Is Direct Assignment Faster than Append for Slice Operations in Go?

Accelerating Slice Append Operations in Go

In Go, slices are crucial data structures that store sequences of values. When appending elements to a slice, it's essential to optimize performance. This article explores the performance difference between two methods of appending to a slice: append and direct assignment (=).

Benchmark Analysis

To compare the performance, we ran two benchmarks using the following code:

func BenchmarkSliceAppend(b *testing.B) {
    a := make([]int, 0, b.N)  // Create an empty slice with sufficient capacity
    for i := 0; i < b.N; i++ {
        a = append(a, i)  // Append i to the slice
    }
}

func BenchmarkSliceSet(b *testing.B) {
    a := make([]int, b.N)  // Create a slice with size b.N
    for i := 0; i < b.N; i++ {
        a[i] = i  // Assign i to the slice's ith element
    }
}

The benchmark results indicate that direct assignment (a[i] = i) is significantly faster than using append:

BenchmarkSliceAppend-4  200000000                7.87 ns/op            8 B/op          0 allocs/op
BenchmarkSliceSet-4     300000000                5.76 ns/op            8 B/op

Why is Assignment Faster than Append?

The performance difference stems from the distinct operations performed by each method:

• Direct Assignment (a[i] = i): This simply sets the value of the ith element of the slice to i.

• Append (a = append(a, i)): This function involves a more complex process:

  1. Calls the built-in append function, creating a new slice header and a temporary slice for the variadic parameter.
  2. Reslices the existing slice if it has sufficient capacity.
  3. Assigns i to the last element of the new slice.
  4. Updates the local variable a to reference the new slice.

In summary, each append operation involves additional overhead in copying the slice header and creating temporary slices, whereas direct assignment simply modifies the existing slice element. This overhead contributes to the observed performance difference.

Recommendation

For optimal performance when appending to slices, it's recommended to pre-allocate sufficient capacity using make rather than repeatedly using the append function. This eliminates the need for dynamic memory allocation and reslicing, significantly improving efficiency.

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