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.
syn.directive.latency 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 compilation time. Refer to the Improving Synthesis
Runtime and Capacity section of the Vitis High-Level Synthesis User
Guide (UG1399) for more information.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: syn.directive.latency Region_All_Loop_A
max=10
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.
Syntax
syn.directive.latency=[OPTIONS] <location>-
<location>is the location (function, loop or region) (in the formatfunction[/label]) to be constrained.
Options
-
max=<integer> - Limits the maximum latency.
-
min=<integer> - Limits the minimum latency.
Examples
Function foo is specified to
have a minimum latency of 4 and a maximum latency of 8.
syn.directive.latency=min=4 max=8 fooIn function foo, loop loop_row is specified to have a maximum latency of
12.
syn.directive.latency=max=12 foo/loop_row