pass: support priority in register allocator (#180)

Updates #156

pass/alloc_test.go

@@ -44,3 +44,55 @@ func TestAllocatorImpossible(t *testing.T) {
 		t.Fatal("expected allocation error")
 	}
 }
+
+func TestAllocatorPriority(t *testing.T) {
+	const n = 4
+
+	// Create an allocator with custom priorities.
+	a, err := NewAllocatorForKind(reg.KindVector)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	a.SetPriority(reg.X0.ID(), -1)
+	a.SetPriority(reg.X7.ID(), 1)
+	a.SetPriority(reg.X13.ID(), 1)
+	a.SetPriority(reg.X3.ID(), 2)
+
+	// The expected n highest priority registers.
+	expect := [n]reg.Physical{
+		reg.X3,  // priority 2, id 3
+		reg.X7,  // priority 1, id 7
+		reg.X13, // priority 1, id 13
+		reg.X1,  // priority 0, id 1 (X0 has priority -1)
+	}
+
+	// Setup allocation problem with n conflicting registers.
+	c := reg.NewCollection()
+	x := make([]reg.Virtual, n)
+	for i := range x {
+		x[i] = c.XMM()
+	}
+
+	for i := range x {
+		a.Add(x[i].ID())
+	}
+
+	for i := 0; i < n; i++ {
+		for j := i + 1; j < n; j++ {
+			a.AddInterference(x[i].ID(), x[j].ID())
+		}
+	}
+
+	// Allocate and confirm expectation.
+	alloc, err := a.Allocate()
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	for i := range x {
+		if got := alloc.LookupRegister(x[i]); got != expect[i] {
+			t.Errorf("x[%d] allocated %s; expected %s", i, got.Asm(), expect[i].Asm())
+		}
+	}
+}