slices: amortize allocations in Insert

Fixes #54948

Change-Id: I467afb940b539b100dcce687b05914a9da7b9ed2
Reviewed-on: https://go-review.googlesource.com/c/go/+/484159
Run-TryBot: Ian Lance Taylor <[email protected]>
Reviewed-by: Bryan Mills <[email protected]>
Run-TryBot: Ian Lance Taylor <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>
Auto-Submit: Ian Lance Taylor <[email protected]>
Reviewed-by: Ian Lance Taylor <[email protected]>
Reviewed-by: Valentin Deleplace <[email protected]>

Author: ianlancetaylor

src/slices/slices.go

@@ -88,7 +88,11 @@ func Insert[S ~[]E, E any](s S, i int, v ...E) S {
 		copy(s2[i:], v)
 		return s2
 	}
-	s2 := make(S, tot)
+	// Use append rather than make so that we bump the size of
+	// the slice up to the next storage class.
+	// This is what Grow does but we don't call Grow because
+	// that might copy the values twice.
+	s2 := append(S(nil), make(S, tot)...)
 	copy(s2, s[:i])
 	copy(s2[i:], v)
 	copy(s2[i+len(v):], s[i:])

src/slices/slices_test.go

@@ -256,6 +256,20 @@ func TestInsert(t *testing.T) {
 			t.Errorf("Insert(%v, %d, %v...) = %v, want %v", test.s, test.i, test.add, got, test.want)
 		}
 	}
+
+	if !testenv.OptimizationOff() && !race.Enabled {
+		// Allocations should be amortized.
+		const count = 50
+		n := testing.AllocsPerRun(10, func() {
+			s := []int{1, 2, 3}
+			for i := 0; i < count; i++ {
+				s = Insert(s, 0, 1)
+			}
+		})
+		if n > count/2 {
+			t.Errorf("too many allocations inserting %d elements: got %v, want less than %d", count, n, count/2)
+		}
+	}
 }
 
 var deleteTests = []struct {