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From: "rguenth at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug tree-optimization/57199] [4.8/4.9 Regression] Bogus warning: iteration NNNN invokes undefined behavior -Waggressive-loop-optimizations
Date: Wed, 08 May 2013 08:58:00 -0000
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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=57199

Richard Biener <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |RESOLVED
         Resolution|                            |INVALID

--- Comment #1 from Richard Biener <rguenth at gcc dot gnu.org> 2013-05-08 08:57:45 UTC ---
Note that there seems to be an error in the source:

    void resize(size_t n) {
        size_t s = size();
        DestroyElements(mutable_array() + n, s - n);

for the call with n == size () + 1 the function calls DestroyElements
with -1U elements which we figure out and optimize the caller to

  <bb 3>:
  _13 = i_12 * 80;
  _14 = _11 + _13;
  Foo::~Foo (_14);
  i_15 = i_12 + 1;

  <bb 4>:
  # i_12 = PHI <0(2), i_15(3)>
  if (i_12 != 18446744073709551615)
    goto <bb 3>;
  else
    goto <bb 5>;

which then causes the addition to overflow and invoke undefined behavior
and we warn.

Thus, the warning, while not 100% helpful, points at a real problem.