GCC: program doesn't work with compilation option -O3

Question

I\'m writing a C++ program that doesn\'t work (I get a segmentation fault) when I compile it with optimizations (options -O1, -O2, -O3, etc.), but it works just fine when I

Answer 1

Ran into the same problem a few days ago, in my case it was aliasing. And GCC does it differently, but not wrongly, when compared to other compilers. GCC has become what some might call a rules-lawyer of the C++ standard, and their implementation is correct, but you also have to be really correct in you C++, or it'll over optimize somethings, which is a pain. But you get speed, so can't complain.

Answer 2

It is very seldom the compiler fault, but compiler do have bugs in them, and them often manifest themselves at different optimization levels (if there is a bug in an optimization pass, for example).

In general when reporting programming problems: provide a minimal code sample to demonstrate the issue, such that people can just save the code to a file, compile and run it. Make it as easy as possible to reproduce your problem.

Also, try different versions of GCC (compiling your own GCC is very easy, especially on Linux). If possible, try with another compiler. Intel C has a compiler which is more or less GCC compatible (and free for non-commercial use, I think). This will help pinpointing the problem.

Answer 3

It's almost (almost) never the compiler.

First, make sure you're compiling warning-free, with -Wall.

If that didn't give you a "eureka" moment, attach a debugger to the least optimized version of your executable that crashes and see what it's doing and where it goes.

5 will get you 10 that you've fixed the problem by this point.

Answer 4

Wow, I didn't expect answers so quicly, and so many...

The error occurs upon sorting a std::vector of pointers using std::sort()

I provide the strict-weak-ordering functor.

But I know the functor I provide is correct because I've used it a lot and it works fine.

Plus, the error cannot be some invalid pointer in the vector becasue the error occurs just when I sort the vector. If I iterate through the vector without applying std::sort first, the program works fine.

I just used GDB to try to find out what's going on. The error occurs when std::sort invoke my functor. Aparently std::sort is passing an invalid pointer to my functor. (of course this happens with the optimized version only, any level of optimization -O, -O2, -O3)

Answer 5

The true answer is hidden somewhere inside all the comments in this thread. First of all: it is not a bug in the compiler.

The problem has to do with floating point precision. distanceToPointSort should be a function that should never return true for both the arguments (a,b) and (b,a), but that is exactly what can happen when the compiler decides to use higher precision for some data paths. The problem is especially likely on, but by no means limited to, x86 without -mfpmath=sse. If the comparator behaves that way, the sort function can become confused, and the segmentation fault is not surprising.

I consider -ffloat-store the best solution here (already suggested by CesarB).

Answer 6

You may be running into an aliasing problem (or it could be a million other things). Look up the -fstrict-aliasing option.

This kind of question is impossible to answer properly without more information.