Re: [PATCH] Re: Bignum performance

[Po Lu]

Emanuel Berg <incal@dataswamp.org> writes:

> Ihor Radchenko wrote:
>
>>>> In practice, as more generic benchmarks demonstrated, we
>>>> only had 10% performance hit. Not something to claim that
>>>> Elisp is much slower compared to CL.
>>>
>>> What do you mean, generic +10% is a huge improvement.
>>
>> It is, but it is also tangent to comparison between Elisp
>> and CL. The main (AFAIU) difference between Elisp and CL is
>> in how the bignums are stored. Elisp uses its own internal
>> object type while CL uses GMP's native format.
>
> GMP = GNU Multiple Precision Arithmetic Library.
>
> https://en.wikipedia.org/wiki/GNU_Multiple_Precision_Arithmetic_Library
>
>> And we have huge overheads converting things back-and-forth
>> between GMP and Elisp formats. It is by choice. And my patch
>> did not do anything about this difference.

AFAIU, no conversion takes place between ``Elisp formats'' and GMP
formats.  Our bignums rely on GMP for all data storage and memory
allocation.

struct Lisp_Bignum
{
  union vectorlike_header header;
  mpz_t value; <-------------------- GMP type
} GCALIGNED_STRUCT;

and finally:

INLINE mpz_t const *
bignum_val (struct Lisp_Bignum const *i)
{
  return &i->value;
}
INLINE mpz_t const *
xbignum_val (Lisp_Object i)
{
  return bignum_val (XBIGNUM (i));
}