[release-branch.go1.22] cmd/compile: fix copying SSA-able variables o…

…ptimization

CL 541715 added an optimization to copy SSA-able variables.

When handling m[k] = append(m[k], ...) case, it uses ir.SameSafeExpr to
check that m[k] expressions are the same, then doing type assertion to
convert the map index to ir.IndexExpr node. However, this assertion is
not safe for m[k] expression in append(m[k], ...), since it may be
wrapped by ir.OCONVNOP node.

Fixing this by un-wrapping any ir.OCONVNOP before doing type assertion.

Fixes #66178

Change-Id: I9ff7165ab97bc7f88d0e9b7b31604da19a8ca206
Reviewed-on: https://go-review.googlesource.com/c/go/+/569716
Auto-Submit: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Reviewed-on: https://go-review.googlesource.com/c/go/+/569818
Reviewed-by: David Chase <drchase@google.com>

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

@@ -643,7 +643,12 @@ func (o *orderState) stmt(n ir.Node) {
 			indexLHS.Index = o.cheapExpr(indexLHS.Index)
 
 			call := n.Y.(*ir.CallExpr)
-			indexRHS := call.Args[0].(*ir.IndexExpr)
+			arg0 := call.Args[0]
+			// ir.SameSafeExpr skips OCONVNOPs, so we must do the same here (#66096).
+			for arg0.Op() == ir.OCONVNOP {
+				arg0 = arg0.(*ir.ConvExpr).X
+			}
+			indexRHS := arg0.(*ir.IndexExpr)
 			indexRHS.X = indexLHS.X
 			indexRHS.Index = indexLHS.Index
 

test/fixedbugs/issue66096.go

@@ -0,0 +1,17 @@
+// compile
+
+// Copyright 2024 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 Message struct {
+	Header map[string][]string
+}
+
+func f() {
+	m := Message{Header: map[string][]string{}}
+	m.Header[""] = append([]string(m.Header[""]), "")
+	_ = m
+}