cmd/compile: move Type.Maplineno to separate data structure

Relatively few types are ever used as map keys,
so tracking this separately is a net win.

Passes toolstash -cmp.

name       old alloc/op     new alloc/op     delta
Template       55.9MB ± 0%      55.5MB ± 0%  -0.71%        (p=0.000 n=10+10)
Unicode        37.8MB ± 0%      37.7MB ± 0%  -0.27%        (p=0.000 n=10+10)
GoTypes         180MB ± 0%       179MB ± 0%  -0.52%         (p=0.000 n=7+10)
Compiler        806MB ± 0%       803MB ± 0%  -0.41%        (p=0.000 n=10+10)

CPU and number of allocs are unchanged.

Change-Id: I6d60d74a4866995a231dfed3dd5792d75d904292
Reviewed-on: https://go-review.googlesource.com/21622
Reviewed-by: Matthew Dempsky <mdempsky@google.com>

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

@@ -27,7 +27,7 @@ func TestSizeof(t *testing.T) {
 		{Name{}, 52, 80},
 		{Node{}, 92, 144},
 		{Sym{}, 60, 112},
-		{Type{}, 56, 88},
+		{Type{}, 52, 80},
 		{MapType{}, 20, 40},
 		{ForwardType{}, 16, 32},
 		{FuncType{}, 28, 48},

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

@@ -390,8 +390,8 @@ func checkMapKeyType(key *Type) {
 		// before key is fully defined, the error
 		// will only be printed for the first one.
 		// good enough.
-		if key.Maplineno == 0 {
+		if maplineno[key] == 0 {
-			key.Maplineno = lineno
+			maplineno[key] = lineno
 		}
 	}
 }

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

@@ -153,8 +153,6 @@ type Type struct {
 	Vargen int32 // unique name for OTYPE/ONAME
 	Lineno int32 // line at which this type was declared, implicitly or explicitly
 
-	Maplineno int32 // first use of this type as a map key
-
 	Etype      EType // kind of type
 	Noalg      bool  // suppress hash and eq algorithm generation
 	Trecur     uint8 // to detect loops

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

@@ -3513,7 +3513,11 @@ func domethod(n *Node) {
 	checkwidth(n.Type)
 }
 
-var mapqueue []*Node
+var (
+	mapqueue []*Node
+	// maplineno tracks the line numbers at which types are first used as map keys
+	maplineno = map[*Type]int32{}
+)
 
 func copytype(n *Node, t *Type) {
 	if t.Etype == TFORW {
@@ -3522,7 +3526,7 @@ func copytype(n *Node, t *Type) {
 		return
 	}
 
-	maplineno := n.Type.Maplineno
+	mapline := maplineno[n.Type]
 	embedlineno := n.Type.ForwardType().Embedlineno
 	l := n.Type.ForwardType().Copyto
 
@@ -3559,8 +3563,8 @@ func copytype(n *Node, t *Type) {
 	lineno = lno
 
 	// Queue check for map until all the types are done settling.
-	if maplineno != 0 {
+	if mapline != 0 {
-		t.Maplineno = maplineno
+		maplineno[t] = mapline
 		mapqueue = append(mapqueue, n)
 	}
 }
@@ -3609,7 +3613,7 @@ ret:
 		}
 
 		for _, n := range mapqueue {
-			lineno = n.Type.Maplineno
+			lineno = maplineno[n.Type]
 			checkMapKeyType(n.Type)
 		}