Joaquín Mª López Muñoz <joaquin@tid.es> writes:
While in the process of replacing iterator helpers from <boost/operators.hpp> in favor of boost::iterator_facade as my internal iterator generation lib in Boost.MultiIndex, I've discovered an unpleasant increase in compilation times.
The attached program shows the problem. Compilation times when using boost::iterator_facade are *way* higher than with good old boost::bidirectional_iterator_helper (to test it yourself, just comment/uncomment the line "#define USE_BOOST_ITERATOR" in the program):
MSVC++ 6.0: iterator_facade: ~7 sec. bidirectional_iterator_helper: less than 1 sec. GCC 3.2 iterator_facade: 4.9 sec. bidirectional_iterator_helper: 2.5 sec.
I had to modify the test to eliminate undefined behavior and make sure dead code elimination didn't kick in (enclosed). Results: GCC 3.4.4: Test 1 discarded Test 2 with operator library: 1.469 with iterator library: 2.282 Test 3 with operator library: 1.438 with iterator library: 2.297 GCC 4.0.2: Test 1 discarded Test 2 with operator library: 1.468 with iterator library: 1.579 Test 3 with operator library: 1.453 with iterator library: 1.563 MSVC 7.1 Test 1 discarded Test 2 with operator library: 1.219 with iterator library: 1.187 Test 3 with operator library: 1.203 with iterator library: 1.188 Unfortunately I couldn't get vc6 or vc7 to compile it :( Now, here's the interesting thing: If you change the needless postincrement you're using into preincrement, you get: GCC 3.4.4: Test 1 discarded Test 2 with operator library: 1.485 with iterator library: 1.453 Test 3 with operator library: 1.437 with iterator library: 1.453
My questions: 1. Maybe the test program is flawed and has some (unseen by me) bias in favor of bidirectional_iterator_helper?
The postincrement comes to mind.
2. If not so, why is iterator_facade so very expensive? Is there any way to alleviate the problem?
I don't know; you could experiment with defining postincrement in terms of preincrement. You could also look at what type postfix_increment_result is computing in iterator_facade.hpp
As it stands, I'm not so sure it's a good idea to adopt boost::iterator_facade: my local tests for Boost.MultiIndex show an overall increase in compilation time of around 25% :(
If your code is correct with bidirectional_iterator_helper, of course there's no need to switch. -- Dave Abrahams Boost Consulting www.boost-consulting.com