pragma HLS latency

Description

Specifies a minimum or maximum latency value, or both, for the completion of functions, loops, and regions. Latency is defined as the number of clock cycles required to produce an output. Function latency is the number of clock cycles required for the function to compute all output values, and return. Loop latency is the number of cycles to execute all iterations of the loop. See "Performance Metrics Example" of Vivado Design Suite User Guide: High-Level Synthesis (UG902).

Vivado HLS always tries to the minimize latency in the design. When the LATENCY pragma is specified, the tool behavior is as follows:
  • Latency is greater than the minimum, or less than the maximum: The constraint is satisfied. No further optimizations are performed.
  • Latency is less than the minimum: If Vivado HLS can achieve less than the minimum specified latency, it extends the latency to the specified value, potentially increasing sharing.
  • Latency is greater than the maximum: If Vivado 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, and produces a design with the smallest achievable latency in excess of the maximum.
Tip: You can also use the LATENCY pragma 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 improves tool runtime. Refer to "Improving Run Time and Capacity" of Vivado Design Suite User Guide: High-Level Synthesis (UG902) for more information.

Syntax

Place the pragma within the boundary of a function, loop, or region of code where the latency must be managed.

#pragma HLS latency min=<int> max=<int>

Where:

  • min=<int>: Optionally specifies the minimum latency for the function, loop, or region of code.
  • max=<int>: Optionally specifies the maximum latency for the function, loop, or region of code.
    Note: Although both min and max are described as optional, one must be specified.

Example 1

Function foo is specified to have a minimum latency of 4 and a maximum latency of 8:
int foo(char x, char a, char b, char c) {
  #pragma HLS latency min=4 max=8
  char y;
  y = x*a+b+c;
  return y
}

Example 2

In the following example loop_1 is specified to have a maximum latency of 12. Place the pragma in the loop body as shown:
void foo (num_samples, ...) {
  int i;
  ...
  loop_1: for(i=0;i< num_samples;i++) { 
  #pragma HLS latency max=12
    ...
    result = a + b;
  }
}

Example 3

The following example creates a code region and groups signals that need to change in the same clock cycle by specifying zero latency:
// create a region { } with a latency = 0
{
  #pragma HLS LATENCY max=0 min=0
  *data = 0xFF;
  *data_vld = 1;
}

See Also

  • Vivado Design Suite User Guide: High-Level Synthesis (UG902)