pragma HLS stream

Description

By default, array variables are implemented as RAM:

  • Top-level function array parameters are implemented as a RAM interface port.
  • General arrays are implemented as RAMs for read-write access.
  • In sub-functions involved in DATAFLOW optimizations, the array arguments are implemented using a RAM pingpong buffer channel.
  • Arrays involved in loop-based DATAFLOW optimizations are implemented as a RAM pingpong buffer channel

If the data stored in the array is consumed or produced in a sequential manner, a more efficient communication mechanism is to use streaming data as specified by the STREAM pragma, where FIFOs are used instead of RAMs.

Important: When an argument of the top-level function is specified as INTERFACE type ap_fifo, the array is automatically implemented as streaming.

Syntax

Place the pragma in the C source within the boundaries of the required location.

#pragma HLS stream variable=<variable> depth=<int> dim=<int> off 

Where:

  • variable=<variable>: Specifies the name of the array to implement as a streaming interface.
  • depth=<int>: Relevant only for array streaming in DATAFLOW channels. By default, the depth of the FIFO implemented in the RTL is the same size as the array specified in the C code. This options lets you modify the size of the FIFO and specify a different depth.

    When the array is implemented in a DATAFLOW region, it is common to the use the depth= option to reduce the size of the FIFO. For example, in a DATAFLOW region when all loops and functions are processing data at a rate of II=1, there is no need for a large FIFO because data is produced and consumed in each clock cycle. In this case, the depth= option may be used to reduce the FIFO size to 1 to substantially reduce the area of the RTL design.

    Tip: The config_dataflow -depth command provides the ability to stream all arrays in a DATAFLOW region. The depth= option specified here overrides the config_dataflow command for the assigned variable.
  • dim=<int>: Specifies the dimension of the array to be streamed. The default is dimension 1. Specified as an integer from 0 to N, for an array with N dimensions.
  • off: Disables streaming data. Relevant only for array streaming in dataflow channels.
    Tip: The config_dataflow -default_channel fifo command globally implies a STREAM pragma on all arrays in the design. The off option specified here overrides the config_dataflow command for the assigned variable, and restores the default of using a RAM pingpong buffer based channel.

Example 1

The following example specifies array A[10] to be streaming, and implemented as a FIFO:
#pragma HLS STREAM variable=A

Example 2

In this example array B is set to streaming with a FIFO depth of 12:
#pragma HLS STREAM variable=B depth=12

Example 3

Array C has streaming disabled. It is assumed to be enabled by config_dataflow in this example:
#pragma HLS STREAM variable=C off

See Also

  • Vivado Design Suite User Guide: High-Level Synthesis (UG902)