[commit: packages/integer-gmp] master: Allocate initial 1-limb mpz_t on the Stack and introduce MPZ# type (7bdcadd)

[Herbert Valerio Riedel]

On 2014-01-13 at 21:57:03 +0100, Simon Marlow wrote:
> On 13/01/14 13:25, git at git.haskell.org wrote:
>> Repository : ssh://git@git.haskell.org/integer-gmp
>>
>> On branch  : master
>> Link       : http://ghc.haskell.org/trac/ghc/changeset/7bdcadda7e884edffb1427f0685493f3a2e5c5fa/integer-gmp
>>
>>> ---------------------------------------------------------------
>>
>> commit 7bdcadda7e884edffb1427f0685493f3a2e5c5fa
>> Author: Herbert Valerio Riedel <hvr at gnu.org>
>> Date:   Thu Jan 9 00:19:31 2014 +0100
>>
>>      Allocate initial 1-limb mpz_t on the Stack and introduce MPZ# type
>>
>>      We now allocate a 1-limb mpz_t on the stack instead of doing a more
>>      expensive heap-allocation (especially if the heap-allocated copy becomes
>>      garbage right away); this addresses #8647.
>
> While this is quite cool (turning some J# back into S#), I don't
> understand why you've done it this way.  Couldn't it be done in the
> Haskell layer rather than modifying the primops?  The ByteArray# has
> already been allocated by GMP, so you don't lose anything by returning
> it to Haskell and checking the size there.  Then all the
> DUMMY_BYTEARRAY stuff would go away.

Actually there isn't always a ByteArray# allocated; the patch got rid of
all mpz_init() calls for the result-mpz_t (which would have allocated
1-limb ByteArray#s);

Now instead, the single word-sized limb that would have been
heap-allocated via mpz_init() before calling the actual GMP operation,
is allocated on the stack instead, and only if the GMP routines need to
grow the passed in mpz_t's an actual ByteArray# is allocated.

That's why I needed a way to return either a single stack-allocated limb
(hence the word#), *or* an heap-allocated 'ByteArray#', which lead to
the MPZ# 3-tuple.

Greetings,
  hvr