set_directive_latency - 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

Specifies a maximum or minimum latency value, or both, on a function, loop, or region.

Vitis HLS always aims for minimum latency. The behavior of the tool when minimum and maximum latency values are specified is as follows:

  • Latency is less than the minimum: If Vitis HLS can achieve less than the minimum specified latency, it extends the latency to the specified value, potentially enabling increased sharing.
  • Latency is greater than the minimum: The constraint is satisfied. No further optimizations are performed.
  • Latency is less than the maximum: The constraint is satisfied. No further optimizations are performed.
  • Latency is greater than the maximum: If Vitis HLS cannot schedule within the maximum limit, it increases effort to achieve the specified constraint. If it still fails to meet the maximum latency, it issues a warning. Vitis HLS then produces a design with the smallest achievable latency.
Tip: You can also use the LATENCY pragma or directive to limit the efforts of the tool to find an optimum solution. Specifying latency constraints for scopes within the code: loops, functions, or regions, reduces the possible solutions within that scope and can improve tool runtime. Refer to Improving Synthesis Runtime and Capacity for more information.

Syntax

set_directive_latency [OPTIONS] <location>
  • <location> is the location (function, loop or region) (in the format function[/label]) to be constrained.

Options

-max <integer>
Specifies the maximum latency.
-min <integer>
Specifies the minimum latency.

Examples

Function foo is specified to have a minimum latency of 4 and a maximum latency of 8.

set_directive_latency -min=4 -max=8 foo

In function foo, loop loop_row is specified to have a maximum latency of 12. Place the pragma in the loop body.

set_directive_latency -max=12 foo/loop_row