__builtin_expect usage and g++

[Anders Fugmann <afu at fugmann dot dhs dot org>]

Hi,

While trying to squeeze the last cycle out of a program, I stumbled over 
that using __builtin_expect yields worse code than when not using it.
Tests on the subject reveild that the problem only existed when 
compiling with the g++ compiler. The gcc compiler did, as expected, 
yield better code (in terms of runtime).

The program below exposes the irregularity:

#if 1
#define LIKELY(expression) (__builtin_expect(!!(expression), 1))
#define UNLIKELY(expression) (__builtin_expect(!!(expression), 0))
#else
#define LIKELY(expression) (expression)
#define UNLIKELY(expression) (expression)
#endif

int main(void)
{
    unsigned int i,j;
    for(i = 0; i < (1 << 30); i++) {
	if (UNLIKELY(i%10 == 0)) {
	    for (j=0; j<10; j++) {
		++i;
	    }
	}
	if (LIKELY(i%5 != 0)) {
	    ++i;
	}
    }
}

When compiling this program with
g++ -O3 -o test test.c, the result is (best case of all runs):

afu@marvin:~$ time ./test
real    0m4.246s
user    0m4.205s
sys     0m0.008s

However setting #if 0, gives the result (worst case):

afu@marvin:~$ time ./test
real    0m4.131s
user    0m4.094s
sys     0m0.009s

Is this to be expected when compiling c++ programs, and are there other 
issues to be aware of?

Btw. The compiler is:
gcc (GCC) 3.3.2 (Debian)
Copyright (C) 2003 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is 
NO warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR 
PURPOSE.

Machine: Athlon 1.4 GHz

Regards
Anders Fugmann