set_directive_pipeline - 2021.2 English

Vitis High-Level Synthesis User Guide (UG1399)

Document ID
UG1399
ft:locale
English (United States)
Release Date
2021-12-15
Version
2021.2 English

Description

Reduces the initiation interval (II) for a function or loop by allowing the concurrent execution of operations as described in Function and Loop Pipelining. A pipelined function or loop can process new inputs every N clock cycles, where N is the initiation interval (II). An II of 1 processes a new input every clock cycle.

As a default behavior, with the PIPELINE pragma or directive Vitis HLS will generate the minimum II for the design according to the specified clock period constraint. The emphasis will be on meeting timing, rather than on achieving II unless the -II option is specified.

The default type of pipeline is defined by the config_compile -pipeline_style command, but can be overridden in the PIPELINE pragma or directive.

If Vitis HLS cannot create a design with the specified II, it:

  • Issues a warning.
  • Creates a design with the lowest possible II.

You can then analyze this design with the warning messages to determine what steps must be taken to create a design that satisfies the required initiation interval.

Syntax

set_directive_pipeline [OPTIONS] <location>

Where:

  • <location> is the location (in the format function[/label]) to be pipelined.

Options

-II <integer>
Specifies the desired initiation interval for the pipeline. Vitis HLS tries to meet this request. Based on data dependencies, the actual result might have a larger II.
-off
Turns off pipeline for a specific loop or function. This can be used when config_compile -pipeline_loops is used to globally pipeline loops.
-rewind
Note: Applicable only to a loop.
Optional keyword. Enables rewinding as described in Rewinding Pipelined Loops for Performance. This enables continuous loop pipelining with no pause between one execution of the loop ending and the next execution starting. Rewinding is effective only if there is one single loop (or a perfect loop nest) inside the top-level function. The code segment before the loop:
  • Is considered as initialization.
  • Is executed only once in the pipeline.
  • Cannot contain any conditional operations (if-else).
-style <stp | frp | flp>
Specifies the type of pipeline to use for the specified function or loop. For more information on pipeline styles refer to Flushing Pipelines. The types of pipelines include:
stp
Stall pipeline. Runs only when input data is available otherwise it stalls. This is the default setting, and is the type of pipeline used by Vitis HLS for both loop and function pipelining. Use this when a flushable pipeline is not required. For example, when there are no performance or deadlock issue due to stalls.
flp
This option defines the pipeline as a flushable pipeline as described in Flushing Pipelines. This type of pipeline typically consumes more resources and/or can have a larger II because resources cannot be shared among pipeline iterations.
frp
Free-running, flushable pipeline. Runs even when input data is not available. Use this when you need better timing due to reduced pipeline control signal fanout, or when you need improved performance to avoid deadlocks. However, this pipeline style can consume more power as the pipeline registers are clocked even if there is no data.
Important: This is a hint not a hard constraint. The tool checks design conditions for enabling pipelining. Some loops might not conform to a particular style and the tool reverts to the default style (stp) if necessary.

Examples

Function func is pipelined with the specified initiation interval.

set_directive_pipeline func II=1