This is the mail archive of the gcc-patches@gcc.gnu.org mailing list for the GCC project.


Index Nav: [Date Index] [Subject Index] [Author Index] [Thread Index]
Message Nav: [Date Prev] [Date Next] [Thread Prev] [Thread Next]
Other format: [Raw text]

Re: Improve insert/emplace robustness to self insertion


On 06/07/16 21:46 +0200, François Dumont wrote:
On 05/07/2016 12:47, Jonathan Wakely wrote:
On 04/07/16 15:55 +0100, Jonathan Wakely wrote:
I'm getting nervous about the smart insertion trick to avoid making a
copy, I have a devious testcase in mind which will break with that
change. I'll share the testcase later today.

Here's a testcase which passes with libstdc++ but fails with libc++
because libc++ doesn't make a copy when inserting a T lvalue into
std::vector<T>:

#include <vector>
#include <memory>
#include <cassert>

struct T
{
T(int v = 0) : value(v) { }
T(const T& t);
T& operator=(const T& t);
void make_child() { child = std::make_unique<T>(value + 10); }
std::unique_ptr<T> child;
int value;
};

T::T(const T& t) : value(t.value)
{
if (t.child)
  child.reset(new T(*t.child));
}

T& T::operator=(const T& t)
{
value = t.value;
if (t.child)
{
  if (child)
    *child = *t.child;
  else
    child.reset(new T(*t.child));
}
else
  child.reset();
return *this;
}

int main()
{
std::vector<T> v;
v.reserve(3);
v.push_back(T(1));
v.back().make_child();
v.push_back(T(2));
v.back().make_child();

assert(v[1].child->value == 12);
assert(v[1].child->child == nullptr);

v.insert(v.begin(), *v[1].child);

assert(v[0].value == 12);
assert(v[0].child == nullptr);
}

The problem is that the object being inserted (*v[1].child) is not an
element of the vector, so the optimization assumes it is unchanged by
shuffling the existing elements. That assumption is wrong.

As far as I can see, this program is perfectly valid. It's slightly
contrived to prove a point, but it's not entirely unrealistic code.


Don't you plan to add it to the testsuite ?

Yes, I'm just very busy with other things, I've only been doing
anything on std::vector so you're not waiting too long for responses
:-)

On my side I rebase part of my patch to reorganize a little bit code. I reintroduced _M_realloc_insert which isolates the code of _M_insert_aux used when we need to reallocate memory. So _M_insert_aux is used only when insertion can be done in place. It is a nice replacement for _M_emplace_back_aux that have been removed. In most of vector modifiers we start checking if we need to reallocate or not.

Great, I was thinking of doing that kind of refactoring.

I'll review it ASAP.


Index Nav: [Date Index] [Subject Index] [Author Index] [Thread Index]
Message Nav: [Date Prev] [Date Next] [Thread Prev] [Thread Next]