[Bug c++/57199] New: [4.8, 4.9] Bogus warning: iteration NNNN invokes undefined behavior -Waggressive-loop-optimizations

[ppluzhnikov at google dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=57199

             Bug #: 57199
           Summary: [4.8, 4.9] Bogus warning: iteration NNNN invokes
                    undefined behavior -Waggressive-loop-optimizations
    Classification: Unclassified
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
        AssignedTo: unassig...@gcc.gnu.org
        ReportedBy: ppluzhni...@google.com


Google ref: b/8856275

Test case:



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();
    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);
}



Using trunk r198689:

g++ -std=c++11 -O2 -c tt.cc
tt.cc: In member function ‘void CP::Add()’:
tt.cc:21:7: warning: iteration 230584300921369396ul invokes undefined behavior
[-Waggressive-loop-optimizations]
       ptr[i].~T();
       ^
tt.cc:20:5: note: containing loop
     for (size_t i = 0; i < num; i++) {
     ^