Thanks.

Yes. I know it can be done using the ludicrously verbose, Camel_Case_And_Underscores inline stack macros. I was trying to understand why I can't use the neater and more concise XS macros from Inline C. I've found I can reduce the requirements to:

#! perl -slw
use strict;
use Inline C => Config => BUILD_NOISY => 1;
use Inline C => <<'END_C',  NAME => 'monkeys', CLEAN_AFTER_BUILD => 0;

void rnd64( int n ) {
    Inline_Stack_Vars;
    static unsigned __int64 y = 88172645463325252i64;

    EXTEND( SP, n );
    while( n-- ) {
        y ^= y << 13;
        y ^= y >> 7;
        y ^= y << 17;
        mPUSHu( y );
    }
    Inline_Stack_Done;
    return;
}
[download]

But looking at the C produced by the above, it looks like there is a path through the generated wrapper function that avoids both the XSRETURN_EMPTY and the PUTBACK, thus returning whatever has been pushed:

XS(XS_main_rnd64); /* prototype to pass -Wmissing-prototypes */
XS(XS_main_rnd64)
{
#ifdef dVAR
    dVAR; dXSARGS;
#else
    dXSARGS;
#endif
    if (items != 1)
       croak_xs_usage(cv,  "n");
    PERL_UNUSED_VAR(ax); /* -Wall */
    SP -= items;
    {
    int    n = (int)SvIV(ST(0));
#line 46 "monkeys.xs"
    I32* temp;
#line 117 "monkeys.c"
#line 48 "monkeys.xs"
    temp = PL_markstack_ptr++;
    rnd64(n);
    if (PL_markstack_ptr != temp) {
          /* truly void, because dXSARGS not invoked */
      PL_markstack_ptr = temp;
      XSRETURN_EMPTY; /* return empty stack */
        }
        /* must have used dXSARGS; list context implied */
    return; /* assume stack size is correct */                 ### Her
+e
#line 128 "monkeys.c"
    PUTBACK;
    return;
    }
}
[download]

But I'm obviously missing something in the OP code that would cause (or allow) it to follow that path?