cmd/compile: simplify walk OCONVIFACE

n.Type and n.Left.Type are used heavily. Give them useful names.

We generate the type word frequently. Make it a closure.
(We don't want to generate it up front, since there are some code
paths that don't need it, and generating it has side-effects.)

Simplify and document the final call construction.

Follow-up to address feedback on CL 147360.

Change-Id: I251134a55cf80d8b1676280a345d150f2288c09a
Reviewed-on: https://go-review.googlesource.com/c/147538
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Martin Möhrmann <moehrmann@google.com>

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

@@ -815,16 +815,21 @@ opswitch:
 	case OCONVIFACE:
 		n.Left = walkexpr(n.Left, init)
 
-		// Optimize convT2E or convT2I as a two-word copy when T is pointer-shaped.
-		if isdirectiface(n.Left.Type) {
-			var t *Node
-			if n.Type.IsEmptyInterface() {
-				t = typename(n.Left.Type)
-			} else {
-				t = itabname(n.Left.Type, n.Type)
+		fromType := n.Left.Type
+		toType := n.Type
+
+		// typeword generates the type word of the interface value.
+		typeword := func() *Node {
+			if toType.IsEmptyInterface() {
+				return typename(fromType)
+			}
+			return itabname(fromType, toType)
 		}
-			l := nod(OEFACE, t, n.Left)
-			l.Type = n.Type
+
+		// Optimize convT2E or convT2I as a two-word copy when T is pointer-shaped.
+		if isdirectiface(fromType) {
+			l := nod(OEFACE, typeword(), n.Left)
+			l.Type = toType
 			l.SetTypecheck(n.Typecheck())
 			n = l
 			break
@@ -844,11 +849,11 @@ opswitch:
 		// or creating one on the stack.
 		var value *Node
 		switch {
-		case n.Left.Type.Size() == 0:
+		case fromType.Size() == 0:
 			// n.Left is zero-sized. Use zerobase.
 			cheapexpr(n.Left, init) // Evaluate n.Left for side-effects. See issue 19246.
 			value = zerobase
-		case n.Left.Type.IsBoolean() || (n.Left.Type.Size() == 1 && n.Left.Type.IsInteger()):
+		case fromType.IsBoolean() || (fromType.Size() == 1 && fromType.IsInteger()):
 			// n.Left is a bool/byte. Use staticbytes[n.Left].
 			n.Left = cheapexpr(n.Left, init)
 			value = nod(OINDEX, staticbytes, byteindex(n.Left))
@@ -856,23 +861,17 @@ opswitch:
 		case n.Left.Class() == PEXTERN && n.Left.Name != nil && n.Left.Name.Readonly():
 			// n.Left is a readonly global; use it directly.
 			value = n.Left
-		case !n.Left.Type.IsInterface() && n.Esc == EscNone && n.Left.Type.Width <= 1024:
+		case !fromType.IsInterface() && n.Esc == EscNone && fromType.Width <= 1024:
 			// n.Left does not escape. Use a stack temporary initialized to n.Left.
-			value = temp(n.Left.Type)
+			value = temp(fromType)
 			init.Append(typecheck(nod(OAS, value, n.Left), Etop))
 		}
 
 		if value != nil {
 			// Value is identical to n.Left.
 			// Construct the interface directly: {type/itab, &value}.
-			var t *Node
-			if n.Type.IsEmptyInterface() {
-				t = typename(n.Left.Type)
-			} else {
-				t = itabname(n.Left.Type, n.Type)
-			}
-			l := nod(OEFACE, t, typecheck(nod(OADDR, value, nil), Erv))
-			l.Type = n.Type
+			l := nod(OEFACE, typeword(), typecheck(nod(OADDR, value, nil), Erv))
+			l.Type = toType
 			l.SetTypecheck(n.Typecheck())
 			n = l
 			break
@@ -884,9 +883,9 @@ opswitch:
 		//    tmp = tmp.type
 		// }
 		// e = iface{tmp, i.data}
-		if n.Type.IsEmptyInterface() && n.Left.Type.IsInterface() && !n.Left.Type.IsEmptyInterface() {
+		if toType.IsEmptyInterface() && fromType.IsInterface() && !fromType.IsEmptyInterface() {
 			// Evaluate the input interface.
-			c := temp(n.Left.Type)
+			c := temp(fromType)
 			init.Append(nod(OAS, c, n.Left))
 
 			// Get the itab out of the interface.
@@ -900,51 +899,41 @@ opswitch:
 
 			// Build the result.
 			e := nod(OEFACE, tmp, ifaceData(c, types.NewPtr(types.Types[TUINT8])))
-			e.Type = n.Type // assign type manually, typecheck doesn't understand OEFACE.
+			e.Type = toType // assign type manually, typecheck doesn't understand OEFACE.
 			e.SetTypecheck(1)
 			n = e
 			break
 		}
 
-		fnname, needsaddr := convFuncName(n.Left.Type, n.Type)
+		fnname, needsaddr := convFuncName(fromType, toType)
 
-		if !needsaddr && !n.Left.Type.IsInterface() {
+		if !needsaddr && !fromType.IsInterface() {
 			// Use a specialized conversion routine that only returns a data pointer.
 			// ptr = convT2X(val)
 			// e = iface{typ/tab, ptr}
 			fn := syslook(fnname)
-			dowidth(n.Left.Type)
-			fn = substArgTypes(fn, n.Left.Type)
+			dowidth(fromType)
+			fn = substArgTypes(fn, fromType)
 			dowidth(fn.Type)
 			call := nod(OCALL, fn, nil)
 			call.List.Set1(n.Left)
 			call = typecheck(call, Erv)
 			call = walkexpr(call, init)
 			call = safeexpr(call, init)
-			var tab *Node
-			if n.Type.IsEmptyInterface() {
-				tab = typename(n.Left.Type)
-			} else {
-				tab = itabname(n.Left.Type, n.Type)
-			}
-			e := nod(OEFACE, tab, call)
-			e.Type = n.Type
+			e := nod(OEFACE, typeword(), call)
+			e.Type = toType
 			e.SetTypecheck(1)
 			n = e
 			break
 		}
 
-		var ll []*Node
-		if n.Type.IsEmptyInterface() {
-			if !n.Left.Type.IsInterface() {
-				ll = append(ll, typename(n.Left.Type))
-			}
-		} else {
-			if n.Left.Type.IsInterface() {
-				ll = append(ll, typename(n.Type))
+		var tab *Node
+		if fromType.IsInterface() {
+			// convI2I
+			tab = typename(toType)
 		} else {
-				ll = append(ll, itabname(n.Left.Type, n.Type))
-			}
+			// convT2x
+			tab = typeword()
 		}
 
 		v := n.Left
@@ -960,14 +949,13 @@ opswitch:
 			}
 			v = nod(OADDR, v, nil)
 		}
-		ll = append(ll, v)
 
-		dowidth(n.Left.Type)
+		dowidth(fromType)
 		fn := syslook(fnname)
-		fn = substArgTypes(fn, n.Left.Type, n.Type)
+		fn = substArgTypes(fn, fromType, toType)
 		dowidth(fn.Type)
 		n = nod(OCALL, fn, nil)
-		n.List.Set(ll)
+		n.List.Set2(tab, v)
 		n = typecheck(n, Erv)
 		n = walkexpr(n, init)