cmd/compile: avoid allocating a Nodes for readonly method receivers

We were allocating a Nodes for common method calls
that did not modify the Nodes.
Though there is no clear wall time impact,
this significantly reduces the number of allocations,
so it seems worth doing.

Passes toolstash -cmp.

name       old alloc/op    new alloc/op    delta
Template      59.0MB ± 0%     58.6MB ± 0%   -0.81%        (p=0.000 n=25+25)
Unicode       41.4MB ± 0%     41.3MB ± 0%   -0.18%        (p=0.000 n=25+25)
GoTypes        198MB ± 0%      197MB ± 0%   -0.80%        (p=0.000 n=24+25)
Compiler       875MB ± 0%      865MB ± 0%   -1.09%        (p=0.000 n=25+25)

name       old allocs/op   new allocs/op   delta
Template        581k ± 0%       520k ± 0%  -10.42%        (p=0.000 n=25+25)
Unicode         413k ± 0%       403k ± 0%   -2.30%        (p=0.000 n=25+25)
GoTypes        1.78M ± 0%      1.58M ± 0%  -11.18%        (p=0.000 n=25+25)
Compiler       7.66M ± 0%      6.47M ± 0%  -15.51%        (p=0.000 n=25+25)

Change-Id: I012a9f4b333821bdf61b4f2bdff4ce5c3b5d3057
Reviewed-on: https://go-review.googlesource.com/21056Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/cmd/compile/internal/gc/syntax.go

@@ -376,7 +376,7 @@ type Nodes struct{ slice *[]*Node }
 // Slice returns the entries in Nodes as a slice.
 // Changes to the slice entries (as in s[i] = n) will be reflected in
 // the Nodes.
-func (n *Nodes) Slice() []*Node {
+func (n Nodes) Slice() []*Node {
 	if n.slice == nil {
 		return nil
 	}
@@ -384,7 +384,7 @@ func (n *Nodes) Slice() []*Node {
 }
 
 // Len returns the number of entries in Nodes.
-func (n *Nodes) Len() int {
+func (n Nodes) Len() int {
 	if n.slice == nil {
 		return 0
 	}
@@ -393,19 +393,19 @@ func (n *Nodes) Len() int {
 
 // Index returns the i'th element of Nodes.
 // It panics if n does not have at least i+1 elements.
-func (n *Nodes) Index(i int) *Node {
+func (n Nodes) Index(i int) *Node {
 	return (*n.slice)[i]
 }
 
 // First returns the first element of Nodes (same as n.Index(0)).
 // It panics if n has no elements.
-func (n *Nodes) First() *Node {
+func (n Nodes) First() *Node {
 	return (*n.slice)[0]
 }
 
 // Second returns the second element of Nodes (same as n.Index(1)).
 // It panics if n has fewer than two elements.
-func (n *Nodes) Second() *Node {
+func (n Nodes) Second() *Node {
 	return (*n.slice)[1]
 }
 
@@ -432,13 +432,13 @@ func (n *Nodes) MoveNodes(n2 *Nodes) {
 
 // SetIndex sets the i'th element of Nodes to node.
 // It panics if n does not have at least i+1 elements.
-func (n *Nodes) SetIndex(i int, node *Node) {
+func (n Nodes) SetIndex(i int, node *Node) {
 	(*n.slice)[i] = node
 }
 
 // Addr returns the address of the i'th element of Nodes.
 // It panics if n does not have at least i+1 elements.
-func (n *Nodes) Addr(i int) **Node {
+func (n Nodes) Addr(i int) **Node {
 	return &(*n.slice)[i]
 }