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Subject: [Bug c/50975] New: Logical operators evaluated in wrong order if no side effects
Date: Thu, 03 Nov 2011 07:19:00 -0000
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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=50975

             Bug #: 50975
           Summary: Logical operators evaluated in wrong order if no side
                    effects
    Classification: Unclassified
           Product: gcc
           Version: 4.6.2
            Status: UNCONFIRMED
          Severity: major
          Priority: P3
         Component: c
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: gcc.hall@gmail.com


gcc 4.6.2  -Os 

If the expressions either side of a logical operator (|| or &&) have no
side effects, gcc sometimes evaluates them in the wrong order.    See
the example below.  Of course the code works, but the standard is clear,
logical operators GUARANTEE left to right evaluation.  In this case 
people use the feature as a micro optimization, putting the most likely to
succeed case first with || or vice-versa with &&.

-----------------------------------
#include <stdio.h>
int
main( int argc, char *argv[] )
{
  char *last = argv[1];

  if( *last == 10 || *last == 13 )
    --last;

  printf("last = %p\n", last );
  return 0;
}
---------------------------------------------
    movb    (%eax), %dl # *last_3, D.2517
    cmpb    $13, %dl    #, D.2517
    je  .L4 #,
    cmpb    $10, %dl    #, D.2517
    jne .L2 #,
.L4:
    decl    %eax    # last
.L2:
--------------------------------------------