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From: "zhongyunde at huawei dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c++/101119] New: Missing the check about modify global variable
 for __attribute__((const)) function
Date: Fri, 18 Jun 2021 10:56:15 +0000
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X-Bugzilla-Version: 10.3.0
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D101119

            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 che=
ck.


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

=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D
#include <stdio.h>
int gValue =3D 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 =3D const_cast<NPair*>(this);
    pt->bins[0] =3D 1;
    printf ("after %d\n", bins[0]);
    return gValue++; // expect a error
  }
  NPair (void) {
     bins[0] =3D 3;
  }
};

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

int main (void)
{
  int tmp =3D nPair.bins[0];

  (void)nPair.exclusion(1);

  return tmp =3D=3D nPair.bins[0];
}=