Add optimization for join of length 1

Author: msimacek

graalpython/com.oracle.graal.python.test/src/tests/unittest_tags/test_unicode.txt

@@ -13,6 +13,7 @@
 *graalpython.lib-python.3.test.test_unicode.UnicodeTest.test_additional_rsplit
 *graalpython.lib-python.3.test.test_unicode.UnicodeTest.test_additional_split
 *graalpython.lib-python.3.test.test_unicode.UnicodeTest.test_ascii
+*graalpython.lib-python.3.test.test_unicode.UnicodeTest.test_bug1001011
 *graalpython.lib-python.3.test.test_unicode.UnicodeTest.test_bytes_comparison
 *graalpython.lib-python.3.test.test_unicode.UnicodeTest.test_capitalize
 *graalpython.lib-python.3.test.test_unicode.UnicodeTest.test_capitalize_nonascii

graalpython/com.oracle.graal.python/src/com/oracle/graal/python/builtins/objects/str/StringNodes.java

@@ -256,7 +256,8 @@ static String doPSequence(VirtualFrame frame, String self, PSequence sequence,
                         @Cached @SuppressWarnings("unused") IsBuiltinClassProfile listProfile,
                         @Cached SequenceNodes.GetSequenceStorageNode getSequenceStorageNode,
                         @Cached SequenceStorageNodes.LenNode lenNode,
-                        @Cached("createBinaryProfile()") ConditionProfile isEmptyProfile,
+                        @Cached ConditionProfile isEmptyProfile,
+                        @Cached ConditionProfile isSingleItemProfile,
                         @Cached SequenceStorageNodes.GetItemNode getItemNode,
                         @Cached CastToJavaStringCheckedNode castToJavaStringNode,
                         @Cached PRaiseNode raise) {
@@ -275,6 +276,10 @@ static String doPSequence(VirtualFrame frame, String self, PSequence sequence,
             try {
                 // manually peel first iteration
                 Object item = getItemNode.execute(frame, storage, i);
+                // shortcut
+                if (isSingleItemProfile.profile(len == 1)) {
+                    return castToJavaStringNode.cast(item, INVALID_SEQ_ITEM, i, item);
+                }
                 StringUtils.append(sb, castToJavaStringNode.cast(item, INVALID_SEQ_ITEM, i, item));
 
                 for (i = 1; i < len; i++) {