I have noticed a significant speed difference between foo1 and foo2 (D2 code):
import std.c.stdio: printf; int foo1(int x, int y) { static int[int[2]] cache; int[2] args = [x, y]; cache[args] = x; return x; } int foo2(int x, int y) { static struct Pair { int x, y; } static int[Pair] cache; Pair args = Pair(x, y); cache[args] = x; return x; } void main() { enum int N = 600; int tot; foreach (x; 1 .. N) foreach (y; 1 .. N) tot += foo2(x, y); // use foo1 or foo2 here printf("%d\n", tot); } Bye, bearophile