cmd/compile: fix load of interface{}-typed OpIData in expand_calls

In certain cases, the declkared type of an OpIData is interface{}.
This was not expected (since interface{} is a pair, right?) and
thus caused a crash.  What is intended is that these be treated as
a byteptr, so do that instead (this is what happens in 1.15).

Fixes #42568.

Change-Id: Id7c9e5dc2cbb5d7c71c6748832491ea62b0b339f
Reviewed-on: https://go-review.googlesource.com/c/go/+/270057
Trust: David Chase <drchase@google.com>
Run-TryBot: David Chase <drchase@google.com>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>

src/cmd/compile/internal/amd64/ssa.go

@@ -76,7 +76,7 @@ func storeByType(t *types.Type) obj.As {
 			return x86.AMOVQ
 		}
 	}
-	panic("bad store type")
+	panic(fmt.Sprintf("bad store type %v", t))
 }
 
 // moveByType returns the reg->reg move instruction of the given type.
@@ -101,7 +101,7 @@ func moveByType(t *types.Type) obj.As {
 		case 16:
 			return x86.AMOVUPS // int128s are in SSE registers
 		default:
-			panic(fmt.Sprintf("bad int register width %d:%s", t.Size(), t))
+			panic(fmt.Sprintf("bad int register width %d:%v", t.Size(), t))
 		}
 	}
 }

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

@@ -196,6 +196,9 @@ 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

test/fixedbugs/issue42568.go

@@ -0,0 +1,25 @@
+// 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.
+
+// Ensure that late expansion correctly handles an OpIData with type interface{}
+
+package p
+
+type S struct{}
+
+func (S) M() {}
+
+type I interface {
+	M()
+}
+
+func f(i I) {
+	o := i.(interface{})
+	if _, ok := i.(*S); ok {
+		o = nil
+	}
+	println(o)
+}