Question

I am programming an algorithm that contains 4 nested for loops. The problem is at at each level a pointer is updated. The innermost loop only uses 1 of the pointers. The algorithm does a complicated count. When I include a debugging statement that logs the combination of the indexes and the results of the count I get the correct answer. When the debugging statement is omitted, the count is incorrect. The program is compiled with the -O3 option on gcc. Why would this happen?

Answer 1

Always put your code through something like valgrind, Purify, etc, before blaming the optimizer. Especially when blaming things related to pointers.

It’s not to say the optimizer isn’t broken, but more than likely, it’s you. I’ve worked on various C++ compilers and seen my share of seg faults that only happen with optimized code. Quite often, people do things like forget to count the \0 when allocating space for a string, etc. And it’s just luck at that point on which pages you’re allocated when the program runs with different -O settings.

Also, important questions: are you dealing with restricted pointers at all?