I'm running a 64-bit build of Perl, making IVs & UVs 64-bit. As you pointed out to me a long time ago, a 64-bit double can hold at most a 53-bit integer. Therefore there is a potential that assigning either of the former to the latter, could result in loss of data. I expected a warning.

And indeed, if I try this with straight C, I get those warnings:

void test2 ( SV *sv ) {
    double d = SvIV( sv );
    double d2 = SvUV( sv );
    return;
}

IC_sort.c
IC_sort.xs(42) : warning C4244: 'initializing' : conversion from 'IV' 
+to 'double', possible loss of data
IC_sort.xs(43) : warning C4244: 'initializing' : conversion from 'UV' 
+to 'double', possible loss of data
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Hence, I attribute their absence to "the magic of XS".