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, 16 insertions, 9 deletions

diff --git a/test/append.go b/test/append.go
index 3f6251ee50..3d16063406 100644
--- a/test/append.go
+++ b/test/append.go
@@ -13,14 +13,12 @@ import (
 	"reflect"
 )
 
-
 func verify(name string, result, expected interface{}) {
 	if !reflect.DeepEqual(result, expected) {
 		panic(name)
 	}
 }
 
-
 func main() {
 	for _, t := range tests {
 		verify(t.name, t.result, t.expected)
@@ -30,6 +28,10 @@ func main() {
 	verifyType()
 }
 
+var (
+	zero int = 0
+	one  int = 1
+)
 
 var tests = []struct {
 	name             string
@@ -49,7 +51,6 @@ var tests = []struct {
 	{"bool i", append([]bool{true, false, true}, []bool{true}...), []bool{true, false, true, true}},
 	{"bool j", append([]bool{true, false, true}, []bool{true, true, true}...), []bool{true, false, true, true, true, true}},
 
-
 	{"byte a", append([]byte{}), []byte{}},
 	{"byte b", append([]byte{}, 0), []byte{0}},
 	{"byte c", append([]byte{}, 0, 1, 2, 3), []byte{0, 1, 2, 3}},
@@ -84,7 +85,6 @@ var tests = []struct {
 	{"int16 i", append([]int16{0, 1, 2}, []int16{3}...), []int16{0, 1, 2, 3}},
 	{"int16 j", append([]int16{0, 1, 2}, []int16{3, 4, 5}...), []int16{0, 1, 2, 3, 4, 5}},
 
-
 	{"uint32 a", append([]uint32{}), []uint32{}},
 	{"uint32 b", append([]uint32{}, 0), []uint32{0}},
 	{"uint32 c", append([]uint32{}, 0, 1, 2, 3), []uint32{0, 1, 2, 3}},
@@ -99,7 +99,6 @@ var tests = []struct {
 	{"uint32 i", append([]uint32{0, 1, 2}, []uint32{3}...), []uint32{0, 1, 2, 3}},
 	{"uint32 j", append([]uint32{0, 1, 2}, []uint32{3, 4, 5}...), []uint32{0, 1, 2, 3, 4, 5}},
 
-
 	{"float64 a", append([]float64{}), []float64{}},
 	{"float64 b", append([]float64{}, 0), []float64{0}},
 	{"float64 c", append([]float64{}, 0, 1, 2, 3), []float64{0, 1, 2, 3}},
@@ -114,7 +113,6 @@ var tests = []struct {
 	{"float64 i", append([]float64{0, 1, 2}, []float64{3}...), []float64{0, 1, 2, 3}},
 	{"float64 j", append([]float64{0, 1, 2}, []float64{3, 4, 5}...), []float64{0, 1, 2, 3, 4, 5}},
 
-
 	{"complex128 a", append([]complex128{}), []complex128{}},
 	{"complex128 b", append([]complex128{}, 0), []complex128{0}},
 	{"complex128 c", append([]complex128{}, 0, 1, 2, 3), []complex128{0, 1, 2, 3}},
@@ -129,7 +127,6 @@ var tests = []struct {
 	{"complex128 i", append([]complex128{0, 1, 2}, []complex128{3}...), []complex128{0, 1, 2, 3}},
 	{"complex128 j", append([]complex128{0, 1, 2}, []complex128{3, 4, 5}...), []complex128{0, 1, 2, 3, 4, 5}},
 
-
 	{"string a", append([]string{}), []string{}},
 	{"string b", append([]string{}, "0"), []string{"0"}},
 	{"string c", append([]string{}, "0", "1", "2", "3"), []string{"0", "1", "2", "3"}},
@@ -143,8 +140,19 @@ var tests = []struct {
 
 	{"string i", append([]string{"0", "1", "2"}, []string{"3"}...), []string{"0", "1", "2", "3"}},
 	{"string j", append([]string{"0", "1", "2"}, []string{"3", "4", "5"}...), []string{"0", "1", "2", "3", "4", "5"}},
-}
 
+	{"make a", append([]string{}, make([]string, 0)...), []string{}},
+	{"make b", append([]string(nil), make([]string, 0)...), []string(nil)},
+
+	{"make c", append([]struct{}{}, make([]struct{}, 0)...), []struct{}{}},
+	{"make d", append([]struct{}{}, make([]struct{}, 2)...), make([]struct{}, 2)},
+
+	{"make e", append([]int{0, 1}, make([]int, 0)...), []int{0, 1}},
+	{"make f", append([]int{0, 1}, make([]int, 2)...), []int{0, 1, 0, 0}},
+
+	{"make g", append([]*int{&zero, &one}, make([]*int, 0)...), []*int{&zero, &one}},
+	{"make h", append([]*int{&zero, &one}, make([]*int, 2)...), []*int{&zero, &one, nil, nil}},
+}
 
 func verifyStruct() {
 	type T struct {
@@ -185,7 +193,6 @@ func verifyStruct() {
 	verify("struct m", append(s, e...), r)
 }
 
-
 func verifyInterface() {
 	type T interface{}
 	type S []T