Very poor II with double buffering

You have conditional swapping of "w" and "r" on line 106; I am not sure if it is reasonable to expect the compiler to be able to pipeline the operation in this case. What I can recommend is physically separating your buffer into two (e.g. B_buffer_1 and B_buffer_2) and using conditional statements to choose which one to read from or write to (maybe with a flag that gets set or reset in the conditional statement at the end), rather than swapping the side of the buffer.