https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114821
Bug ID: 114821
Summary: _M_realloc_append should use memcpy instead of loop to
copy data when possible
Product: gcc
Version: 14.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: libstdc++
Assignee: unassigned at gcc dot gnu.org
Reporter: hubicka at gcc dot gnu.org
Target Milestone: ---
In thestcase
#include <vector>
typedef unsigned int uint32_t;
std::pair<uint32_t, uint32_t> pair;
void
test()
{
std::vector<std::pair<uint32_t, uint32_t>> stack;
stack.push_back (pair);
while (!stack.empty()) {
std::pair<uint32_t, uint32_t> cur = stack.back();
stack.pop_back();
if (!cur.first)
{
cur.second++;
stack.push_back (cur);
stack.push_back (cur);
}
if (cur.second > 10000)
break;
}
}
int
main()
{
for (int i = 0; i < 10000; i++)
test();
}
We produce _M_reallloc_append which uses loop to copy data instead of memcpy.
This is bigger and slower. The reason why __relocate_a does not use memcpy
seems to be fact that pair has copy constructor. It still can be pattern
matched by ldist but it fails with:
(compute_affine_dependence
ref_a: *__first_1, stmt_a: *__cur_37 = *__first_1;
ref_b: *__cur_37, stmt_b: *__cur_37 = *__first_1;
) -> dependence analysis failed
So we can not disambiguate old and new vector memory and prove that loop is
indeed memcpy loop. I think this is valid since operator new is not required to
return new memory, but I think adding __restrict should solve this.
Problem is that I got lost on where to add them, since relocate_a uses
iterators instead of pointers
