3 ms·

I'm curious, what is usually the cause of code starting to work when it is compiled with -O0?

• bostik 9y ago
  I don't have an answer to that, but I have seen - and worked with - several codebases which work fine when compiled with -g, but will crash (in good cases) or behave irrationally (in bad cases) without. The crashing ones at least are easy. Somewhere a list or variable-argument array is missing the NULL terminator...
  • tonyarkles 9y ago
    This was on Windows with VC++, but same deal. The code that some cow-orkers had written was copying strings like "LAX" and "ORD" into `char airport[3]` using strcpy(). In debug builds, VC was allocating a whole 32-bit word, but in release builds it was packing everything on the stack. Write to it, and the terminating null ends up overwriting a byte of the next variable on the stack. Urgh. Of course, these were several hundred line functions, so the strcpy and the subsequent use of the trashed variable were a long ways away.
• ryl00 9y ago
  In my experience, it often means variables that are used before they are initialized, or dangling pointers.
• gvb 9y ago
  * Optimizer bugs. Thankfully these are less common nowadays. It the "good old days" of new compilers this happened quite a bit. Embedded code: * Missing "volatiles" which allow the optimizer to optimize out "unused" loads and stores to hardware or multitasking shared variables. * Race conditions (e.g. unsynchronized access to multitasking shared variables). Making the code run slower changes the access pattern, often times obscuring the bug.
• gpderetta 9y ago
  Crap code that hit undefined behaviour every other line (like accessing dead temporaries or freed memory, assuming stack layout, out of bound accesses, etc.).