[Bug tree-optimization/19637] New: Missed constant propagation with placement new

[rguenth at tat dot physik dot uni-tuebingen dot de]

For the following testcase with three similar functions we do different
tree optimizations:

#include <new>

struct Foo {
        Foo() { i[0] = 1; }
        int i[2];
};

int foo_char(void)
{
        int i[2];
        new (reinterpret_cast<char *>(i)) Foo();
        return reinterpret_cast<Foo *>(i)->i[0];
}

int foo_void(void)
{
        int i[2];
        new (reinterpret_cast<void *>(i)) Foo();
        return reinterpret_cast<Foo *>(i)->i[0];
}

int foo_void_offset(void)
{
        int i[2];
        new (reinterpret_cast<void *>(&i[0])) Foo();
        return reinterpret_cast<Foo *>(&i[0])->i[0];
}

We only can optimize the foo_void_offset() variant to return 1, the
foo_void() variant results in

<bb 0>:
  this = (struct Foo *) &i[0];
  this->i[0] = 1;
  i.6 = (struct Foo *) &i;
  return i.6->i[0];

where the difference starts in what the frontend produces:

  (void) (TARGET_EXPR <D.1791, (struct Foo *) operator new (8, (void *) &i[0])>;
and return <retval> = ((struct Foo *) &i[0])->i[0];

vs.   (void) (TARGET_EXPR <D.1783, (struct Foo *) operator new (8, (void *) (int
*) &i)>;
and return <retval> = ((struct Foo *) (int *) &i)->i[0];

note that mixing &i[0] and i does not allow folding.


For the char* variant we even cannot prove that &i is non-null (!?):

<bb 0>:
  i.2 = (char *) &i;
  __p = i.2;
  this = (struct Foo *) __p;
  if (__p != 0B) goto <L1>; else goto <L3>;

<L1>:;
  this->i[0] = 1;

<L3>:;
  i.4 = (struct Foo *) &i;
  return i.4->i[0];

though this might be somehow related to type-based aliasing rules(?).
Note that the char variant does not care if &i[0] or plain i is specified.

-- 
           Summary: Missed constant propagation with placement new
           Product: gcc
           Version: 4.0.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P2
         Component: tree-optimization
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: rguenth at tat dot physik dot uni-tuebingen dot de
                CC: gcc-bugs at gcc dot gnu dot org


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=19637