[dev.regabi] cmd/compile: fix map assignment order

After the previous cleanup/optimization CLs, ascompatee now correctly
handles map assignments too. So remove the code from order.mapAssign,
which causes us to assign to the map at the wrong point during
execution. It's not every day you get to fix an issue by only removing
code.

Thanks to Cuong Manh Le for test cases and continually following up on
this issue.

Passes toolstash -cmp. (Apparently the standard library never uses
tricky map assignments. Go figure.)

Fixes #23017.

Change-Id: Ie0728103d59d884d00c1c050251290a2a46150f9
Reviewed-on: https://go-review.googlesource.com/c/go/+/281172
Trust: Matthew Dempsky <mdempsky@google.com>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>

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

@@ -537,21 +537,7 @@ func (o *orderState) call(nn ir.Node) {
 	}
 }
 
-// mapAssign appends n to o.out, introducing temporaries
-// to make sure that all map assignments have the form m[k] = x.
-// (Note: expr has already been called on n, so we know k is addressable.)
-//
-// If n is the multiple assignment form ..., m[k], ... = ..., x, ..., the rewrite is
-//	t1 = m
-//	t2 = k
-//	...., t3, ... = ..., x, ...
-//	t1[t2] = t3
-//
-// The temporaries t1, t2 are needed in case the ... being assigned
-// contain m or k. They are usually unnecessary, but in the unnecessary
-// cases they are also typically registerizable, so not much harm done.
-// And this only applies to the multiple-assignment form.
-// We could do a more precise analysis if needed, like in walk.go.
+// mapAssign appends n to o.out.
 func (o *orderState) mapAssign(n ir.Node) {
 	switch n.Op() {
 	default:
@@ -572,28 +558,7 @@ func (o *orderState) mapAssign(n ir.Node) {
 
 	case ir.OAS2, ir.OAS2DOTTYPE, ir.OAS2MAPR, ir.OAS2FUNC:
 		n := n.(*ir.AssignListStmt)
-		var post []ir.Node
-		for i, m := range n.Lhs {
-			switch {
-			case m.Op() == ir.OINDEXMAP:
-				m := m.(*ir.IndexExpr)
-				if !ir.IsAutoTmp(m.X) {
-					m.X = o.copyExpr(m.X)
-				}
-				if !ir.IsAutoTmp(m.Index) {
-					m.Index = o.copyExpr(m.Index)
-				}
-				fallthrough
-			case base.Flag.Cfg.Instrumenting && n.Op() == ir.OAS2FUNC && !ir.IsBlank(m):
-				t := o.newTemp(m.Type(), false)
-				n.Lhs[i] = t
-				a := ir.NewAssignStmt(base.Pos, m, t)
-				post = append(post, typecheck.Stmt(a))
-			}
-		}
-
 		o.out = append(o.out, n)
-		o.out = append(o.out, post...)
 	}
 }
 

test/fixedbugs/issue23017.go

@@ -0,0 +1,113 @@
+// run
+
+// 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.
+
+// assignment order in multiple assignments.
+// See issue #23017
+
+package main
+
+import "fmt"
+
+func main() {}
+
+func init() {
+	var m = map[int]int{}
+	var p *int
+
+	defer func() {
+		recover()
+		check(1, len(m))
+		check(42, m[2])
+	}()
+	m[2], *p = 42, 2
+}
+
+func init() {
+	var m = map[int]int{}
+	p := []int{}
+
+	defer func() {
+		recover()
+		check(1, len(m))
+		check(2, m[2])
+	}()
+	m[2], p[1] = 2, 2
+}
+
+func init() {
+	type P struct{ i int }
+	var m = map[int]int{}
+	var p *P
+
+	defer func() {
+		recover()
+		check(1, len(m))
+		check(3, m[2])
+	}()
+	m[2], p.i = 3, 2
+}
+
+func init() {
+	type T struct{ i int }
+	var x T
+	p := &x
+	p, p.i = new(T), 4
+	check(4, x.i)
+}
+
+func init() {
+	var m map[int]int
+	var a int
+	var p = &a
+
+	defer func() {
+		recover()
+		check(5, *p)
+	}()
+	*p, m[2] = 5, 2
+}
+
+var g int
+
+func init() {
+	var m map[int]int
+	defer func() {
+		recover()
+		check(0, g)
+	}()
+	m[0], g = 1, 2
+}
+
+func init() {
+	type T struct{ x struct{ y int } }
+	var x T
+	p := &x
+	p, p.x.y = new(T), 7
+	check(7, x.x.y)
+	check(0, p.x.y)
+}
+
+func init() {
+	type T *struct{ x struct{ y int } }
+	x := struct{ y int }{0}
+	var q T = &struct{ x struct{ y int } }{x}
+	p := q
+	p, p.x.y = nil, 7
+	check(7, q.x.y)
+}
+
+func init() {
+	x, y := 1, 2
+	x, y = y, x
+	check(2, x)
+	check(1, y)
+}
+
+func check(want, got int) {
+	if want != got {
+		panic(fmt.Sprintf("wanted %d, but got %d", want, got))
+	}
+}