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

On 2014-01-13 at 21:57:03 +0100, Simon Marlow wrote:

On 13/01/14 13:25, git@git.haskell.org wrote:

...

Repository : ssh://git@git.haskell.org/integer-gmp

On branch : master Link : http://ghc.haskell.org/trac/ghc/changeset/7bdcadda7e884edffb1427f0685493f3a2...

...

---------------------------------------------------------------

commit 7bdcadda7e884edffb1427f0685493f3a2e5c5fa Author: Herbert Valerio Riedel 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