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 bug33;

extern(C) int memcmp(void*,void*,size_t);

private int string_cmp(char[] s1, char[] s2)
{
    auto len = s1.length;
    if (s2.length < len)
        len = s2.length;
    int result = memcmp(s1.ptr, s2.ptr, len);
    if (result == 0)
        result = cast(int)(cast(ptrdiff_t)s1.length - cast(ptrdiff_t)s2.length);
    return result;
}

struct S
{
    char[] toString()
    {
        return "S";
    }
}

int func()
{
    S a,b;
    return string_cmp(a.toString(),b.toString());
}

void main()
{
    assert(func() == 0);
}