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basic_string.tcc (_M_mutate, _M_clone): Implement exponential growth policy to meet linear amortized time requirements...

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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
This commit is contained in:
Paolo Carlini 2001-12-07 10:07:55 +01:00 committed by Benjamin Kosnik
parent d385b9dd9d
commit 79f57f2322
2 changed files with 46 additions and 7 deletions
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8
libstdc++-v3/ChangeLog
View File

@ -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

45
libstdc++-v3/include/bits/basic_string.tcc
View File

@ -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