Revert "Revert "cmd/compile: inline convI2E""

This reverts commit 7dd9c385.

Reason for revert: Reverting the revert, which will re-enable the convI2E optimization.  We originally reverted the convI2E optimization because it was making the builder fail, but the underlying cause was later determined to be unrelated.

Original CL: https://go-review.googlesource.com/31260
Revert CL: https://go-review.googlesource.com/31310
Real bug: https://go-review.googlesource.com/c/25159
Real fix: https://go-review.googlesource.com/c/31316

Change-Id: I17237bb577a23a7675a5caab970ccda71a4124f2
Reviewed-on: https://go-review.googlesource.com/32023
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>

src/cmd/compile/internal/gc/builtin/runtime.go

@@ -58,7 +58,6 @@ func slicecopy(to any, fr any, wid uintptr) int
 func slicestringcopy(to any, fr any) int
 
 // interface conversions
-func convI2E(elem any) (ret any)
 func convI2I(typ *byte, elem any) (ret any)
 func convT2E(typ *byte, elem *any) (ret any)
 func convT2I(tab *byte, elem *any) (ret any)

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

@@ -410,16 +410,14 @@ func walkexprlistcheap(s []*Node, init *Nodes) {
 	}
 }
 
-// Build name of function: convI2E etc.
+// Build name of function for interface conversion.
 // Not all names are possible
-// (e.g., we'll never generate convE2E or convE2I).
+// (e.g., we'll never generate convE2E or convE2I or convI2E).
 func convFuncName(from, to *Type) string {
 	tkind := to.iet()
 	switch from.iet() {
 	case 'I':
 		switch tkind {
-		case 'E':
-			return "convI2E"
 		case 'I':
 			return "convI2I"
 		}
@@ -1080,6 +1078,34 @@ opswitch:
 			break
 		}
 
+		// Implement interface to empty interface conversion.
+		// tmp = i.itab
+		// if tmp != nil {
+		//    tmp = tmp.type
+		// }
+		// e = iface{tmp, i.data}
+		if n.Type.IsEmptyInterface() && n.Left.Type.IsInterface() && !n.Left.Type.IsEmptyInterface() {
+			// Evaluate the input interface.
+			c := temp(n.Left.Type)
+			init.Append(nod(OAS, c, n.Left))
+
+			// Get the itab out of the interface.
+			tmp := temp(ptrto(Types[TUINT8]))
+			init.Append(nod(OAS, tmp, typecheck(nod(OITAB, c, nil), Erv)))
+
+			// Get the type out of the itab.
+			nif := nod(OIF, typecheck(nod(ONE, tmp, nodnil()), Erv), nil)
+			nif.Nbody.Set1(nod(OAS, tmp, itabType(tmp)))
+			init.Append(nif)
+
+			// Build the result.
+			e := nod(OEFACE, tmp, ifaceData(c, ptrto(Types[TUINT8])))
+			e.Type = n.Type // assign type manually, typecheck doesn't understand OEFACE.
+			e.Typecheck = 1
+			n = e
+			break
+		}
+
 		var ll []*Node
 		if n.Type.IsEmptyInterface() {
 			if !n.Left.Type.IsInterface() {

src/runtime/iface.go

@@ -264,16 +264,6 @@ func assertE2T2(t *_type, e eface, r unsafe.Pointer) bool {
 	return true
 }
 
-func convI2E(i iface) (r eface) {
-	tab := i.tab
-	if tab == nil {
-		return
-	}
-	r._type = tab._type
-	r.data = i.data
-	return
-}
-
 func assertI2E(inter *interfacetype, i iface, r *eface) {
 	tab := i.tab
 	if tab == nil {