PR66414 optimize std::string::find

2017-01-09  Jonathan Wakely  <jwakely@redhat.com>
	    Aditya Kumar  <hiraditya@msn.com>

	PR libstdc++/66414
	* include/bits/basic_string.tcc
	(basic_string::find(const CharT*, size_type, size_type)): Optimize.

Co-Authored-By: Aditya Kumar <hiraditya@msn.com>

From-SVN: r244225

libstdc++-v3/ChangeLog

+2017-01-09  Jonathan Wakely  <jwakely@redhat.com>
+	    Aditya Kumar  <hiraditya@msn.com>
+
+	PR libstdc++/66414
+	* include/bits/basic_string.tcc
+	(basic_string::find(const CharT*, size_type, size_type)): Optimize.
+
 2017-01-06  Jonathan Wakely  <jwakely@redhat.com>
 
 	* testsuite/21_strings/basic_string/operations/find/char/6.cc: New.

libstdc++-v3/include/bits/basic_string.tcc

@@ -1190,18 +1190,30 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     {
       __glibcxx_requires_string_len(__s, __n);
       const size_type __size = this->size();
-      const _CharT* __data = _M_data();
 
       if (__n == 0)
 	return __pos <= __size ? __pos : npos;
+      if (__pos >= __size)
+	return npos;
 
-      if (__n <= __size)
+      const _CharT __elem0 = __s[0];
+      const _CharT* const __data = data();
+      const _CharT* __first = __data + __pos;
+      const _CharT* const __last = __data + __size;
+      size_type __len = __size - __pos;
+
+      while (__len >= __n)
 	{
-	  for (; __pos <= __size - __n; ++__pos)
-	    if (traits_type::eq(__data[__pos], __s[0])
-		&& traits_type::compare(__data + __pos + 1,
-					__s + 1, __n - 1) == 0)
-	      return __pos;
+	  // Find the first occurrence of __elem0:
+	  __first = traits_type::find(__first, __len - __n + 1, __elem0);
+	  if (!__first)
+	    return npos;
+	  // Compare the full strings from the first occurrence of __elem0.
+	  // We already know that __first[0] == __s[0] but compare them again
+	  // anyway because __s is probably aligned, which helps memcmp.
+	  if (traits_type::compare(__first, __s, __n) == 0)
+	    return __first - __data;
+	  __len = __last - ++__first;
 	}
       return npos;
     }