cmd/compile: fix evaluation of "" < s

Fixes #24817

Change-Id: Ifa79ab3dfe69297eeef85f7193cd5f85e5982bc5
Reviewed-on: https://go-review.googlesource.com/106655
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>

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

@@ -1275,6 +1275,13 @@ opswitch:
 		}
 		if cs != nil {
 			cmp := n.SubOp()
+			// Our comparison below assumes that the non-constant string
+			// is on the left hand side, so rewrite "" cmp x to x cmp "".
+			// See issue 24817.
+			if Isconst(n.Left, CTSTR) {
+				cmp = brrev(cmp)
+			}
+
 			// maxRewriteLen was chosen empirically.
 			// It is the value that minimizes cmd/go file size
 			// across most architectures.

test/fixedbugs/issue24817.go

@@ -0,0 +1,64 @@
+// run
+
+// Copyright 2018 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.
+
+// Check all ways to compare a non-constant string to the empty string.
+
+package main
+
+import (
+	"fmt"
+	"os"
+)
+
+var (
+	s      = "abc"
+	e      = ""
+	failed bool
+)
+
+func main() {
+	want(true, "" < s, `"" < s`)
+	want(false, s < "", `s < ""`)
+	want(false, "" < e, `"" < e`)
+	want(false, e < "", `e < ""`)
+
+	want(true, "" <= s, `"" <= s`)
+	want(false, s <= "", `s <= ""`)
+	want(true, "" <= e, `"" <= e`)
+	want(true, e <= "", `e <= ""`)
+
+	want(false, "" > s, `"" > s`)
+	want(true, s > "", `s > ""`)
+	want(false, "" > e, `"" > e`)
+	want(false, e > "", `e > ""`)
+
+	want(false, "" >= s, `"" >= s`)
+	want(true, s >= "", `s >= ""`)
+	want(true, "" >= e, `"" >= e`)
+	want(true, e >= "", `e >= ""`)
+
+	want(false, "" == s, `"" == s`)
+	want(false, s == "", `s == ""`)
+	want(true, "" == e, `"" == e`)
+	want(true, e == "", `e == ""`)
+
+	want(true, "" != s, `"" != s`)
+	want(true, s != "", `s != ""`)
+	want(false, "" != e, `"" != e`)
+	want(false, e != "", `e != ""`)
+
+	if failed {
+		os.Exit(1)
+	}
+}
+
+//go:noinline
+func want(b bool, have bool, msg string) {
+	if b != have {
+		fmt.Println(msg)
+		failed = true
+	}
+}