I was about to leave this topic not to swamp the list with something that appears to go nowere. But now I feel that I must answer the comments, so here it goes. By "agressive optimisation" I mean an optimisation that drastically reduces run-time performance of (some part of) the program. So I guess automatic vectorisation could fall into this term. In my scenario the original programmer did not have any reason for the strange code -- it just happened. I'm sure we all write bad/suboptimal code sometimes. Hence there would be no comment describing this in the code. And compiling without optimisations would not prevent the above -- only if the original programmer(s) had compiled without optimisations but this was obviously not the case. We seem to agree that the only way to know what a change will do to the performance of a program is testing (using whatever tools are available). That is what makes me a bit uncomfortable because then we don't really understand it. Disabling all optimisations would definitely be a step backwards and not where we want to go. Then again, maybe this problem is mostly of theoretical concern and not at all common in practice. I guess it **is** far more likely that clean code will trigger optimisations. But we cannot really know for sure... Regards Johan