mpz_get_d precision

[Roberto Bagnara]

Kevin Ryde wrote:
> Updated words on the get_d functions will be as follows.
> 
> The bits about hardware traps are because I don't want to try to
> guarantee they will or won't occur.  I'm fairly sure in the past some
> cases trapped and some not, so this should be a no-change.
> 
> 
>  -- Function: double mpz_get_d (mpz_t OP)
>      Convert OP to a `double', truncating if necessary (ie. rounding
>      towards zero).
> 
>      If the exponent from the conversion is too big, the result is
>      system dependent.  An infinity is returned where available.  A
>      hardware overflow trap may or may not occur.
> 
>  -- Function: double mpz_get_d_2exp (signed long int *EXP, mpz_t OP)
>      Convert OP to a `double', truncating if necessary (ie. rounding
>      towards zero), and returning the exponent separately.
> 
>      The return value is in the range 0.5<=abs(D)<1 and the exponent is
>      stored to `*EXP'.  D * 2^EXP is the (truncated) OP value.  If OP
>      is zero, the return is 0.0 and 0 is stored to `*EXP'.
> 
>      This is similar to the standard C `frexp' function (*note
>      Normalization Functions: (libc)Normalization Functions.).
> 
> 
>  -- Function: double mpq_get_d (mpq_t OP)
>      Convert OP to a `double', truncating if necessary (ie. rounding
>      towards zero).
> 
>      If the exponent from the conversion is too big or too small to fit
>      a `double' then the result is system dependent.  For too big an
>      infinity is returned when available.  For too small 0.0 is
>      returned.  Hardware overflow, underflow and denorm traps may or
>      may not occur.
> 
> 
>  -- Function: double mpf_get_d (mpf_t OP)
>      Convert OP to a `double', truncating if necessary (ie. rounding
>      towards zero).
> 
>      If the exponent in OP is too big or too small to fit a `double'
>      then the result is system dependent.  For too big an infinity is
>      returned when available.  For too small 0.0 is returned.  Hardware
>      overflow, underflow and denorm traps may or may not occur.
> 
>  -- Function: double mpf_get_d_2exp (signed long int *EXP, mpf_t OP)
>      Convert OP to a `double', truncating if necessary (ie. rounding
>      towards zero), and with an exponent returned separately.
> 
>      The return value is in the range 0.5<=abs(D)<1 and the exponent is
>      stored to `*EXP'.  D * 2^EXP is the (truncated) OP value.  If OP
>      is zero, the return is 0.0 and 0 is stored to `*EXP'.
> 
>      This is similar to the standard C `frexp' function (*note
>      Normalization Functions: (libc)Normalization Functions.).

I see this extra documentation is not present in the GMP 4.1.4 distribution.
Since I am very interested about these guarantees (in particular
for what concerns mpz_get_d and mpq_get_d), I would like to ask:

1) Do these words apply to GMP 4.1.4?
2) (In any case) Will they apply to GMP 4.2?

Many thanks in advance,

      Roberto

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