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 HLS 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