cmd/compile: remove check that Zero's arg has the correct base type

It doesn't have to. The type in the aux field is authoritative.
There are cases involving casting from interface{} where pointers
have a placeholder pointer type (because the type is not known when
the IData op is generated).

The check was introduced in CL 13447.

Fixes #39459

Change-Id: Id77a57577806a271aeebd20bea5d92d08ee7aa6b
Reviewed-on: https://go-review.googlesource.com/c/go/+/239817
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: David Chase <drchase@google.com>

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

@@ -73,9 +73,11 @@ func dse(f *Func) {
 		}
 
 		// Walk backwards looking for dead stores. Keep track of shadowed addresses.
-		// An "address" is an SSA Value which encodes both the address and size of
-		// the write. This code will not remove dead stores to the same address
-		// of different types.
+		// A "shadowed address" is a pointer and a size describing a memory region that
+		// is known to be written. We keep track of shadowed addresses in the shadowed
+		// map, mapping the ID of the address to the size of the shadowed region.
+		// Since we're walking backwards, writes to a shadowed region are useless,
+		// as they will be immediately overwritten.
 		shadowed.clear()
 		v := last
 
@@ -93,17 +95,13 @@ func dse(f *Func) {
 				sz = v.AuxInt
 			}
 			if shadowedSize := int64(shadowed.get(v.Args[0].ID)); shadowedSize != -1 && shadowedSize >= sz {
-				// Modify store into a copy
+				// Modify the store/zero into a copy of the memory state,
+				// effectively eliding the store operation.
 				if v.Op == OpStore {
 					// store addr value mem
 					v.SetArgs1(v.Args[2])
 				} else {
 					// zero addr mem
-					typesz := v.Args[0].Type.Elem().Size()
-					if sz != typesz {
-						f.Fatalf("mismatched zero/store sizes: %d and %d [%s]",
-							sz, typesz, v.LongString())
-					}
 					v.SetArgs1(v.Args[1])
 				}
 				v.Aux = nil

test/fixedbugs/issue39459.go

@@ -0,0 +1,22 @@
+// 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 T struct { // big enough to be an unSSAable type
+	a, b, c, d, e, f int
+}
+
+func f(x interface{}, p *int) {
+	_ = *p // trigger nil check here, removing it from below
+	switch x := x.(type) {
+	case *T:
+		// Zero twice, so one of them will be removed by the deadstore pass
+		*x = T{}
+		*p = 0 // store op to prevent Zero ops from being optimized by the earlier opt pass rewrite rules
+		*x = T{}
+	}
+}