Scope storage class [Was: DMD 1.037 and 2.020 releases]

[bearophile]

To test the new scoping features of D 2.021 I have used a small stressing 
program, coming from this page, originally by Knuth:

http://www.rosettacode.org/wiki/Man_or_boy_test

More info:
http://en.wikipedia.org/wiki/Man_or_boy_test

My purpose is to see the D2 compiler being able to compute results up to N=25 
on my PC (that has 2 GB RAM and a 32 bit operating system).

---------------------------

This is the code I have used for DMD 1.037:

import std.c.stdio: printf;

int a(int k, lazy int x1, lazy int x2, lazy int x3, lazy int x4, lazy int x5) {
    int delegate() b;
    b = {
        k -= 1;
        return a(k, b(), x1, x2, x3, x4);
    };
    return k <= 0 ? x4 + x5 : b();
}

void main() {
    printf("%d\n", a(15, 1, -1, -1, 1, 0)); // N is 15
}

---------------------------

This is the code I have used for DMD 2.021. I was not sure how to use the 
scope, so if you have improvements please tell me:

import std.c.stdio: printf;

int a(scope int k, lazy int x1, lazy int x2, lazy int x3, lazy int x4, lazy int 
x5) {
    scope int delegate() b;
    b = {
        k -= 1;
        return a(k, b(), x1, x2, x3, x4);
    };
    return k <= 0 ? x4 + x5 : b();
}

void main() {
    printf("%d\n", a(15, 1, -1, -1, 1, 0)); // N is 15
}

---------------------------

The results for higher values of N are:
k   21           22            23         24          25
A   -389695     -865609     -1922362    -4268854    -9479595

In both cases I have compiled the code with:
dmd -O -release -inline -L/STACK:1700000000 man_or_boy.d

The results:

DMD V.1.037 (exe: 168_476: bytes):
  N=24:  788 MB RAM, 3 seconds
  N=25: 1.57 GB RAM, 6.42 seconds

DMD V.2.021 (exe: 99_356 bytes):

The code was too much slow in D2, so I have stopped it. Do you know if the D2 
code can be improved to have performance similar to D1 ones?

Bye,
bearophile