set_directive_latency - 2023.1 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.

If the intention is to limit the total latency of all loop iterations, the latency directive should be applied to a region that encompasses the entire loop, as in this example: set_directive Region_All_Loop_A

Region_All_Loop_A: {
Loop_A: for (i=0; i<N; i++) 
  { 
  ..Loop Body... 
  }
}

In this case, even if the loop is unrolled, the latency directive sets a maximum limit on all loop operations.

If Vitis HLS cannot meet a maximum latency constraint it relaxes the latency constraint and tries to achieve the best possible result.

If a minimum latency constraint is set and Vitis HLS can produce a design with a lower latency than the minimum required it inserts dummy clock cycles to meet the minimum latency.

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

See Also

• pragma HLS latency