[dev.typeparams] cmd/compile: cache mapped types during irgen

If we see the exact same types2.Type a second time, we can map it to
the same *types.Type instance. Not strictly necessary, but reduces
memory usage and plays better with the rest of the compiler given the
current state of things.

Change-Id: I53686d072c7c7834b0c97417bc8d5f2cd24572b2
Reviewed-on: https://go-review.googlesource.com/c/go/+/284692
Trust: Matthew Dempsky <mdempsky@google.com>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>

src/cmd/compile/internal/noder/irgen.go

@@ -79,6 +79,7 @@ func check2(noders []*noder) {
 		info:   &info,
 		posMap: m,
 		objs:   make(map[types2.Object]*ir.Name),
+		typs:   make(map[types2.Type]*types.Type),
 	}
 	g.generate(noders)
 
@@ -94,6 +95,7 @@ type irgen struct {
 
 	posMap
 	objs   map[types2.Object]*ir.Name
+	typs   map[types2.Type]*types.Type
 	marker dwarfgen.ScopeMarker
 }
 

src/cmd/compile/internal/noder/types.go

@@ -26,6 +26,20 @@ func (g *irgen) pkg(pkg *types2.Package) *types.Pkg {
 }
 
 func (g *irgen) typ(typ types2.Type) *types.Type {
+	// Caching type mappings isn't strictly needed, because typ0 preserves
+	// type identity; but caching minimizes memory blow-up from mapping the
+	// same composite type multiple times, and also plays better with the
+	// current state of cmd/compile (e.g., haphazard calculation of type
+	// sizes).
+	res, ok := g.typs[typ]
+	if !ok {
+		res = g.typ0(typ)
+		g.typs[typ] = res
+	}
+	return res
+}
+
+func (g *irgen) typ0(typ types2.Type) *types.Type {
 	switch typ := typ.(type) {
 	case *types2.Basic:
 		return g.basic(typ)