re PR libstdc++/41267 (vector<>::resize() from an empty vector calls memmove for…

re PR libstdc++/41267 (vector<>::resize() from an empty vector calls memmove for 0 bytes (twice), wasting a lot of cpu time in a production PowerDNS)

2009-09-06  Paolo Carlini  <paolo.carlini@oracle.com>

	PR libstdc++/41267
	* include/bits/stl_algobase.h (__copy_move::__copy_m,
	__copy_move_backward::__copy_move_b): Don't call __builtin_memmove
	with a null third argument.

From-SVN: r151459

libstdc++-v3/ChangeLog

+2009-09-06  Paolo Carlini  <paolo.carlini@oracle.com>
+
+	PR libstdc++/41267
+	* include/bits/stl_algobase.h (__copy_move::__copy_m,
+	__copy_move_backward::__copy_move_b): Don't call __builtin_memmove
+	with a null third argument.
+
 2009-09-04  Benjamin Kosnik  <bkoz@redhat.com>
 
 	Revert.

libstdc++-v3/include/bits/stl_algobase.h

@@ -375,9 +375,10 @@ _GLIBCXX_BEGIN_NAMESPACE(std)
         static _Tp*
         __copy_m(const _Tp* __first, const _Tp* __last, _Tp* __result)
         {
-	  __builtin_memmove(__result, __first,
-			    sizeof(_Tp) * (__last - __first));
-	  return __result + (__last - __first);
+	  const ptrdiff_t _Num = __last - __first;
+	  if (_Num)
+	    __builtin_memmove(__result, __first, sizeof(_Tp) * _Num);
+	  return __result + _Num;
 	}
     };
 
@@ -572,7 +573,8 @@ _GLIBCXX_BEGIN_NAMESPACE(std)
         __copy_move_b(const _Tp* __first, const _Tp* __last, _Tp* __result)
         {
 	  const ptrdiff_t _Num = __last - __first;
-	  __builtin_memmove(__result - _Num, __first, sizeof(_Tp) * _Num);
+	  if (_Num)
+	    __builtin_memmove(__result - _Num, __first, sizeof(_Tp) * _Num);
 	  return __result - _Num;
 	}
     };