Why is _mp_size not of type mp_size_t?

Roberto Bagnara bagnara at cs.unipr.it
Thu Nov 3 12:04:36 CET 2005

Hi there,

we have been bitten by a problem that can be explained as follows:
on machines that are not Crays, `mp_size_t' is of type `long int':

#if defined (_CRAY) && ! defined (_CRAYMPP)
/* plain `int' is much faster (48 bits) */
#define __GMP_MP_SIZE_T_INT     1
typedef int                     mp_size_t;
typedef int                     mp_exp_t;
#define __GMP_MP_SIZE_T_INT     0
typedef long int                mp_size_t;
typedef long int                mp_exp_t;

However, the `_mp_size' field of `_mpz_struct' is of type `int':

typedef struct
   int _mp_alloc;                /* Number of *limbs* allocated and pointed
                                    to by the _mp_d field.  */
   int _mp_size;                 /* abs(_mp_size) is the number of limbs the
                                    last field points to.  If _mp_size is
                                    negative this is a negative number.  */
   mp_limb_t *_mp_d;             /* Pointer to the limbs.  */
} __mpz_struct;
#endif /* __GNU_MP__ */

For some reason (assonance, quite likely), we were assuming that `_mp_size'
was of type `mp_size_t' but this wrong assumption fails whenever `int' and
`long int' are different types (which happens, e.g., on x86_64-unknown-linux-gnu).

Notice that I am not posting this to gmp-bugs because it is not, I believe,
a bug of GMP: we are fiddling with the internal details of GMP and this is
something that goes beyond the API.  Nonetheless, I wonder why many GMP
functions accepts parameters of type `long int' (on non-Crays) when they
can never handle anything that does not fit `int'.
Is this apparent discrepancy intended?  (I know that a size of 2^31-1 fits
all present and future purposes, but this does not explain why mp_size_t
can go beyond that limit.)
All the best,


Prof. Roberto Bagnara
Computer Science Group
Department of Mathematics, University of Parma, Italy
mailto:bagnara at cs.unipr.it

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