my first D program (and benchmark against perl)

[perlancar via Digitalmars-d-learn]

Here's my first non-hello-world D program, which is a direct 
translation from the Perl version. I was trying to get a feel 
about D's performance:

---BEGIN asciitable.d---
import std.string;
import std.stdio;

string fmttable(ref string[][] table) {
     string res = "";

     // column widths
     int[] widths;

     if (table.length == 0) return "";

     widths.length = table[0].length;

     for (int colnum=0; colnum < table[0].length; colnum++) {
         int width = 0;
         for (int rownum=0; rownum < table.length; rownum++) {
             if (table[rownum][colnum].length > width)
                 width = cast(int) table[rownum][colnum].length;
         }
         widths[colnum] = width;
     }

     for (int rownum=0; rownum < table.length; rownum++) {
         res ~= "|";
         for (int colnum=0; colnum < table[rownum].length; 
colnum++) {
             res ~= leftJustify(table[rownum][colnum], 
widths[colnum]);
             res ~= "|";
         }
         res ~= "\n";
     }

     return res;
}

void main() {
     // tiny table (1x1)
     /*
     string[][] table = [
         ["row1.1"],
     ];
     */

     // small table (3x5)
     string[][] table = [
         ["row1.1", "row1.2  ", "row1.3"],
         ["row2.1", "row2.2", "row2.3"],
         ["row3.1", "row3.2", "row3.3  "],
         ["row4.1", "row4.2", "row4.3"],
         ["row5.1", "row5.2", "row5.3"],
     ];

     write(fmttable(table));
     for (int i=0; i < 1000000; i++) {
         fmttable(table);
     }
}
---END asciitable.d---

Perl version:

---BEGIN asciitable.pl---
#!/usr/bin/env perl

sub fmttable {
     my $table = shift;

     my $res = "";

     # column widths
     my @widths;

     if (@$table == 0) { return "" }

     for my $colnum (0 .. $#{$table->[0]}) {
         my $width = 0;
         for my $rownum (0 .. $#{$table}) {
             if (length($table->[$rownum][$colnum]) > $width) {
                 $width = length($table->[$rownum][$colnum]);
             }
         }
         $widths[$colnum] = $width;
     }

     for my $rownum (0..$#{$table}) {
         $res .= "|";
         for my $colnum (0..$#{$table->[$rownum]}) {
             $res .= sprintf("%-".$widths[$colnum]."s|", 
$table->[$rownum][$colnum]);
         }
         $res .= "\n";
     }
     $res;
}

# tiny table (1x1)
#my $table = [["row1.1"]];

# small table (3x5)
my $table = [
     ["row1.1", "row1.2", "row1.3"],
     ["row2.1", "row2.2  ", "row2.3"],
     ["row3.1", "row3.2", "row3.3  "],
     ["row4.1", "row4.2", "row4.3"],
     ["row5.1", "row5.2", "row5.3"],
];

print fmttable($table);

for (1..1_000_000) {
     fmttable($table);
}
---END asciitable.pl---

While I am quite impressed with how easy I was able to write D, I 
am not so impressed with the performance. Using rdmd (build 
20151103), the D program runs in 17.127s while the Perl version 
runs in 11.391s (so the D version is quite a bit *slower* than 
Perl's). While using gdc (Debian 4.9.2-10), I am able to run it 
in 3.988s (only about 3x faster than Perl's version).

I understand that string processing (concatenation, allocation) 
is quite optimized in Perl, I was wondering if the D version 
could still be sped up significantly?