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From: "ppluzhnikov at google dot com" <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 12:55:00 -0000
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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=57199

Paul Pluzhnikov <ppluzhnikov at google dot com> changed:

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

--- Comment #2 from Paul Pluzhnikov <ppluzhnikov at google dot com> 2013-05-08 12:55:31 UTC ---
> Thus, the warning, while not 100% helpful, points at a real problem.

There is no real problem in the original source, which reads:

  size_t s = size();
  if (n < s)
    DestroyElements(mutable_array() + n, s - n);
  else ...

and produces the same warning.

Here is updated test with above condition; confirmed to still show the warning
with trunk @r198709.



typedef decltype(sizeof(0)) size_t;

struct Foo {
  ~Foo();      // comment out -> problem disappears
  int x[20];
};

template <typename T, int N>
struct InlinedVector {
  inline InlinedVector() : size_(0) { }
  inline size_t size() const {
    return size_;
  }
  T* mutable_array();
  void resize(size_t n) {
    size_t s = size();
    if (n < s)
      DestroyElements(mutable_array() + n, s - n);
  }
  static void DestroyElements(T* ptr, size_t num) {
    for (size_t i = 0; i < num; i++) {
      ptr[i].~T();
    }
  }

  size_t size_;
};

struct CP {
  void Add();
  InlinedVector<Foo,1> foo_;
};

void CP::Add() {
  foo_.resize(foo_.size() + 1);
}