Specifying Arrays as Ping-Pong Buffers or FIFOs - 2021.1 English

Vitis High-Level Synthesis User Guide (UG1399)

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2021.1 English

All arrays are implemented by default as ping-pong to enable random access. These buffers can also be sized if needed. For example, in some circumstances, such as when a task is being bypassed, a performance degradation is possible. To mitigate this affect on performance, you can give more slack to the producer and consumer by increasing the size of these buffers by using the STREAM directive as shown below.

void top ( ... ) {
#pragma HLS dataflow
  int A[1024];
#pragma HLS stream off variable=A depth=3
  producer(A, B, …);  // producer writes A and B
  middle(B, C, ...);  // middle reads B and writes C
  consumer(A, C, …);  // consumer reads A and C

In the interface, arrays are automatically specified as streaming if an array on the top-level function interface is set as interface type ap_fifo, axis or ap_hs, it is automatically set as streaming.

Inside the design, all arrays must be specified as streaming using the STREAM directive if a FIFO is desired for the implementation.

Note: When the STREAM directive is applied to an array, the resulting FIFO implemented in the hardware contains as many elements as the array. The -depth option can be used to specify the size of the FIFO.

The STREAM directive is also used to change any arrays in a DATAFLOW region from the default implementation specified by the config_dataflow configuration.

  • If the config_dataflow default_channel is set as ping-pong, any array can be implemented as a FIFO by applying the STREAM directive to the array.
    Note: To use a FIFO implementation, the array must be accessed in a streaming manner.
  • If the config_dataflow default_channel is set to FIFO or Vitis HLS has automatically determined the data in a DATAFLOW region is accessed in a streaming manner, any array can still be implemented as a ping-pong implementation by applying the STREAM directive to the array with the -off option.
Important: To preserve the accesses, it might be necessary to prevent compiler optimizations (dead code elimination particularly) by using the volatile qualifier.

When an array in a DATAFLOW region is specified as streaming and implemented as a FIFO, the FIFO is typically not required to hold the same number of elements as the original array. The tasks in a DATAFLOW region consume each data sample as soon as it becomes available. The config_dataflow command with the -fifo_depth option or the STREAM directive with the -depth can be used to set the size of the FIFO to the minimum number of elements required to ensure flow of data never stalls. If the -off option is selected, the -depth option sets the depth (number of blocks) of the PIPO. The depth should be at least 2.