Re^2: XS, C doubles and -Duselongdouble

I'm trying to figure out what your concern might actually be

The GMP C library can assign longs (signed and unsigned), doubles and strings to the mpz_t (integer) struct. But it has no inbuilt way of assigning long longs and long doubles to an mpz_t.

I've merely been looking at how to deal with some of the gotchas that arise when you interface a -Duse64bitint build of perl (with/without -Duselongdouble) to that GMP library.

Let's say I have a perl built with 64-bit int support (but no long double) and I've done:

use warnings;
use Math::GMPz qw(:mpz);
$num = 144115188075868217;# this is an IV
$obj = Math::GMPz->new($num);
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What should happen there ? Should it croak with the message "Hey ... the GMP library doesn't handle ints bigger than 32-bit!!" ? Should it silently assign the value as a 32-bit int ? I've chosen to have the new() XS function read $num as a string using SvPV_nolen(), and then have the GMP library's mpz_set_str() function assign from that string - and that seems to assign the correct 64-bit value.

Unfortunately, it doesn't appear to be so simple when it comes to NV's and -Duselongdouble builds of perl:

use warnings;
use Math::GMPz qw(:mpz);
$num = 2 ** 57 + 12345;# this is an NV
$obj = Math::GMPz->new($num);
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If, under a long double build of perl, the XS function now reads $num using SvPV_nolen(), and the value gets assigned using mpz_set_str(), then it gets the value wrong. There is no simple way (that I can see) for $obj to be assigned the value represented by $num - using the existing assignment operations provided by the GMP library. Of course, if $num fits into a C double anyway, then there's no problem. So ... I started to think along the lines that, since the GMP library did not support long doubles, it was unreasonable to expect that Math::GMPz should support long doubles (even though the build of perl did support them) - and that a croak/warning should be emitted iff precision was being lost.

But I'm now not so sure about that - your mention of sprintf() made me aware that I can probably make use of the C sprintf() function. That is, the new() XS function assigns the $num arg to a long double, sprintf() converts that long double to a string, and *that* string is then handed over to the GMP library's mpz_set_str() function ... and $obj ends up with the correct value and no loss of precision.

I'll give that a go ... in the meantime, thoughts and comments welcome.

Thanks ysth.

Cheers,
Rob

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Re^3: XS, C doubles and -Duselongdouble by ysth (Canon) on Jun 11, 2007 at 07:40 UTC
It's really difficult to get XS code right that does different things for different types of input; if I were you, I'd document that Math::GMPz->new takes a string of digits, and let the caller worry about anything beyond that. Update: or provide a perl new_from_number that looks something like this: `use Config; sub new_from_number { my $class = shift; my $num = shift; my $fmt = '%.0f'; if (exists($Config{nv_preserves_uv_bits}) && $Config{nv_preserves_uv_bits} < 8 * $Config{uvsize} ) { $fmt = $num < 0 ? '%d' : '%u'; } $class->new(sprintf($fmt, $num)); }` [download] I'm probably forgetting something important there, though. Update 2: yes, I was forgetting to revert to %.0f for numbers outside [IV_MIN, UV_MAX].	[reply] [d/l]

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Re^3: XS, C doubles and -Duselongdouble
by ysth (Canon) on Jun 11, 2007 at 07:40 UTC

Update: or provide a perl new_from_number that looks something like this:

use Config;
sub new_from_number {
   my $class = shift;
   my $num = shift;
   my $fmt = '%.0f';
   if (exists($Config{nv_preserves_uv_bits})
      && $Config{nv_preserves_uv_bits} < 8 * $Config{uvsize}
   ) {
      $fmt = $num < 0 ? '%d' : '%u';
   }
   $class->new(sprintf($fmt, $num));
}
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Update 2: yes, I was forgetting to revert to %.0f for numbers outside [IV_MIN, UV_MAX].

[reply]
[d/l]