Small performance problem

[bearophile]

In D there are several things to implement and fix that have a priority higher 
than tuning the performance of the DMD back-end (a work may be wasted time 
anyway). But arrays are common, so the following may interest anyway.

On LDC this synthetic benchmark shows the same run time with both values of the 
use_dynamic_array constant, while on DMD the version that uses dynamic arrays 
is something like 70% faster (on both D1 compiler and last D2 compiler):

version (Tango)
    import tango.stdc.stdio: printf;
else
    version (D_Version2)
        import std.c.stdio: printf;

const bool use_dynamic_array = true; // change this

void main() {
    const int n = 100_000;
    const int nloop = 6_000;

    int[] aux1 = new int[n];
    int[] aux2 = new int[n];

    static if (use_dynamic_array) {
        alias aux1 a1;
        alias aux2 a2;
    } else {
        int* a1 = aux1.ptr;
        int* a2 = aux2.ptr;
    }

    for (int i; i < nloop; i++) {
        for (int j; j < n; j++)
            a1[j] += a2[j];
        for (int j; j < n; j++)
            a2[j] += a1[j];
    }

    printf("%d\n", a1[10]);
}

Asm of the inner loop with dynamic arrays (DMD):
L59:    mov     ECX,[EBX*4][ESI]
        add     [EBX*4][EDX],ECX
        inc     EBX
        cmp     EBX,0186A0h
        jb      L59

Asm of the inner loop with pointers (DMD):
L47:    mov     ESI,01Ch[ESP]
        mov     EAX,014h[ESP]
        mov     EDI,[EBX*4][ESI]
        add     [EBX*4][EAX],EDI
        inc     EBX
        cmp     EBX,0186A0h
        jb      L47

Bye,
bearophile