cmd/compile: add x.Uses==1 test to load combiners

We need to make sure that when we combine loads, we only do
so if there are no other uses of the load.  We can't split
one load into two because that can then lead to inconsistent
loaded values in the presence of races.

Add some aggressive copy removal code so that phantom
"dead copy" uses of values are cleaned up promptly.  This lets
us use x.Uses==1 conditions reliably.

Change-Id: I9037311db85665f3868dbeb3adb3de5c20728b38
Reviewed-on: https://go-review.googlesource.com/21853
Reviewed-by: Todd Neal <todd@tneal.org>

2 files changed, 50 insertions, 0 deletions

diff --git a/src/cmd/compile/internal/gc/testdata/dupLoad.go b/src/cmd/compile/internal/gc/testdata/dupLoad.go
new file mode 100644
index 0000000000..d12c26355a
--- /dev/null
+++ b/src/cmd/compile/internal/gc/testdata/dupLoad.go
@@ -0,0 +1,46 @@
+// run
+
+// Copyright 2016 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.
+
+// This test makes sure that we don't split a single
+// load up into two separate loads.
+
+package main
+
+import "fmt"
+
+//go:noinline
+func read(b []byte) (uint16, uint16) {
+	// There is only a single read of b[0].  The two
+	// returned values must have the same low byte.
+	v := b[0]
+	return uint16(v), uint16(v) | uint16(b[1])<<8
+}
+
+const N = 100000
+
+func main() {
+	done := make(chan struct{})
+	b := make([]byte, 2)
+	go func() {
+		for i := 0; i < N; i++ {
+			b[0] = byte(i)
+			b[1] = byte(i)
+		}
+		done <- struct{}{}
+	}()
+	go func() {
+		for i := 0; i < N; i++ {
+			x, y := read(b)
+			if byte(x) != byte(y) {
+				fmt.Printf("x=%x y=%x\n", x, y)
+				panic("bad")
+			}
+		}
+		done <- struct{}{}
+	}()
+	<-done
+	<-done
+}
diff --git a/src/cmd/compile/internal/gc/testdata/namedReturn.go b/src/cmd/compile/internal/gc/testdata/namedReturn.go
index dafb5d719f..19ef8a7e43 100644
--- a/src/cmd/compile/internal/gc/testdata/namedReturn.go
+++ b/src/cmd/compile/internal/gc/testdata/namedReturn.go
@@ -1,5 +1,9 @@
 // run
 
+// Copyright 2016 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.
+
 // This test makes sure that naming named
 // return variables in a return statement works.
 // See issue #14904.