[Bug c++/66411] New: False positive in array bound check in a for loop

[dougkwan at google dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66411

            Bug ID: 66411
           Summary: False positive in array bound check in a for loop
           Product: gcc
           Version: 4.9.2
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: dougkwan at google dot com
  Target Milestone: ---

gcc-4.9.2 produces a bogus out of bound warning for the following test case

----
class A;

class C {
  void m_fn1();
  A *a_;
};

class A {
public:
  void m_fn2();
  int elements_[8];
  int num_elements_;
};

int a;
void A::m_fn2() {
  int b = 0;
  if (num_elements_)
    ++b;
  for (int i = b + 1; i < num_elements_; ++i) {
    if (elements_[i])
      ++a;
  }
}

void C::m_fn1() {
  a_->m_fn2();
}
----


sh-4.3$ ~/gcc-4.9.2/install/bin/g++ -O3  -Werror=array-bounds bug.cc -c -o
/dev/null
bug.cc: In member function ‘void A::m_fn2()’:
bug.cc:21:20: error: array subscript is above array bounds
[-Werror=array-bounds]
     if (elements_[i])
                    ^
bug.cc: In member function ‘void C::m_fn1()’:
bug.cc:21:20: error: array subscript is above array bounds
[-Werror=array-bounds]
     if (elements_[i])
                    ^
cc1plus: some warnings being treated as errors
sh-4.3$ 

gcc config:

sh-4.3$ ~/gcc-4.9.2/install/bin/g++ -v
Using built-in specs.
COLLECT_GCC=/usr/local/google/home/dougkwan/gcc-4.9.2/install/bin/g++
COLLECT_LTO_WRAPPER=/usr/local/google/home/dougkwan/gcc-4.9.2/install/bin/../libexec/gcc/x86_64-unknown-linux-gnu/4.9.2/lto-wrapper
Target: x86_64-unknown-linux-gnu
Configured with: ../gcc-4.9.2/configure --enable-languages=c,c++
--disable-bootstrap --disable-multilib
Thread model: posix
gcc version 4.9.2 (GCC)