[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 <andybons@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: David Chase <drchase@google.com>

4 files changed, 47 insertions, 11 deletions

diff --git a/src/cmd/compile/internal/ssa/gen/generic.rules b/src/cmd/compile/internal/ssa/gen/generic.rules
index 944a84df85..10e49fc4f7 100644
--- a/src/cmd/compile/internal/ssa/gen/generic.rules
+++ b/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)
diff --git a/src/cmd/compile/internal/ssa/rewritegeneric.go b/src/cmd/compile/internal/ssa/rewritegeneric.go
index c67e4f90eb..fddfebdf42 100644
--- a/src/cmd/compile/internal/ssa/rewritegeneric.go
+++ b/src/cmd/compile/internal/ssa/rewritegeneric.go
@@ -5670,6 +5670,8 @@ func rewriteValuegeneric_OpArg_10(v *Value) bool {
 	return false
 }
 func rewriteValuegeneric_OpArraySelect_0(v *Value) bool {
+	b := v.Block
+	_ = b
 	// match: (ArraySelect (ArrayMake1 x))
 	// cond:
 	// result: x
@@ -5684,23 +5686,26 @@ func rewriteValuegeneric_OpArraySelect_0(v *Value) bool {
 		v.AddArg(x)
 		return true
 	}
-	// match: (ArraySelect [0] (Load ptr mem))
+	// match: (ArraySelect [0] x:(Load ptr mem))
 	// cond:
-	// result: (Load ptr mem)
+	// result: @x.Block (Load <v.Type> ptr mem)
 	for {
 		if v.AuxInt != 0 {
 			break
 		}
-		v_0 := v.Args[0]
-		if v_0.Op != OpLoad {
+		x := v.Args[0]
+		if x.Op != OpLoad {
 			break
 		}
-		_ = v_0.Args[1]
-		ptr := v_0.Args[0]
-		mem := v_0.Args[1]
-		v.reset(OpLoad)
-		v.AddArg(ptr)
-		v.AddArg(mem)
+		_ = x.Args[1]
+		ptr := x.Args[0]
+		mem := x.Args[1]
+		b = x.Block
+		v0 := b.NewValue0(v.Pos, OpLoad, v.Type)
+		v.reset(OpCopy)
+		v.AddArg(v0)
+		v0.AddArg(ptr)
+		v0.AddArg(mem)
 		return true
 	}
 	// match: (ArraySelect [0] x:(IData _))
diff --git a/test/fixedbugs/issue22683.go b/test/fixedbugs/issue22683.go
new file mode 100644
index 0000000000..a59a0edaf4
--- /dev/null
+++ b/test/fixedbugs/issue22683.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)
+		}
+	}
+}
diff --git a/test/fixedbugs/issue22683.out b/test/fixedbugs/issue22683.out
new file mode 100644
index 0000000000..27ba77ddaf
--- /dev/null
+++ b/test/fixedbugs/issue22683.out
@@ -0,0 +1 @@
+true