Streaming Library - Blocking Behaviour

Note that the fifo read() and write() calls are blocking. Hence if a module attempts to read from a FIFO that is empty, it will be stalled. Similarly, if it attempts to write to a FIFO that is full, it will be stalled. If you want non-blocking behaviour, you can check if the FIFO is empty (with empty()) before calling read(), and likewise, check if the FIFO is full (with full()) before calling write() (see Streaming Library - Non-Blocking Behaviour).

With the blocking behaviour, if the depths of FIFOs are not sized properly, it can cause a deadlock. SmartHLS™ prints out messages to alert the user that a FIFO is causing stalls.

In hardware simulation, the following messages are shown.

Warning: fifo_write() has been stalled for     1000000 cycles due to FIFO being full.
Warning: fifo_read() has been stalled for     1000000 cycles due to FIFO being empty.
Warning: fifo_read() has been stalled for     1000000 cycles due to FIFO being empty.
Warning: fifo_write() has been stalled for     1000000 cycles due to FIFO being full.
Warning: fifo_read() has been stalled for     1000000 cycles due to FIFO being empty.
Warning: fifo_read() has been stalled for     1000000 cycles due to FIFO being empty.

If you continue to see these messages, you can suspect that there is a deadlock. In this case, we recommend making sure there is no blocking read from an empty FIFO or blocking write to a full FIFO, and potentially increasing the depth of the FIFOs.