Implement first D-specific optimization pass: -simplify-drtcalls. It uses the machinery of the standard -simplify-libcalls pass, but optimizes calls to the D runtime instead of calls to C libraries. At the moment, these optimizations are implemented by this pass: - Avoid the runtime call for `arr.length = newlen` if it can determine that the new length isn't longer than the old one. - Ditto for `cast(T[]) arr` if it will clearly always succeed. (e.g. if the length of the original array is zero, or if the old element size is a multiple of the new element size)

module classes6;
extern(C) int printf(char*, ...);

class C
{
    void f()
    {
        printf("world\n");
    }
}

class D : C
{
    void f()
    {
        printf("moon\n");
    }
}


extern(C)
{
    void srand(uint seed);
    int rand();
}

import ldc.intrinsics;

void main()
{
    C c;
    srand(readcyclecounter());
    if (rand() % 2)
        c = new C;
    else
        c = new D;
    c.f();
}