[Bug c++/57072] New: bogus "is used uninitialized" warning

[Bug c++/57072] New: bogus "is used uninitialized" warning

[ppluzhnikov at google dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=57072

             Bug #: 57072
           Summary: bogus "is used uninitialized" warning
    Classification: Unclassified
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: ppluzhnikov@google.com


Google ref: b/8700048

Test:


struct A {
  ~A() __attribute__((noreturn));
  A& stream();
  template <typename T> void operator<<(T&);
};

struct C {
  enum State { S1, S2 };
  static bool Fn(State s) {
    switch (s) {
      case S2: A().stream() << s;
    }
  }
};

void TestFn()
{
  for (int i = 0; i < 2; ++i) {
    C::Fn ? (void) 0 : A().stream() << " ";
    C::Fn(static_cast<C::State>(i));
  }
}



Using g++ (GCC) 4.9.0 20130412 (experimental) (r197936):

g++ -c -Wuninitialized -O2 -fno-tree-vrp t.cc

t.cc: In function ‘void TestFn()’:
t.cc:16:6: warning: ‘s’ is used uninitialized in this function
[-Wuninitialized]
 void TestFn()
      ^

This warning doesn't trigger on 4_7-branch, but a slightly less reduced test
case does, so it's likely not a recent regression.
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From: "burnus at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug middle-end/57073] New: __builtin_powif (-1.0, k) should be optimized to "1.0 - 2.0 * (K%2)"
Date: Thu, 25 Apr 2013 18:04:00 -0000
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http://gcc.gnu.org/bugzilla/show_bug.cgi?idW073

             Bug #: 57073
           Summary: __builtin_powif (-1.0, k) should be optimized to "1.0
                    - 2.0 * (K%2)"
    Classification: Unclassified
           Product: gcc
           Version: 4.9.0
            Status: UNCONFIRMED
          Keywords: missed-optimization
          Severity: normal
          Priority: P3
         Component: middle-end
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: burnus@gcc.gnu.org


Motivated by PR57071.

In numerical code, it is not unlike to find code of the form "(-1.0) raised to
the power of k", in Fortran: (-1.0)**k.

That translates into:
  __builtin_powif (-1.0e+0, k)
which stays that way even with -O3.

Expected: It gets optimized to "1.0 - 2.0 * (K%2)"


Fortran test case:

real function f(k)
  integer, value :: k
  f = (-1.0)**k
end

[Bug c++/57072] bogus "is used uninitialized" warning

[law at redhat dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=57072

Jeffrey A. Law <law at redhat dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |RESOLVED
         Resolution|---                         |DUPLICATE

--- Comment #1 from Jeffrey A. Law <law at redhat dot com> ---
Accurate warnings from -Wuninitialized depends on various optimizers being
allowed to do their job.  -fno-tree-vrp in this case disables collpasing a
conditional so a singularity and disables the first jump threading pass.

In combination these two prevent the CFG from being optimized enough to prove
the value can't be used uninitialized.

*** This bug has been marked as a duplicate of bug 42145 ***