[release-branch.go1.10] bytes, strings: fix comparison of long byte slices on s390x

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 #26117

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/124016
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 6b71830557..8b7a98d0c8 100644
--- a/src/runtime/asm_s390x.s
+++ b/src/runtime/asm_s390x.s
@@ -1032,6 +1032,8 @@ 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)