[dev.typeparams] cmd/compile: rewrite method call into method expression during escape analysis

CL 330331 extended escape analysis to analyze method expression calls
the same as normal method calls. We can now simply desugar method calls
into function calls in escape analysis.

To do this, two things must be changed:

 - Folding the rewrite method call to method expression call into an
   export function in typecheck package, so others can re-use it.

 - walkCall now have to call usemethod for method expression calls.
   (It seems to me this is a bug in current tip, because if one write
   (*rtype).Method(typ, i) in package "reflect", then the function won't
   be marked with AttrReflectMethod)

Passes toolstash -cmp.

Change-Id: I4745ab6110b417c7fd32949cc799811a882cd2ec
Reviewed-on: https://go-review.googlesource.com/c/go/+/330671
Trust: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Run-TryBot: Cuong Manh Le <cuong.manhle.vn@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>

src/cmd/compile/internal/devirtualize/devirtualize.go

@@ -50,7 +50,6 @@ func Call(call *ir.CallExpr) {
 		if base.Flag.LowerM != 0 {
 			base.WarnfAt(call.Pos(), "devirtualizing %v to %v", sel, typ)
 		}
-		call.SetOp(ir.OCALLMETH)
 		call.X = x
 	case ir.ODOTINTER:
 		// Promoted method from embedded interface-typed field (#42279).

src/cmd/compile/internal/escape/call.go

@@ -46,6 +46,7 @@ func (e *escape) callCommon(ks []hole, call ir.Node, init *ir.Nodes, wrapper *ir
 	case ir.OCALLFUNC, ir.OCALLMETH, ir.OCALLINTER:
 		call := call.(*ir.CallExpr)
 		typecheck.FixVariadicCall(call)
+		typecheck.FixMethodCall(call)
 
 		// Pick out the function callee, if statically known.
 		//

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

@@ -15,7 +15,7 @@ import (
 	"go/token"
 )
 
-// package all the arguments that match a ... T parameter into a []T.
+// MakeDotArgs package all the arguments that match a ... T parameter into a []T.
 func MakeDotArgs(pos src.XPos, typ *types.Type, args []ir.Node) ir.Node {
 	var n ir.Node
 	if len(args) == 0 {
@@ -57,6 +57,25 @@ func FixVariadicCall(call *ir.CallExpr) {
 	call.IsDDD = true
 }
 
+// FixMethodCall rewrites a method call t.M(...) into a function call T.M(t, ...).
+func FixMethodCall(call *ir.CallExpr) {
+	if call.X.Op() != ir.ODOTMETH {
+		return
+	}
+
+	dot := call.X.(*ir.SelectorExpr)
+
+	fn := Expr(ir.NewSelectorExpr(dot.Pos(), ir.OXDOT, ir.TypeNode(dot.X.Type()), dot.Selection.Sym))
+
+	args := make([]ir.Node, 1+len(call.Args))
+	args[0] = dot.X
+	copy(args[1:], call.Args)
+
+	call.SetOp(ir.OCALLFUNC)
+	call.X = fn
+	call.Args = args
+}
+
 // ClosureType returns the struct type used to hold all the information
 // needed in the closure for clo (clo must be a OCLOSURE node).
 // The address of a variable of the returned type can be cast to a func.

src/cmd/compile/internal/walk/expr.go

@@ -489,7 +489,7 @@ func walkAddString(n *ir.AddStringExpr, init *ir.Nodes) ir.Node {
 
 // walkCall walks an OCALLFUNC, OCALLINTER, or OCALLMETH node.
 func walkCall(n *ir.CallExpr, init *ir.Nodes) ir.Node {
-	if n.Op() == ir.OCALLINTER || n.Op() == ir.OCALLMETH {
+	if n.Op() == ir.OCALLINTER || n.X.Op() == ir.OMETHEXPR {
 		// We expect both interface call reflect.Type.Method and concrete
 		// call reflect.(*rtype).Method.
 		usemethod(n)
@@ -549,20 +549,8 @@ func walkCall1(n *ir.CallExpr, init *ir.Nodes) {
 	}
 	n.SetWalked(true)
 
-	// If this is a method call t.M(...),
-	// rewrite into a function call T.M(t, ...).
-	// TODO(mdempsky): Do this right after type checking.
 	if n.Op() == ir.OCALLMETH {
-		withRecv := make([]ir.Node, len(n.Args)+1)
-		dot := n.X.(*ir.SelectorExpr)
-		withRecv[0] = dot.X
-		copy(withRecv[1:], n.Args)
-		n.Args = withRecv
-
-		dot = ir.NewSelectorExpr(dot.Pos(), ir.OXDOT, ir.TypeNode(dot.X.Type()), dot.Selection.Sym)
-
-		n.SetOp(ir.OCALLFUNC)
-		n.X = typecheck.Expr(dot)
+		typecheck.FixMethodCall(n)
 	}
 
 	args := n.Args