Does it *have* to be Math::BigInt::GMP ?
It's very simple with Math::GMP or Math::GMPz objects:

use Math::GMP;
use Math::GMPz;

use Inline C => Config =>
    LIBS => '-lgmp',
    BUILD_NOISY => 1,
    USING => ParseRecDescent;

use Inline C => <<'EOC';

#include <gmp.h>

void my_mul_si(SV * rop, SV * op, SV * si) {
     mpz_mul_si
     (*(INT2PTR(mpz_t *, SvIV(SvRV(rop)))),
      *(INT2PTR(mpz_t *, SvIV(SvRV(op)))),
      SvIV(si));
}

EOC

my $ x = -10;

my $g_rop = Math::GMP->new();
my $g_op  = Math::GMP->new(1234567);

my $z_rop = Math::GMPz->new();
my $z_op  = Math::GMPz->new(1234567);

my_mul_si($g_rop, $g_op, $x);
my_mul_si($z_rop, $z_op, $x);

my_mul_si($g_op, $g_op, $x);
my_mul_si($z_op, $z_op, $x);

print $g_rop, "\n", $z_rop, "\n",
      $g_op,  "\n", $z_op, "\n";

__END__
Outputs:
-12345670
-12345670
-12345670
-12345670
[download]

I feel that it should also be do-able with Math::BigInt::GMP, too - but I'm currently stuck on trying to work out how that module's mpz_from_sv_nofail() passes its own sv_derived_from() check.
Every object I pass to it in my own (speculative) code turns out to be a Math::BigInt (not the required Math::BigInt::GMP) object:

use Math::BigInt lib => 'GMP';
use warnings;
use strict;

use Inline C => Config =>
    BUILD_NOISY => 1;

use Inline C => <<'EOC';
      
void derivation(SV * obj) {
     if(sv_derived_from(obj, "Math::BigInt::GMP"))
       printf("A Math::BigInt::GMP object\n");
     else printf("Not a Math::BigInt::GMP object\n");

     if(sv_derived_from(obj, "Math::BigInt"))
       printf("A Math::BigInt object\n");
     else printf("Not a Math::BigInt object\n");
}

EOC

my $o = Math::BigInt->new(1234);
derivation($o);

__END__
Outputs:
Not a Math::BigInt::GMP object
A Math::BigInt object
[download]

What's the trick ?

Cheers,
Rob

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