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 vararg1;

import tango.stdc.stdarg;

extern(C) int add(int n, ...)
{
    va_list ap=void;
    va_start(ap, n);
    int r;
    for (int i=0; i<n; i++)
        r += va_arg!(int)(ap);
    va_end(ap);
    return r;
}

void main()
{
    int i = add(4, 1,2,3,4);
    assert(i == 10);
}