syn.directive.dataflow - 2025.2 English - UG1702

Vitis Reference Guide (UG1702)
Document ID
UG1702
Release Date
2025-11-20
Version
2025.2 English

Description

This directive performs all operations sequentially in a C/C++ description. In the absence of any directives that limit resources (such as set_directive_allocation), Vitis HLS seeks to minimize latency and improve concurrency. Data dependencies can limit this. For example, functions or loops that access arrays must finish all read/write accesses to the arrays before they complete. This prevents the next function or loop that consumes the data from starting operation.

However, it is possible for the operations in a function or loop to start operation before the previous function or loop completes all its operations. syn.directive.dataflow specifies that dataflow optimization be performed on the functions or loops, improving the concurrency of the RTL implementation. When syn.directive.dataflow is specified, the HLS tool analyzes the dataflow between sequential functions or loops and creates channels. The channels are based on ping-pong RAMs or FIFOs that allow consumer functions or loops to start operation before the producer functions or loops complete. Functions or loops can operate in parallel, decreasing latency and improving the throughput of the RTL.

Tip: The syn.dataflow.xxx command specifies the default memory channel and FIFO depth used by syn.directive.dataflow as explained in Dataflow Configuration.

If no initiation interval is specified, Vitis HLS attempts to minimize the initiation interval and start operation as soon as data is available. The initiation interval is number of cycles between the start of one function or loop and the next. For the DATAFLOW optimization to work, the data must flow through the design from one task to the next. The following coding styles prevent the HLS tool from performing the DATAFLOW optimization.

  • Single-producer-consumer violations
  • Feedback between tasks
  • Conditional execution of tasks
  • Loops with multiple exit conditions
Important: If any of these coding styles are present, the HLS tool issues a message and does not perform DATAFLOW optimization.

Finally, the DATAFLOW optimization has no hierarchical implementation. If a sub-function or loop contains additional tasks that might benefit from the DATAFLOW optimization, apply optimization to the loop and the sub-function. You can also inline the sub-function.

Syntax

syn.directive.dataflow=<location> disable_start_propagation
  • <location> is the location (in the format function[/label]) at which dataflow optimization is to be performed.
  • disable_start_propagation disables the creation of a start FIFO used to propagate a start token to an internal process. Such FIFOs can sometimes be a bottleneck for performance.

Examples

Specifies DATAFLOW optimization within function foo.

syn.directive.dataflow=foo

See Also