pragma HLS stream - 2020.2 English

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.
Arrays involved in sub-functions, or loop-based DATAFLOW optimizations are implemented as a RAM ping pong 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. See Managing Interface Synthesis for more information.

Syntax

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

#pragma HLS stream variable=<variable> depth=<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 option lets you modify the size of the FIFO to specify a different depth.
When the array is implemented in a DATAFLOW region, it is common to 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 can 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 in the STREAM pragma overrides the config_dataflow -depth setting for the specified <variable>.
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 specified <variable>, and restores the default of using a RAM ping pong 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. In this example, it is assumed to be enabled by config_dataflow -default_channel.

#pragma HLS STREAM variable=C off

pragma HLS stream - 2020.2 English - UG1399

Vitis High-Level Synthesis User Guide (UG1399)