Avoid the KMP overhead for simple indexOf() operations. Will look into...

Avoid the KMP overhead for simple indexOf() operations.  Will look into evaluating the best cutoff between a simple search and KMP in the future.  This improves some simple operations ~1.5x.

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@349 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

src/runtime.cc

@@ -911,12 +911,12 @@ static Object* Runtime_StringIndexOf(Arguments args) {
   CONVERT_CHECKED(String, pat, args[1]);
   Object* index = args[2];
 
-  int subject_length = sub->length();
-  int pattern_length = pat->length();
-
   sub->TryFlatten();
   pat->TryFlatten();
 
+  int subject_length = sub->length();
+  int pattern_length = pat->length();
+
   uint32_t start_index;
   if (!Array::IndexFromObject(index, &start_index)) return Smi::FromInt(-1);
   if (pattern_length == 0) return Smi::FromInt(start_index);
@@ -934,8 +934,23 @@ static Object* Runtime_StringIndexOf(Arguments args) {
     return Smi::FromInt(-1);
   }
 
-  // For patterns with a length larger than one character we use the KMP
-  // algorithm.
+  // For small searches, KMP is not worth the setup overhead.
+  if (subject_length < 100) {
+    // We know our pattern is at least 2 characters, we cache the first so
+    // the common case of the first character not matching is faster.
+    uint16_t pattern_first_char = pat->Get(0);
+    for (int i = start_index; i + pattern_length <= subject_length; i++) {
+      if (sub->Get(i) != pattern_first_char) continue;
+
+      for (int j = 1; j < pattern_length; j++) {
+        if (pat->Get(j) != sub->Get(j + i)) break;
+        if (j == pattern_length - 1) return Smi::FromInt(i);
+      }
+    }
+    return Smi::FromInt(-1);
+  }
+
+  // For patterns with a larger length we use the KMP algorithm.
   //
   // Compute the 'next' table.
   int* next_table = NewArray<int>(pattern_length);

src/string.js

@@ -332,6 +332,7 @@ function ApplyReplacementFunction(replace, captures, subject) {
 // ECMA-262 section 15.5.4.7
 %AddProperty($String.prototype, "indexOf", function(searchString /* position */) {  // length == 1
   var str = ToString(this);
+  var str_len = str.length;
   var searchStr = ToString(searchString);
   var index = 0;
   if (%_ArgumentsLength() > 1) {
@@ -339,8 +340,8 @@ function ApplyReplacementFunction(replace, captures, subject) {
     index = TO_INTEGER(arg1);
   }
   if (index < 0) index = 0;
-  if (index > str.length) index = str.length;
-  if (searchStr.length + index > str.length) return -1;
+  if (index > str_len) index = str_len;
+  if (searchStr.length + index > str_len) return -1;
   return %StringIndexOf(str, searchStr, index);
 }, DONT_ENUM);