basic_string.tcc (_M_mutate, _M_clone): Implement exponential growth policy to meet...

2001-12-06  Paolo Carlini  <pcarlini@unitus.it>
	    Loren J. Rittle <ljrittle@acm.org>

	* include/bits/basic_string.tcc (_M_mutate, _M_clone): Implement
	exponential growth policy to meet linear amortized time
	requirements of the standard.
        (_S_create): Adjust comment.

Co-Authored-By: Loren J. Rittle <ljrittle@acm.org>
From-SVN: r47750

diff --git a/libstdc++-v3/ChangeLog b/libstdc++-v3/ChangeLog
index 4348d33f44de79f43dd5379de196943df00454db..afbb23417411e3bcc26ac9c23c727599cec03e20 100644 (file)
--- a/libstdc++-v3/ChangeLog
+++ b/libstdc++-v3/ChangeLog
@@ -1,3 +1,11 @@
+2001-12-06  Paolo Carlini  <pcarlini@unitus.it>
+           Loren J. Rittle <ljrittle@acm.org>
+
+       * include/bits/basic_string.tcc (_M_mutate, _M_clone): Implement
+       exponential growth policy to meet linear amortized time
+       requirements of the standard.
+        (_S_create): Adjust comment.
+
 2001-12-06  Benjamin Kosnik  <bkoz@redhat.com>
 
        libstdc++/3720

diff --git a/libstdc++-v3/include/bits/basic_string.tcc b/libstdc++-v3/include/bits/basic_string.tcc
index eeaa20aadefdde9789b9cd4efad3cca5fe98d904..cbaf88361da1cc8fc07055077f3556d5fec00ec5 100644 (file)
--- a/libstdc++-v3/include/bits/basic_string.tcc
+++ b/libstdc++-v3/include/bits/basic_string.tcc
@@ -265,6 +265,12 @@ namespace std
       _M_rep()->_M_set_leaked();
     }
 
+  // _M_mutate and, below, _M_clone, include, in the same form, an exponential
+  // growth policy, necessary to meet amortized linear time requirements of
+  // the library: see http://gcc.gnu.org/ml/libstdc++/2001-07/msg00085.html.
+  // The policy is active for allocations requiring an amount of memory above
+  // system pagesize. This is consistent with the requirements of the standard:
+  // see, f.i., http://gcc.gnu.org/ml/libstdc++/2001-07/msg00130.html
   template<typename _CharT, typename _Traits, typename _Alloc>
     void
     basic_string<_CharT, _Traits, _Alloc>::
@@ -279,7 +285,21 @@ namespace std
        {
          // Must reallocate.
          allocator_type __a = get_allocator();
-         _Rep* __r = _Rep::_S_create(__new_size, __a);
+         // See below (_S_create) for the meaning and value of these
+         // constants.
+         const size_type __pagesize = 4096;
+         const size_type __malloc_header_size = 4 * sizeof (void*);
+         // The biggest string which fits in a memory page
+         const size_type __page_capacity = !(__pagesize - __malloc_header_size
+                                             - sizeof(_Rep) - sizeof(_CharT)) 
+                                             / sizeof(_CharT);
+         _Rep* __r;
+         if (__new_size > capacity() && __new_size > __page_capacity)
+           // Growing exponentially.
+           __r = _Rep::_S_create(__new_size > 2*capacity() ?
+                                 __new_size : 2*capacity(), __a);
+         else
+           __r = _Rep::_S_create(__new_size, __a);
          try 
            {
              if (__pos)
@@ -380,11 +400,6 @@ namespace std
       // with tuned parameters to get this perfect for any particular
       // malloc implementation.  Fortunately, generalizations about
       // common features seen among implementations seems to suffice.
-      // This algorithm does not replace the need for an exponential
-      // growth shaper to meet library specification.  Note: THIS IS
-      // NOT THE CORRECT LOCATION FOR AN EXPONENTIAL GROWTH SHAPER
-      // (since this code affect initial allocation as well as
-      // reallocation).
 
       // __pagesize need not match the actual VM page size for good
       // results in practice, thus we pick a common value on the low
@@ -430,7 +445,23 @@ namespace std
     basic_string<_CharT, _Traits, _Alloc>::_Rep::
     _M_clone(const _Alloc& __alloc, size_type __res)
     {
-      _Rep* __r = _Rep::_S_create(_M_length + __res, __alloc);
+      // Requested capacity of the clone.
+      const size_type __requested_cap = _M_length + __res;
+      // See above (_S_create) for the meaning and value of these constants.
+      const size_type __pagesize = 4096;
+      const size_type __malloc_header_size = 4 * sizeof (void*);
+      // The biggest string which fits in a memory page.
+      const size_type __page_capacity =
+        (__pagesize - __malloc_header_size - sizeof(_Rep) - sizeof(_CharT))
+        / sizeof(_CharT);
+      _Rep* __r;
+      if (__requested_cap > _M_capacity && __requested_cap > __page_capacity)
+        // Growing exponentially.
+        __r = _Rep::_S_create(__requested_cap > 2*_M_capacity ?
+                              __requested_cap : 2*_M_capacity, __alloc);
+      else
+        __r = _Rep::_S_create(__requested_cap, __alloc);
+      
       if (_M_length)
        {
          try