A literal MPI corner case

I just spent a day tracking down a nasty MPI bug in some fluids code, and the cause is amusing enough to record. A process was interleaving message receives from two of its neighbors, one above and one to the right. Each message contained a band of data one grid cell thick. Both messages were sending into the same array, so the two regions intersected at the corner of the domain.

Strict aliasing and std::vector

Something just occurred to me about the gcc implementation of std::vector in C++. Internally, an instance of vector needs to store (1) a pointer to the start of the memory, (2) the current vector size, and (3) the current buffer size. However, (2) and (3) can be represented either as integers relative to the start pointer or as absolute pointers past the end of the active and buffer spaces. Specifically, the two representations are