string.c: SHARABLE_SUBSTRING_P

* string.c (SHARABLE_SUBSTRING_P): predicate if substring can be
  shared with the original string.  true if just at the end of the
  original string, for the time being.  all substring will be able to
  be shared in the future.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@45633 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

ChangeLog

@@ -1,3 +1,10 @@
+Sat Apr 19 00:17:20 2014  Nobuyoshi Nakada  <nobu@ruby-lang.org>
+
+	* string.c (SHARABLE_SUBSTRING_P): predicate if substring can be
+	  shared with the original string.  true if just at the end of the
+	  original string, for the time being.  all substring will be able to
+	  be shared in the future.
+
 Fri Apr 18 21:48:24 2014  Nobuyoshi Nakada  <nobu@ruby-lang.org>
 
 	* string.c (rb_str_new_frozen): consider the shared string at

string.c

@@ -121,6 +121,12 @@ VALUE rb_cSymbol;
 
 #define STR_ENC_GET(str) get_encoding(str)
 
+#if 1
+#define SHARABLE_SUBSTRING_P(beg, len, end) ((beg) + (len) == (end))
+#else
+#define SHARABLE_SUBSTRING_P(beg, len, end) 1
+#endif
+
 rb_encoding *rb_enc_get_from_index(int index);
 
 static rb_encoding *
@@ -1790,7 +1796,7 @@ rb_str_subseq(VALUE str, long beg, long len)
 {
     VALUE str2;
 
-    if (RSTRING_EMBED_LEN_MAX < len) {
+    if (RSTRING_EMBED_LEN_MAX < len && SHARABLE_SUBSTRING_P(beg, len, RSTRING_LEN(str))) {
 	long olen;
 	str2 = rb_str_new_shared(rb_str_new_frozen(str));
 	RSTRING(str2)->as.heap.ptr += beg;
@@ -1900,7 +1906,7 @@ rb_str_substr(VALUE str, long beg, long len)
     char *p = rb_str_subpos(str, beg, &len);
 
     if (!p) return Qnil;
-    if (len > RSTRING_EMBED_LEN_MAX) {
+    if (len > RSTRING_EMBED_LEN_MAX && SHARABLE_SUBSTRING_P(p, len, RSTRING_END(str))) {
 	long ofs = p - RSTRING_PTR(str);
 	str2 = rb_str_new_frozen(str);
 	str2 = str_new_shared(rb_obj_class(str2), str2);
@@ -4413,7 +4419,7 @@ str_byte_substr(VALUE str, long beg, long len)
     else
 	p = s + beg;
 
-    if (len > RSTRING_EMBED_LEN_MAX) {
+    if (len > RSTRING_EMBED_LEN_MAX && SHARABLE_SUBSTRING_P(beg, len, n)) {
 	str2 = rb_str_new_frozen(str);
 	str2 = str_new_shared(rb_obj_class(str2), str2);
 	RSTRING(str2)->as.heap.ptr += beg;