[release-branch.go1.9] cmd/compile: fix decomposition of 1-element arrays

The offending rule could move the load to a different block,
which is always a bad idea.

Fixes #22683

Change-Id: I973c88389b2359f734924d9f45c3fb38e166691d
Reviewed-on: https://go-review.googlesource.com/77331
Run-TryBot: Andrew Bonventre <[email protected]>
Reviewed-by: Keith Randall <[email protected]>
Reviewed-by: David Chase <[email protected]>

Author: randall77

src/cmd/compile/internal/ssa/gen/generic.rules

@@ -819,7 +819,7 @@
 (Store _ (ArrayMake0) mem) -> mem
 (Store dst (ArrayMake1 e) mem) -> (Store {e.Type} dst e mem)
 
-(ArraySelect [0] (Load ptr mem)) -> (Load ptr mem)
+(ArraySelect [0] x:(Load ptr mem)) -> @x.Block (Load <v.Type> ptr mem)
 
 // Putting [1]{*byte} and similar into direct interfaces.
 (IMake typ (ArrayMake1 val)) -> (IMake typ val)

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

@@ -0,0 +1,30 @@
+// cmpout
+
+// Copyright 2017 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 main
+
+import (
+	"fmt"
+)
+
+type foo struct {
+	bar [1]*int
+}
+
+func main() {
+	ch := make(chan foo, 2)
+	var a int
+	var b [1]*int
+	b[0] = &a
+	ch <- foo{bar: b}
+	close(ch)
+
+	for v := range ch {
+		for i := 0; i < 1; i++ {
+			fmt.Println(v.bar[0] != nil)
+		}
+	}
+}

test/fixedbugs/issue22683.go

@@ -0,0 +1 @@
+true