[chore] primitive slice, avoid allocation if enough space

[bogdandrutu]

Signed-off-by: Bogdan [email protected]

[bogdandrutu]

I created some benchmarks to prove that append is as fast as copy, so log them here for others to see them:

func BenchmarkArrayAppend(b *testing.B) {
	arr := []int64{1, 2, 3, 5, 6, 7, 8}
	b.ReportAllocs()
	for n := 0; n < b.N; n++ {
		cpArr := getArray(n)
		arr = arr[:0]
		arr = append(arr, cpArr...)
		if len(arr) != len(cpArr) {
			b.Fail()
		}
	}
}

func BenchmarkArrayCopy(b *testing.B) {
	arr := []int64{1, 2, 3, 5, 6, 7, 8}
	b.ReportAllocs()
	for n := 0; n < b.N; n++ {
		cpArr := getArray(n)
		if cap(arr) >= len(cpArr) {
			arr = arr[:len(cpArr)]
		} else {
			arr = make([]int64, len(cpArr))
		}
		copy(arr, cpArr)
		if len(arr) != len(cpArr) {
			b.Fail()
		}
	}
}

func BenchmarkArrayAppendNil(b *testing.B) {
	b.ReportAllocs()
	for n := 0; n < b.N; n++ {
		arr := []int64(nil)
		cpArr := getArray(n)
		arr = append(arr, cpArr...)
		if len(arr) != len(cpArr) {
			b.Fail()
		}
	}
}

func BenchmarkArrayCopyNil(b *testing.B) {
	b.ReportAllocs()
	for n := 0; n < b.N; n++ {
		arr := []int64(nil)
		cpArr := getArray(n)
		if cap(arr) >= len(cpArr) {
			arr = arr[:len(cpArr)]
		} else {
			arr = make([]int64, len(cpArr))
		}
		copy(arr, cpArr)
		if len(arr) != len(cpArr) {
			b.Fail()
		}
	}
}

func BenchmarkArrayAppendShort(b *testing.B) {
	ini := []int64{1, 2, 3}
	b.ReportAllocs()
	for n := 0; n < b.N; n++ {
		arr := ini
		cpArr := getArray(n)
		arr = arr[:0]
		arr = append(arr, cpArr...)
		if len(arr) != len(cpArr) {
			b.Fail()
		}
	}
}

func BenchmarkArrayCopyShort(b *testing.B) {
	ini := []int64{1, 2, 3}
	b.ReportAllocs()
	for n := 0; n < b.N; n++ {
		arr := ini
		cpArr := getArray(n)
		if cap(arr) >= len(cpArr) {
			arr = arr[:len(cpArr)]
		} else {
			arr = make([]int64, len(cpArr))
		}
		copy(arr, cpArr)
		if len(arr) != len(cpArr) {
			b.Fail()
		}
	}
}

var (
	arrLen6 = []int64{6, 5, 4, 3, 2, 1}
	arrLen7 = []int64{7, 6, 5, 4, 4, 3, 2, 1}
)

func getArray(iter int) []int64 {
	if iter%2 == 0 {
		return arrLen6
	}
	return arrLen7
}

Results:

goos: darwin
goarch: amd64
pkg: go.opentelemetry.io/collector/pdata/pcommon
cpu: Intel(R) Core(TM) i9-9880H CPU @ 2.30GHz
BenchmarkArrayAppend
BenchmarkArrayAppend-16         	315533996	         3.492 ns/op	       0 B/op	       0 allocs/op
BenchmarkArrayCopy
BenchmarkArrayCopy-16           	308300316	         3.868 ns/op	       0 B/op	       0 allocs/op
BenchmarkArrayAppendNil
BenchmarkArrayAppendNil-16      	37775329	        29.94 ns/op	      56 B/op	       1 allocs/op
BenchmarkArrayCopyNil
BenchmarkArrayCopyNil-16        	40174813	        29.75 ns/op	      56 B/op	       1 allocs/op
BenchmarkArrayAppendShort
BenchmarkArrayAppendShort-16    	38323144	        31.07 ns/op	      56 B/op	       1 allocs/op
BenchmarkArrayCopyShort
BenchmarkArrayCopyShort-16      	43295260	        35.39 ns/op	      56 B/op	       1 allocs/op
PASS

[codecov]

Codecov Report

Base: 91.78% // Head: 91.77% // Decreases project coverage by -0.00% ⚠️

Coverage data is based on head (6c65e10) compared to base (58c8e09).
Patch coverage: 90.47% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6061      +/-   ##
==========================================
- Coverage   91.78%   91.77%   -0.01%     
==========================================
  Files         216      216              
  Lines       13395    13383      -12     
==========================================
- Hits        12294    12282      -12     
  Misses        878      878              
  Partials      223      223              

Impacted Files	Coverage Δ
pdata/pcommon/common.go	94.48% <33.33%> (ø)
pdata/pcommon/generated_primitive_slice.go	100.00% <100.00%> (ø)

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