c++/4486: incorrect evaluation of boolean expressions

[mvishnu at calix dot com]

>Number:         4486
>Category:       c++
>Synopsis:       incorrect evaluation of boolean expressions
>Confidential:   no
>Severity:       critical
>Priority:       high
>Responsible:    unassigned
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Fri Oct 05 15:06:00 PDT 2001
>Closed-Date:
>Last-Modified:
>Originator:     Meenan Vishnu
>Release:        unknown-1.0
>Organization:
>Environment:
Solaris and Linux
>Description:
The expression such as 
bool a = <bool expression>
a = a && <another bool expression>
is evaluating to false when it should be true
in g++ (3 and 3.01)  In the attached file, the assertion should not be asserted, but is asserted. (note that both
data() and is_set_port() are const member functions).

// make multicast requests to input ports with multicast
// cells destined to the output port.
void ARBITER::make_mc_requests( void )
  {
    for( int opi = 0; opi < NUM_PORTS; ++opi )
      {
        ARB_OUTPUT_PORT& op = output_port[ opi ];
         // compute sum of the weights...
        long sum_w = 0;
        for( int ipi = 0; ipi < NUM_PORTS; ++ipi ) 
          {
            ARB_INPUT_PORT& ip = input_port[ ipi ];
            ARB_INOUT_PORT& io = inout_port[ ipi ][ opi ];

            // there should not have been any prior service. 
            assert( ip.s_i <= ip.a_i );

            if( ( ip.enabled.data() ) && 
                ( ip.mc_fluid_q_len > 0 ) && 
                ( ip.current_rm.is_set_port( opi ) ) &&
                ( ip.s_i < ip.a_i ) )
              {
                sum_w  += io.mc_weight; 
              }
          }
        // compute the requests to the inputs...
        long f = op.a_o - op.s_o;
        bool req_cond1 = op.enabled.data() && 
            ( sum_w > 0 ); 
#define COMPILER_CHECK
#ifdef COMPILER_CHECK
        bool req_cond2 = op.enabled.data() && 
            ( sum_w > 0 ); 
#endif
        for( int ipi = 0; ipi < NUM_PORTS; ++ipi )
         {
           ARB_INPUT_PORT& ip = input_port[ ipi ];
           ARB_INOUT_PORT& io = inout_port[ ipi ][ opi ];
           bool req_cond = req_cond1 &&
               ( ip.enabled.data() ) && 
               ( ip.mc_fluid_q_len > 0 ) && 
               ( ip.current_rm.is_set_port( opi ) ) && 
               ( ip.s_i < ip.a_i );
#ifdef COMPILER_CHECK
           bool req_cond2 = req_cond2 &&
               ( ip.enabled.data() ) && 
               ( ip.mc_fluid_q_len > 0 ) && 
               ( ip.current_rm.is_set_port( opi ) ) && 
               ( ip.s_i < ip.a_i );
           // This assertion is failing indicating a compiler problem 
           assert( req_cond == req_cond2 );
#endif
           if( req_cond )
               io.request = f * io.mc_weight / sum_w;
           else
               io.request  = 0; 
         }
      }
  }
>How-To-Repeat:

>Fix:
A fix is to rewrite the expression as
bool a1 = <bool expression>
bool a = a1 && <another bool expression>
>Release-Note:
>Audit-Trail:
>Unformatted: