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From: "dl.soluz at gmx dot net" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug tree-optimization/58483] New: missing optimization opportunity for const std::vector compared to std::array
Date: Fri, 20 Sep 2013 11:03:00 -0000
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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=58483

            Bug ID: 58483
           Summary: missing optimization opportunity for const std::vector
                    compared to std::array
           Product: gcc
           Version: 4.8.1
            Status: UNCONFIRMED
          Severity: enhancement
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: dl.soluz at gmx dot net

this small testprogram shows a missing optimization opportunity for const
std::vector when using initialization_list - in compared to std::array

#include <vector>
#include <numeric>
#include <array>

static int calc(const std::array<int,3> p_ints, const int& p_init)
//static int calc(const std::vector<int> p_ints, const int& p_init)
{
  return std::accumulate(p_ints.begin(), p_ints.end(), p_init);
}

int main()
{
  const int result = calc({10,20,30},100);
  return result;
}

optimizer-result using std::array

int main() ()
{
  <bb 2>:
  return 160;
}

the result using std::vector

int main() ()
{
  int __init;
  int _2;
  int _11;
  int _32;
  int * _33;

  <bb 2>:
  _33 = operator new (12);
  __builtin_memcpy (_33, &._79, 12);
  _32 = MEM[(const int &)_33];
  __init_28 = _32 + 100;
  _2 = MEM[(const int &)_33 + 4];
  __init_18 = _2 + __init_28;
  _11 = MEM[(const int &)_33 + 8];
  __init_13 = _11 + __init_18;
  if (_33 != 0B)
    goto <bb 3>;
  else
    goto <bb 4>;

  <bb 3>:
  operator delete (_33);

  <bb 4>:
  return __init_13;
}

according to Marc Gliss's answer in
(http://gcc.gnu.org/ml/gcc/2013-09/msg00179.html)

"...
We don't perform such high-level optimizations. But if you expand, inline and
simplify this program, the optimizers sees something like:

p=operator new(12);
memcpy(p,M,12); // M contains {10, 20, 30}
res=100+p[0]+p[1]+p[2];
if(p!=0) operator delete(p);

A few things that go wrong:
* because p is filled with memcpy and not with regular assignments,
the compiler doesn't realize that p[0] is known. 

* the test p != 0 is unnecessary (a patch that should help is pending review) *
we would then be left with: 

p=new(12); delete p; return 160; 

gcc knows how to remove free(malloc(12)) but not the C++ variant (I don't even
know if it is legal, or what conditions and flags are required to make it so).
..."

the pending patch is http://gcc.gnu.org/bugzilla/show_bug.cgi?id=19476

but the question is still can new(delete(12)) also removed like
free(malloc(12)) in this scenario?