Avoid bailing out to runtime for short substrings.

This significantly improves the speed for creating short substrings (less than 13 characters) from slices, flat cons strings and external strings.

TEST=string-external-cached.js, string-slices.js

Review URL: http://codereview.chromium.org/8889012

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@10221 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

test/mjsunit/string-external-cached.js

@@ -59,7 +59,7 @@ function test() {
   } catch (ex) { }
   assertEquals("1", charat_short.charAt(1));
 
-  // Test regexp.
+  // Test regexp and short substring.
   var re = /(A|B)/;
   var rere = /(T.{1,2}B)/;
   var ascii = "ABCDEFGHIJKLMNOPQRST";
@@ -81,6 +81,10 @@ function test() {
     assertEquals(["A", "A"], re.exec(twobyte));
     assertEquals(["B", "B"], re.exec(twobyte_slice));
     assertEquals(["T_AB", "T_AB"], rere.exec(twobyte_cons));
+    assertEquals("DEFG", ascii_slice.substr(2, 4));
+    assertEquals("DEFG", twobyte_slice.substr(2, 4));
+    assertEquals("DEFG", ascii_cons.substr(3, 4));
+    assertEquals("DEFG", twobyte_cons.substr(4, 4));
   }
 }
 

test/mjsunit/string-slices.js

@@ -160,6 +160,23 @@ for ( var i = 0; i < 1000; i++) {
   f(flat, cons, slice, i);
 }
 
+// Short substrings.
+flat = "abcdefghijkl12345";
+cons = flat + flat.toUpperCase();
+/x/.exec(cons);  // Flatten cons
+slice = "abcdefghijklmn12345".slice(1, -1);
+assertEquals("cdefg", flat.substr(2, 5));
+assertEquals("cdefg", cons.substr(2, 5));
+assertEquals("cdefg", slice.substr(1, 5));
+
+flat = "abc\u1234defghijkl12345";
+cons = flat + flat.toUpperCase();
+/x/.exec(cons);  // Flatten cons
+slice = "abc\u1234defghijklmn12345".slice(1, -1);
+assertEquals("c\u1234def", flat.substr(2, 5));
+assertEquals("c\u1234def", cons.substr(2, 5));
+assertEquals("c\u1234def", slice.substr(1, 5));
+
 // Concatenate substrings.
 var ascii = 'abcdefghijklmnop';
 var utf = '\u03B1\u03B2\u03B3\u03B4\u03B5\u03B6\u03B7\u03B8\u03B9\u03BA\u03BB';