[Bug c++/101119] New: Missing the check about modify global variable for __attribute__((const)) function

["zhongyunde at huawei dot com" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101119

            Bug ID: 101119
           Summary: Missing the check about modify global variable for
                    __attribute__((const)) function
           Product: gcc
           Version: 10.3.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: zhongyunde at huawei dot com
  Target Milestone: ---

As __attribute__((const)) function should have no side effect, so it won't
modify any global variable. but with the newest gcc, it doesn't do such check.


test base on the ARM64 gcc 11.1 on https://gcc.godbolt.org/ with option -O3 -S
-fno-inline -g0 -Wall -Werror.

=====================================================
#include <stdio.h>
int gValue = 1;
class NPair /*: protected Pointers */ {
public:
  int bins[10];
  //int exclusion(int n) __attribute__((const));
  __attribute__((const)) int exclusion (int n) const {
    printf ("before %d\n", bins[0]);
    NPair* pt = const_cast<NPair*>(this);
    pt->bins[0] = 1;
    printf ("after %d\n", bins[0]);
    return gValue++; // expect a error
  }
  NPair (void) {
     bins[0] = 3;
  }
};

// extern int * __restrict bins;
class NPair nPair;

int main (void)
{
  int tmp = nPair.bins[0];

  (void)nPair.exclusion(1);

  return tmp == nPair.bins[0];
}