[Bug middle-end/55035] reload1.c:3766:41: error: ‘orig_dup[0]’ may be used uninitialized in this function (for fr30, microblaze, moxie, rl78)

["miyuki at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=55035

Mikhail Maltsev <miyuki at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Last reconfirmed|2015-04-09 00:00:00         |2015-5-20
                 CC|                            |miyuki at gcc dot gnu.org
      Known to fail|                            |6.0

--- Comment #5 from Mikhail Maltsev <miyuki at gcc dot gnu.org> ---
Reduced testcase:

$ cat ./warn2.cc
int arr[1];
int N;

void bar();

void foo()
{
  int temp[2];
  for (int i = 0; i < N; i++)
    temp[i] = arr[i];
  bar();
  for (int i = 0; i < N; i++)
    arr[i] = temp[i];
}


$ /opt/gcc-6.0.0/bin/g++ -c -Wall -O2 -fdump-tree-all ./warn2.cc
./warn2.cc: In function 'void foo()':
./warn2.cc:13:21: warning: 'temp[0]' may be used uninitialized in this function
[-Wmaybe-uninitialized]
     arr[i] = temp[i];

$ /opt/gcc-6.0.0/bin/g++ -c -Wall -O2 -fno-tree-sra ./warn2.cc
(no warning)

GCC performs scalar replacement of "temp" and unrolls both loops. N is global
and can be modified by bar(). The final GIMPLE looks like this:

$ cat ./warn2.cc.190t.optimized

void foo() ()
{
  int temp$0;
  int N.0_18;
  int N.0_24;

  <bb 2>:
  N.0_24 = N;
  if (N.0_24 <= 0)
    goto <bb 4>;
  else
    goto <bb 3>;

  <bb 3>:
  temp$0_25 = arr[0];

  <bb 4>:
  # temp$0_26 = PHI <temp$0_25(3), temp$0_27(D)(2)>
  bar ();
  N.0_18 = N;
  if (N.0_18 <= 0)
    goto <bb 6>;
  else
    goto <bb 5>;

  <bb 5>:
  arr[0] = temp$0_26;

  <bb 6>:
  return;
}

Here temp$0_27(D)(2) is the "uninitialized" case. In the original testcase
recog_data.n_dups is considered escaped and orig_dup[0] is treated the same way
as temp[0].