you've piqued my curiosity :-)

Update: I forgot the package statements. D'oh! (Code updated)

This is what I have so far. Not working yet because of an IC problem that I've encountered before, but can't remember how to fix; and the math is almost certainly wrong as is; but I threw it together to try and get a starting point:

#! perl -slw
use Config; package F128;
use Inline C => Config => BUILD_NOISY => 1;
use Inline C => <<END_C,  NAME => 'F128', CLEAN_AFTER_BUILD =>0, TYPEM
+APS => './F128.typemap';
#include "../C/mytypes.h"
#define CLASS "F128"

typedef struct { U64 scale; U64 precision; } F128;

F128 *new( I64 scale, U64 precision ) {
    F128 *n = malloc( sizeof( F128 ) );
    n->scale = scale;
    n->precision = precision;
    return n;
}

F128 *add( F128 *a, F128 *b ) {
    if( a->scale != b->scale ) {
        while( a->scale > b->scale ) {
            a->scale >>= 1;
            a->precision <<= 1;
        }
        while( a->scale < b->scale ) {
            a->scale <<= 1;
            a->precision >>= 1;
        }
    }
    a->precision += b->precision;
    return a;
}

F128 *multiply( F128 *a, F128 *b ) {
    a->precision *= b->precision;
    a->scale += b->scale;
    return a;
}

SV *toString( F128 *a ) {
    char *out = malloc( a->scale );
    U32 c = sprintf( out, "%I64ue%I64i", a->precision, a->scale );
    return newSVpv( out, c );
}

void DESTROY( F128 *a ) {
    free( a );
    return;
}

END_C
package main;
my $Fa = F128::new( -10, 45 );
my $Fb = F128::new( -10, 45 );
my $Fc = F128::new( -10, 45 );

print $Fa->toString;

$Fa->add( $Fb );
print $Fa->toString;

$Fc->multiply( $Fb );
print $Fc->toString;
[download]

The typemap:

TYPEMAP
const char  * T_PV
F128        * O_OBJECT
U64           T_UV
I64           T_IV
U8            T_UV
U8          * T_PV

INPUT
O_OBJECT
    if( sv_isobject($arg) && ( SvTYPE( SvRV($arg) ) == SVt_PVMG ) )
            $var = INT2PTR( $type, SvIV( (SV*)SvRV( $arg ) ) );
    else{
            warn( \"${Package}::$func_name() -- $var is not a blessed 
+SV reference\" );
            XSRETURN_UNDEF;
    }

OUTPUT

# The Perl object is blessed into 'CLASS', which should be a
# char* having the name of the package for the blessing.
O_OBJECT
        sv_setref_pv( $arg, (char *)CLASS, (void*)$var );
[download]

I'm currently getting:

45e-10
90e-10
2025e-20
[download]

Which is almost there for my requirements.

And I know its a trivial fix, but .... I can't remember what. syphilis Got your ears on?.