Hello,
operator new can clobber memory, it is hard to teach the compiler
otherwise since it is replaceable. Here I cache a couple values before the
call to the allocator. I checked the result on this simple example:
#include <vector>
void f(std::vector<int>&v){ v.push_back(0); }
The patch does not affect the fast path where no reallocation is needed.
Compiling with -O3 (everything gets inlined into f), I see a nice decrease
in code size on x86_64
$ size old.o new.o
text data bss dec hex filename
462 0 0 462 1ce old.o
376 0 0 376 178 new.o
Even at -O2 where _M_realloc_insert is not inlined, I get a slight
decrease in code size (490 -> 470). On x86, that's 531 -> 519 (465 ->
387 at -O3).
I'm not going to modify every function like that, I just happened to be
looking at this example for other reasons, and the size gain is larger
than I expected, so I am posting the patch.
Bootstrap+regtest on powerpc64le-unknown-linux-gnu.
2017-11-13 Marc Glisse <marc.gli...@inria.fr>
* include/bits/vector.tcc (vector::_M_realloc_insert): Cache old
values before the allocation.
--
Marc GlisseIndex: libstdc++-v3/include/bits/vector.tcc
===================================================================
--- libstdc++-v3/include/bits/vector.tcc (revision 254629)
+++ libstdc++-v3/include/bits/vector.tcc (working copy)
@@ -414,20 +414,22 @@ _GLIBCXX_BEGIN_NAMESPACE_CONTAINER
_M_realloc_insert(iterator __position, _Args&&... __args)
#else
template<typename _Tp, typename _Alloc>
void
vector<_Tp, _Alloc>::
_M_realloc_insert(iterator __position, const _Tp& __x)
#endif
{
const size_type __len =
_M_check_len(size_type(1), "vector::_M_realloc_insert");
+ pointer __old_start = this->_M_impl._M_start;
+ pointer __old_finish = this->_M_impl._M_finish;
const size_type __elems_before = __position - begin();
pointer __new_start(this->_M_allocate(__len));
pointer __new_finish(__new_start);
__try
{
// The order of the three operations is dictated by the C++11
// case, where the moves could alter a new element belonging
// to the existing vector. This is an issue only for callers
// taking the element by lvalue ref (see last bullet of C++11
// [res.on.arguments]).
@@ -435,45 +437,44 @@ _GLIBCXX_BEGIN_NAMESPACE_CONTAINER
__new_start + __elems_before,
#if __cplusplus >= 201103L
std::forward<_Args>(__args)...);
#else
__x);
#endif
__new_finish = pointer();
__new_finish
= std::__uninitialized_move_if_noexcept_a
- (this->_M_impl._M_start, __position.base(),
+ (__old_start, __position.base(),
__new_start, _M_get_Tp_allocator());
++__new_finish;
__new_finish
= std::__uninitialized_move_if_noexcept_a
- (__position.base(), this->_M_impl._M_finish,
+ (__position.base(), __old_finish,
__new_finish, _M_get_Tp_allocator());
}
__catch(...)
{
if (!__new_finish)
_Alloc_traits::destroy(this->_M_impl,
__new_start + __elems_before);
else
std::_Destroy(__new_start, __new_finish, _M_get_Tp_allocator());
_M_deallocate(__new_start, __len);
__throw_exception_again;
}
_GLIBCXX_ASAN_ANNOTATE_REINIT;
- std::_Destroy(this->_M_impl._M_start, this->_M_impl._M_finish,
- _M_get_Tp_allocator());
- _M_deallocate(this->_M_impl._M_start,
- this->_M_impl._M_end_of_storage - this->_M_impl._M_start);
+ std::_Destroy(__old_start, __old_finish, _M_get_Tp_allocator());
+ _M_deallocate(__old_start,
+ this->_M_impl._M_end_of_storage - __old_start);
this->_M_impl._M_start = __new_start;
this->_M_impl._M_finish = __new_finish;
this->_M_impl._M_end_of_storage = __new_start + __len;
}
template<typename _Tp, typename _Alloc>
void
vector<_Tp, _Alloc>::
_M_fill_insert(iterator __position, size_type __n, const value_type& __x)
{
