expand the fill benchmark with subtest for different prefix lengths

ipalloc/ipalloc_test.go

@@ -1,6 +1,7 @@
 package ipalloc
 
 import (
+	"fmt"
 	"log"
 	"net/netip"
 	"testing"
@@ -223,11 +224,23 @@ func TestAllocDealloc(t *testing.T) {
 	expectAlloc(t, root, 32, "0.0.0.1/32")
 }
 
-func BenchmarkFill(b *testing.B) {
-	root := &tree{prefix: netip.MustParsePrefix("0.0.0.0/16")}
-	for {
+func benchRunN(b *testing.B, bits int) {
+	root := &tree{
+		prefix: netip.PrefixFrom(netip.MustParseAddr("0.0.0.0"), bits),
+	}
+	for i := 0; i < b.N; i++ {
 		if alloc := root.Alloc(32); alloc == nil {
-			break
+			root = &tree{
+				prefix: netip.PrefixFrom(netip.MustParseAddr("0.0.0.0"), bits),
+			}
 		}
 	}
+
+}
+func BenchmarkFill(b *testing.B) {
+	for bits := 24; bits >= 0; bits -= 8 {
+		b.Run(fmt.Sprintf("BenchmarkFill-%2d", bits), func(b *testing.B) {
+			benchRunN(b, bits)
+		})
+	}
 }