cmd/compile: inline _, ok = i.(T)

We already inlined

_, ok = e.(T)
_, ok = i.(E)
_, ok = e.(E)

The only ok-only variants not inlined are now

_, ok = i.(I)
_, ok = e.(I)

These call getitab, so are non-trivial.

Change-Id: Ie45fd8933ee179a679b92ce925079b94cff0ee12
Reviewed-on: https://go-review.googlesource.com/26658
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>

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

@@ -2317,6 +2317,16 @@ func isdirectiface(t *Type) bool {
 	return false
 }
 
+// itabType loads the _type field from a runtime.itab struct.
+func itabType(itab *Node) *Node {
+	typ := NodSym(ODOTPTR, itab, nil)
+	typ.Type = Ptrto(Types[TUINT8])
+	typ.Typecheck = 1
+	typ.Xoffset = int64(Widthptr) // offset of _type in runtime.itab
+	typ.Bounded = true            // guaranteed not to fault
+	return typ
+}
+
 // iet returns 'T' if t is a concrete type,
 // 'I' if t is an interface type, and 'E' if t is an empty interface type.
 // It is used to build calls to the conv* and assert* runtime routines.

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

@@ -589,11 +589,7 @@ func (s *typeSwitch) walk(sw *Node) {
 
 	if !cond.Right.Type.IsEmptyInterface() {
 		// Load type from itab.
-		typ = NodSym(ODOTPTR, typ, nil)
-		typ.Type = Ptrto(Types[TUINT8])
-		typ.Typecheck = 1
-		typ.Xoffset = int64(Widthptr) // offset of _type in runtime.itab
-		typ.Bounded = true            // guaranteed not to fault
+		typ = itabType(typ)
 	}
 	// Load hash from type.
 	h := NodSym(ODOTPTR, typ, nil)

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

@@ -960,19 +960,27 @@ opswitch:
 		fromKind := from.Type.iet()
 		toKind := t.iet()
 
+		res := n.List.First()
+
 		// Avoid runtime calls in a few cases of the form _, ok := i.(T).
 		// This is faster and shorter and allows the corresponding assertX2X2
 		// routines to skip nil checks on their last argument.
-		if isblank(n.List.First()) {
+		if isblank(res) {
 			var fast *Node
-			switch {
-			case fromKind == 'E' && toKind == 'T':
-				tab := Nod(OITAB, from, nil) // type:eface::tab:iface
-				typ := Nod(OCONVNOP, typename(t), nil)
-				typ.Type = Ptrto(Types[TUINTPTR])
-				fast = Nod(OEQ, tab, typ)
-			case fromKind == 'I' && toKind == 'E',
-				fromKind == 'E' && toKind == 'E':
+			switch toKind {
+			case 'T':
+				tab := Nod(OITAB, from, nil)
+				if fromKind == 'E' {
+					typ := Nod(OCONVNOP, typename(t), nil)
+					typ.Type = Ptrto(Types[TUINTPTR])
+					fast = Nod(OEQ, tab, typ)
+					break
+				}
+				fast = Nod(OANDAND,
+					Nod(ONE, nodnil(), tab),
+					Nod(OEQ, itabType(tab), typename(t)),
+				)
+			case 'E':
 				tab := Nod(OITAB, from, nil)
 				fast = Nod(ONE, nodnil(), tab)
 			}
@@ -987,10 +995,10 @@ opswitch:
 		}
 
 		var resptr *Node // &res
-		if isblank(n.List.First()) {
+		if isblank(res) {
 			resptr = nodnil()
 		} else {
-			resptr = Nod(OADDR, n.List.First(), nil)
+			resptr = Nod(OADDR, res, nil)
 		}
 		resptr.Etype = 1 // addr does not escape
 

src/runtime/iface.go

@@ -218,20 +218,17 @@ func assertI2T(t *_type, i iface, r unsafe.Pointer) {
 	}
 }
 
+// The compiler ensures that r is non-nil.
 func assertI2T2(t *_type, i iface, r unsafe.Pointer) bool {
 	tab := i.tab
 	if tab == nil || tab._type != t {
-		if r != nil {
-			memclr(r, t.size)
-		}
+		memclr(r, t.size)
 		return false
 	}
-	if r != nil {
-		if isDirectIface(t) {
-			writebarrierptr((*uintptr)(r), uintptr(i.data))
-		} else {
-			typedmemmove(t, r, i.data)
-		}
+	if isDirectIface(t) {
+		writebarrierptr((*uintptr)(r), uintptr(i.data))
+	} else {
+		typedmemmove(t, r, i.data)
 	}
 	return true
 }