cmd/compile: avoid applying ARM CMP->CMN rewrite in unsigned context

Fixes #41780.

Change-Id: I1dc7c19a9f057650905da3a96214c2ff4abb51be
Reviewed-on: https://go-review.googlesource.com/c/go/+/259450
Trust: David Chase <drchase@google.com>
Run-TryBot: David Chase <drchase@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>

1 files changed, 39 insertions, 0 deletions

diff --git a/test/fixedbugs/issue41780.go b/test/fixedbugs/issue41780.go
new file mode 100644
index 0000000000..632c144a48
--- /dev/null
+++ b/test/fixedbugs/issue41780.go
@@ -0,0 +1,39 @@
+// 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.
+
+// Checks that conversion of CMP(x,-y) -> CMN(x,y) is only applied in correct context.
+
+package main
+
+type decimal struct {
+	d  [8]byte // digits, big-endian representation
+	dp int     // decimal point
+}
+
+var powtab = []int{1, 3, 6, 9, 13, 16, 19, 23, 26}
+
+//go:noinline
+func foo(d *decimal) int {
+	exp := int(d.d[1])
+	if d.dp < 0 || d.dp == 0 && d.d[0] < '5' {
+		var n int
+		if -d.dp >= len(powtab) {
+			n = 27
+		} else {
+			n = powtab[-d.dp] // incorrect CMP -> CMN substitution causes indexing panic.
+		}
+		exp += n
+	}
+	return exp
+}
+
+func main() {
+	var d decimal
+	d.d[0] = '1'
+	if foo(&d) != 1 {
+		println("FAILURE (though not the one this test was written to catch)")
+	}
+}