[Bug c/26642] New: order of multiple assignments to array-values handled differently from gcc 3.3.5

["nobs at tigress dot com" <gcc-bugzilla at gcc dot gnu dot org>]

Following code gives different results between version 4.0.2 and 3.3.5. The
'fun' thing is: There is a warning about the part that still behaves the same
(with normale variables k and l). But the array-variables behave differently
w/o any warnings. It works when putting the XOR-operations in separate lines of
code. So I guess its a matter of optimising handling arrays, but it even
happens with -O0.

> g++ --version
g++ (GCC) 3.3.5 20050117 (prerelease) (SUSE Linux)
> g++ -O0 -Wall -pedantic flip2.cc
> ./a.out
a[] before: 1 2
kl  before: 1 2
a[] after:  2 1
kl  after:  2 1

> g++ --version
g++ (GCC) 4.0.2 20050901 (prerelease) (SUSE Linux)
> g++ -O0 -Wall -pedantic flip2.cc
flip2.cc: In function ‘int main()’:
flip2.cc:16: warning: operation on ‘k’ may be undefined
> ./a.out
a[] before: 1 2
kl  before: 1 2
a[] after:   1
kl  after:  2 1


#include <iostream>

int main (void){

  using namespace std;

  char a[]={'1','2'};

  char k='1';
  char l='2';

  cout << "a[] before: " << a[0] << " " << a[1] << endl;
  cout << "kl  before: " << k    << " " << l    << endl;

  a[0] ^= a[1] ^= a[0] ^= a[1];
  k    ^= l    ^= k    ^= l;

  cout << "a[] after:  " << a[0] << " " << a[1] << endl;
  cout << "kl  after:  " << k    << " " << l    << endl;

  return (0);
}


-- 
           Summary: order of multiple assignments to array-values handled
                    differently from gcc 3.3.5
           Product: gcc
           Version: 4.0.2
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: nobs at tigress dot com
 GCC build triplet: x86
  GCC host triplet: x86
GCC target triplet: x86


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=26642