cmd/compile: set correct type for OpIData

Since CL 270057, there're many attempts to fix the expand_calls pass
with interface{}-typed. But all of them did not fix the root cause. The
main issue is during SSA conversion in gc/ssa.go, for empty interface
case, we make its type as n.Type, instead of BytePtr.

To fix these, we can just use BytePtr for now, since when itab fields
are treated as scalar.

No significal changes on compiler speed, size.

cmd/compile/internal/ssa
expandCalls.func6 9488 -> 9232  (-2.70%)

file                       before   after    Δ       %
cmd/compile/internal/ssa.s 3992893  3992637  -256    -0.006%
total                      20500447 20500191 -256    -0.001%

Fixes #43112
Updates #42784
Updates #42727
Updates #42568

Change-Id: I0b15d9434e0be5448453e61f98ef9c2d6cd93792
Reviewed-on: https://go-review.googlesource.com/c/go/+/276952
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: Keith Randall <khr@golang.org>

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

@@ -5925,7 +5925,7 @@ func (s *state) dottype(n *Node, commaok bool) (res, resok *ssa.Value) {
 				// Load type out of itab, build interface with existing idata.
 				off := s.newValue1I(ssa.OpOffPtr, byteptr, int64(Widthptr), itab)
 				typ := s.load(byteptr, off)
-				idata := s.newValue1(ssa.OpIData, n.Type, iface)
+				idata := s.newValue1(ssa.OpIData, byteptr, iface)
 				res = s.newValue2(ssa.OpIMake, n.Type, typ, idata)
 				return
 			}
@@ -5947,7 +5947,7 @@ func (s *state) dottype(n *Node, commaok bool) (res, resok *ssa.Value) {
 			bOk.AddEdgeTo(bEnd)
 			bFail.AddEdgeTo(bEnd)
 			s.startBlock(bEnd)
-			idata := s.newValue1(ssa.OpIData, n.Type, iface)
+			idata := s.newValue1(ssa.OpIData, byteptr, iface)
 			res = s.newValue2(ssa.OpIMake, n.Type, s.variable(&typVar, byteptr), idata)
 			resok = cond
 			delete(s.vars, &typVar)

src/cmd/compile/internal/ssa/expand_calls.go

@@ -196,9 +196,6 @@ func expandCalls(f *Func) {
 			}
 			if leaf.Op == OpIData {
 				leafType = removeTrivialWrapperTypes(leaf.Type)
-				if leafType.IsEmptyInterface() {
-					leafType = typ.BytePtr
-				}
 			}
 			aux := selector.Aux
 			auxInt := selector.AuxInt + offset
@@ -247,12 +244,9 @@ func expandCalls(f *Func) {
 			// i.e., the struct select is generated and remains in because it is not applied to an actual structure.
 			// The OpLoad was created to load the single field of the IData
 			// This case removes that StructSelect.
-			if leafType != selector.Type && !selector.Type.IsEmptyInterface() { // empty interface for #42727
+			if leafType != selector.Type {
 				f.Fatalf("Unexpected Load as selector, leaf=%s, selector=%s\n", leaf.LongString(), selector.LongString())
 			}
-			if selector.Type.IsEmptyInterface() {
-				selector.Type = typ.BytePtr
-			}
 			leaf.copyOf(selector)
 			for _, s := range namedSelects[selector] {
 				locs = append(locs, f.Names[s.locIndex])

test/fixedbugs/issue43112.go

@@ -0,0 +1,41 @@
+// compile
+
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+type Symbol interface{}
+
+type Value interface {
+	String() string
+}
+
+type Object interface {
+	String() string
+}
+
+type Scope struct {
+	outer *Scope
+	elems map[string]Object
+}
+
+func (s *Scope) findouter(name string) (*Scope, Object) {
+	return s.outer.findouter(name)
+}
+
+func (s *Scope) Resolve(name string) (sym Symbol) {
+	if _, obj := s.findouter(name); obj != nil {
+		sym = obj.(Symbol)
+	}
+	return
+}
+
+type ScopeName struct {
+	scope *Scope
+}
+
+func (n *ScopeName) Get(name string) (Value, error) {
+	return n.scope.Resolve(name).(Value), nil
+}