>> 
>> I will resist those temptations.  :-)
>> 
>> (I have contemplates using the 2-3 low point bits for some other
>> purposes, most likely to track used allocators.  But I don't think that
>> will happen.)
>>
>>   You can also simply off-load the allocation size to the allocated
>>   area (like to *(size_t)(_mp_d - 8)).  In fact you are already breaking
>>   the ABI in some sort by re-purposing fields given that the internals
>>   of __mpz_struct is exposed.
>> 
>> Unfortunately, while that would be desirable, I have the feeling that it
>> is not 100% portable.
>
> I would be surprised if that wouldn't work everywhere - any reason
> you have doubts?  To clarify, allocation would work like
>
> void *ptr = malloc (... + sizeof (size_t));
> ->_mp_d = (char *)ptr + sizeof (size_t);
> size_t *size = ptr;
>
> if you think the pointer arithmetic on _mp_d is problematic you can
> always go through uintptr_t ...

IIRC that's roughly what MPFR does.

-- 
Marc Glisse