[release-branch.go1.9] bytes, strings: fix comparison of long byte slices on s390xrelease-branch.go1.9

The existing implementation of bytes.Compare on s390x doesn't work properly for slices longer
than 256 elements. This change fixes that. Added tests for long strings and slices of bytes.

Fixes #26118

Change-Id: If6d8b68ee6dbcf99a24f867a1d3438b1f208954f
Reviewed-on: https://go-review.googlesource.com/121495
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-on: https://go-review.googlesource.com/124137
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

3 files changed, 28 insertions, 4 deletions

diff --git a/src/bytes/compare_test.go b/src/bytes/compare_test.go
index 35088a1b2e..3e33c27c9c 100644
--- a/src/bytes/compare_test.go
+++ b/src/bytes/compare_test.go
@@ -6,6 +6,7 @@ package bytes_test
 
 import (
 	. "bytes"
+	"internal/testenv"
 	"testing"
 )
 
@@ -58,10 +59,20 @@ func TestCompareIdenticalSlice(t *testing.T) {
 }
 
 func TestCompareBytes(t *testing.T) {
-	n := 128
+	lengths := make([]int, 0) // lengths to test in ascending order
+	for i := 0; i <= 128; i++ {
+		lengths = append(lengths, i)
+	}
+	lengths = append(lengths, 256, 512, 1024, 1333, 4095, 4096, 4097)
+
+	if !testing.Short() || testenv.Builder() != "" {
+		lengths = append(lengths, 65535, 65536, 65537, 99999)
+	}
+
+	n := lengths[len(lengths)-1]
 	a := make([]byte, n+1)
 	b := make([]byte, n+1)
-	for len := 0; len < 128; len++ {
+	for _, len := range lengths {
 		// randomish but deterministic data. No 0 or 255.
 		for i := 0; i < len; i++ {
 			a[i] = byte(1 + 31*i%254)
diff --git a/src/runtime/asm_s390x.s b/src/runtime/asm_s390x.s
index 20e740b927..686e3db411 100644
--- a/src/runtime/asm_s390x.s
+++ b/src/runtime/asm_s390x.s
@@ -1004,6 +1004,8 @@ loop:
 	BGT	gt
 	BLT	lt
 	SUB	$256, R8
+	MOVD	$256(R3), R3
+	MOVD	$256(R5), R5
 	CMP	R8, $256
 	BGT	loop
 tail:
diff --git a/src/strings/compare_test.go b/src/strings/compare_test.go
index bc12e421b0..712e5a741e 100644
--- a/src/strings/compare_test.go
+++ b/src/strings/compare_test.go
@@ -8,6 +8,7 @@ package strings_test
 // Benchmarks omitted since the underlying implementation is identical.
 
 import (
+	"internal/testenv"
 	. "strings"
 	"testing"
 )
@@ -52,10 +53,20 @@ func TestCompareIdenticalString(t *testing.T) {
 }
 
 func TestCompareStrings(t *testing.T) {
-	n := 128
+	lengths := make([]int, 0) // lengths to test in ascending order
+	for i := 0; i <= 128; i++ {
+		lengths = append(lengths, i)
+	}
+	lengths = append(lengths, 256, 512, 1024, 1333, 4095, 4096, 4097)
+
+	if !testing.Short() || testenv.Builder() != "" {
+		lengths = append(lengths, 65535, 65536, 65537, 99999)
+	}
+
+	n := lengths[len(lengths)-1]
 	a := make([]byte, n+1)
 	b := make([]byte, n+1)
-	for len := 0; len < 128; len++ {
+	for _, len := range lengths {
 		// randomish but deterministic data. No 0 or 255.
 		for i := 0; i < len; i++ {
 			a[i] = byte(1 + 31*i%254)