bytes: vary the input alignment to Compare argument in compare_test.go

Currently there are no tests that vary the alignment of Compare arguments.
Since Compare is written in assembly on most platforms (in internal/bytealg)
we should be testing different input alignments. This change modifies TestCompare
to vary the alignment of the second argument of Compare.

Updates #26129

Change-Id: I4c30a5adf96a41225df748675f4e9beea413b35c
Reviewed-on: https://go-review.googlesource.com/c/122536Reviewed-by: Lotus Fenn <fenn.lotus@gmail.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/bytes/compare_test.go

@@ -41,9 +41,16 @@ var compareTests = []struct {
 
 func TestCompare(t *testing.T) {
 	for _, tt := range compareTests {
-		cmp := Compare(tt.a, tt.b)
-		if cmp != tt.i {
-			t.Errorf(`Compare(%q, %q) = %v`, tt.a, tt.b, cmp)
+		numShifts := 16
+		buffer := make([]byte, len(tt.b)+numShifts)
+		// vary the input alignment of tt.b
+		for offset := 0; offset <= numShifts; offset++ {
+			shiftedB := buffer[offset : len(tt.b)+offset]
+			copy(shiftedB, tt.b)
+			cmp := Compare(tt.a, shiftedB)
+			if cmp != tt.i {
+				t.Errorf(`Compare(%q, %q), offset %d = %v; want %v`, tt.a, tt.b, offset, cmp, tt.i)
+			}
 		}
 	}
 }