b) pass any mildy sizable quantity of data by reference rather than through the stack--including largish scalars;

Despite the plausibility of this, I find it makes very little difference (where XSubs are concerned) whether the data is passed by reference or via the stack.

Here's a demo:

use warnings;
use Benchmark;

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

use Inline C => <<'EOC';

/* These subs taken from           *
*  "Extending and Embedding Perl"  *
*  by Jenness and Cozens           */

double sum_as_ref(AV * avref) {
    int i, len;
    double sum = 0;
    SV ** elem;

    len = av_len(avref) + 1;

    for(i = 0; i < len; i++) {
       elem = av_fetch(avref, i, 0);
       sum += (double)SvIV(*elem);
    }
    return sum;
}

double sum_as_list(SV * arg1, ...) {
   int i, len;
   double sum = 0;
   SV * elem;
   Inline_Stack_Vars;

   len = Inline_Stack_Items;

   for(i = 0; i < len; i++) {
      elem = Inline_Stack_Item(i);
      sum += (double)SvIV(elem);
   }
   return sum;
}

EOC

our @x = (1 .. 2e6);
our($list, $ref);

timethese (1, {
'list' => '$list = sum_as_list(@x);',
'ref'  => '$ref  = sum_as_ref(\@x);',
});

print "$list\n$ref\n";
[download]

And the result:

Benchmark: timing 1 iterations of list, ref...
      list: -1 wallclock secs ( 0.14 usr +  0.00 sys =  0.14 CPU) @  7
+.09/s (n=1
)
            (warning: too few iterations for a reliable count)
       ref:  0 wallclock secs ( 0.14 usr +  0.00 sys =  0.14 CPU) @  7
+.09/s (n=1
)
            (warning: too few iterations for a reliable count)
2000001000000
2000001000000
[download]

Cheers, Rob

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