[kjksf]

String benchmarks are so broken.

They way he uses b.N is wrong. b.N is different for different loops so he's e.g. timing 100 iterations of string '+' with a 1000 iterations of builder.WriteString()

Also the compiler can completely null out no-op functions (without side effects) so in benchmarks it's a good idea to assign the value being calculated into e.g. a global variable.

The corrected code is: https://gist.github.com/kjk/6a7d7135ae1e5fa6cd1f0db23d2eaf4d

An example of correctly benchmarking:

    func BenchmarkConcatString(b *testing.B) {
	for n := 0; n < b.N; n++ {
		var str string
		for i := 0; i < 100; i++ {
			str += "x"
		}
		gStr = str
	}
    }

After fixes it paints significantly different picture:

    go test -bench=. -benchmem
    goos: darwin
    goarch: amd64
    BenchmarkConcatString-8    	  300000	      5148 ns/op	    5728 B/op	      99 allocs/op
    BenchmarkConcatBuffer-8    	 1000000	      1046 ns/op	     368 B/op	       3 allocs/op
    BenchmarkConcatBuilder-8   	 1000000	      1177 ns/op	     248 B/op	       5 allocs/op

[i0exception]

His use of b.N is correct. The code you have is simply multiplying N by 100 with the inner for loop - so your times are 100x of what each "concat" operation (+,WriteString) takes.

You are also allocating a new string/buffer/builder for every run - which is not useful if you want to just benchmark concat.

[dmitrim]

Thanks for pointing it out. Should clearly not depend on the number of iterations. It's fixed now.

[fauigerzigerk]

I think there's another bug in the generateSlice function if the intention is to create a slice with n random numbers.

    func generateSlice(n int) []int {
        s := make([]int, n)
        for i := 0; i < n; i++ {
            s = append(s, rand.Intn(1e9))
        }
        return s
    }

As it is now, the function creates a slice with n zeros followed by n random numbers. I suppose you meant to say make([]int, 0, n). You could just as well assign directly to each slice element instead of using append, which would be more efficient.

I made the exact same mistake quite a few times myself.

[dmitrim]

Yep, that meant to be capacity, not length. Corrected. Thanks!