Q67356: C4127 Warning Message May Be Generated with Loop Optimization

Article: Q67356
Product(s): See article
Version(s): 6.00 6.00a | 6.00 6.00a
Operating System(s): MS-DOS | OS/2
Keyword(s): ENDUSER | | mspl13_c
Last Modified: 4-DEC-1990

Under certain circumstances, a C4127: "conditional expression is
constant" warning may be generated by the compiler when loop
optimization (/Ol) is enabled. The warning message usually involves a
comparison of unsigned numbers, where an unsigned is being checked
against zero because an unsigned value must ALWAYS be greater than or
equal to zero. However, this warning can also occur with loop
optimization; in that case, it is an indication that the loop will be
executed at least once.

When you specify /Ol optimization, the compiler normally places two
tests in the code: one at the beginning of the loop and one at the
end. Consider the following sample program:

unsigned foo (unsigned x, unsigned y)
{
   unsigned i;

   for ( i = 0 ; i <= x ; i++ )
      y += i;

   return y;
}

Without loop optimization, the compiler does generate code that
includes a test before the loop to check the initial condition, and
then a check at the end to see if the loop should be terminated.

In this case, however, the variable i starts out as zero and the
initial test checks whether i is less than or equal to x, which must
be true because x is unsigned. This given condition allows the
compiler to optimize the loop by removing the initial test because the
possible values for i and x guarantee that the loop will always be
executed at least once.

When the compiler optimizes out the first test based on this fact, it
generates the C4127 warning message to inform you of the situation.
This message does not necessarily indicate any problems, it just lets
you know that loop will be executed at least once no matter what the
value of i is. This would be important if you expect a value of zero
to cause the loop to not be executed.

If loop optimization is disabled or signed numbers are used instead of
unsigned, then the warning will not be generated. Also, changing the
comparison from "<=" to just "<" will prevent the compiler from being
able to assume that the comparison will always be true the first time
around.

Since the warning message itself is not very descriptive for what is
actually occurring, a new message is being considered for future
versions of the compiler.

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