strange g++ optimization results on simple code

[Kai-Uwe Bux <bux at kubux dot net>]

I am observing that turning a statement into an inline function
without actually changing the algorithm can improve code when g++
is told to optimize. I am using g++ version 3.4.0 (April 20 release)
on a Pentium 4.

I tried the following two files. Here is the "optimized" version:

  // as.cc
  // =====

  #include <iostream>


  inline
  static
  bool comp ( const unsigned int & a,
	      const unsigned int & b ) {
    return( a < b );
  }
  
  int main ( void ) {
    unsigned int A = 1000000000;
    for ( unsigned i = 0; i < A; ++i ) {
      if ( comp( A-2, i ) ) {
        std::cout << i << std::endl;
      }
    }
  }

  // end of file

and this is the plain version:

  // bs.cc
  // =====

  #include <iostream>

  int main ( void ) {
    unsigned int A = 1000000000;
    for ( unsigned i = 0; i < A; ++i ) {
      if ( A-2 < i ) {
        std::cout << i << std::endl;
      }
    }
  }

  // end of file



Unoptimized I get:

  gcc> time as.cc.out 
  999999999

  real    0m8.326s
  user    0m7.790s
  sys     0m0.050s
  gcc> time bs.cc.out 
  999999999

  real    0m2.823s
  user    0m2.670s
  sys     0m0.010s


with -O, I get:

  gcc> time as.cc.out 
  999999999

  real    0m1.458s
  user    0m1.360s
  sys     0m0.010s
  gcc> time bs.cc.out 
  999999999

  real    0m1.432s
  user    0m1.330s
  sys     0m0.000s


with -O2, I get:

  gcc> time as.cc.out
  999999999

  real    0m0.711s
  user    0m0.650s
  sys     0m0.000s
  gcc> time bs.cc.out
  999999999

  real    0m1.265s
  user    0m1.250s
  sys     0m0.000s


and with -O3, I get:

  gcc> time as.cc.out 
  999999999

  real    0m0.529s
  user    0m0.500s
  sys     0m0.010s
  gcc> time bs.cc.out 
  999999999

  real    0m1.329s
  user    0m1.250s
  sys     0m0.000s


I do not understand, why as.cc appears to optimize better than bs.cc.
Actually, I do not understand, why different code is generated at all.

If someone who understands the internal magic of g++'s code generation
has an explanation as to why this happens, I would be very interested
in learning about it. Pointers would be highly appreciated: I searched
for optimization in gcc's bugzilla database and I tried a Google
search on optimization in g++, but I couldn't spot results pertinent
to my observation.


Thanks for your consideration
and best regards, Yours

Kai-Uwe Bux