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.
-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.
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.