cmd/compile: optimize append(x, make([]T, y)...) slice extension

Changes the compiler to recognize the slice extension pattern

  append(x, make([]T, y)...)

and replace it with growslice and an optional memclr to avoid an allocation for make([]T, y).

Memclr is not called in case growslice already allocated a new cleared backing array
when T contains pointers.

amd64:
name                      old time/op    new time/op    delta
ExtendSlice/IntSlice         103ns ± 4%      57ns ± 4%   -44.55%  (p=0.000 n=18+18)
ExtendSlice/PointerSlice     155ns ± 3%      77ns ± 3%   -49.93%  (p=0.000 n=20+20)
ExtendSlice/NoGrow          50.2ns ± 3%     5.2ns ± 2%   -89.67%  (p=0.000 n=18+18)

name                      old alloc/op   new alloc/op   delta
ExtendSlice/IntSlice         64.0B ± 0%     32.0B ± 0%   -50.00%  (p=0.000 n=20+20)
ExtendSlice/PointerSlice     64.0B ± 0%     32.0B ± 0%   -50.00%  (p=0.000 n=20+20)
ExtendSlice/NoGrow           32.0B ± 0%      0.0B       -100.00%  (p=0.000 n=20+20)

name                      old allocs/op  new allocs/op  delta
ExtendSlice/IntSlice          2.00 ± 0%      1.00 ± 0%   -50.00%  (p=0.000 n=20+20)
ExtendSlice/PointerSlice      2.00 ± 0%      1.00 ± 0%   -50.00%  (p=0.000 n=20+20)
ExtendSlice/NoGrow            1.00 ± 0%      0.00       -100.00%  (p=0.000 n=20+20)

Fixes #21266

Change-Id: Idc3077665f63cbe89762b590c5967a864fd1c07f
Reviewed-on: https://go-review.googlesource.com/109517
Run-TryBot: Martin Möhrmann <moehrmann@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>

1 files changed, 30 insertions, 0 deletions

diff --git a/src/runtime/slice_test.go b/src/runtime/slice_test.go
index 46db071ebe..c2dfb7afd1 100644
--- a/src/runtime/slice_test.go
+++ b/src/runtime/slice_test.go
@@ -72,6 +72,36 @@ func BenchmarkGrowSlice(b *testing.B) {
 	})
 }
 
+var (
+	SinkIntSlice        []int
+	SinkIntPointerSlice []*int
+)
+
+func BenchmarkExtendSlice(b *testing.B) {
+	var length = 4 // Use a variable to prevent stack allocation of slices.
+	b.Run("IntSlice", func(b *testing.B) {
+		s := make([]int, 0, length)
+		for i := 0; i < b.N; i++ {
+			s = append(s[:0:length/2], make([]int, length)...)
+		}
+		SinkIntSlice = s
+	})
+	b.Run("PointerSlice", func(b *testing.B) {
+		s := make([]*int, 0, length)
+		for i := 0; i < b.N; i++ {
+			s = append(s[:0:length/2], make([]*int, length)...)
+		}
+		SinkIntPointerSlice = s
+	})
+	b.Run("NoGrow", func(b *testing.B) {
+		s := make([]int, 0, length)
+		for i := 0; i < b.N; i++ {
+			s = append(s[:0:length], make([]int, length)...)
+		}
+		SinkIntSlice = s
+	})
+}
+
 func BenchmarkAppend(b *testing.B) {
 	b.StopTimer()
 	x := make([]int, 0, N)