Description
The loop tripcount is the total number of iterations performed by a loop. Vitis HLS reports the total latency of each loop (the number of cycles to execute all iterations of the loop). This loop latency is therefore a function of the tripcount (number of loop iterations).
The tripcount can be a constant value. It might depend on the value
of variables used in the loop expression (for example, x<y) or control statements used inside the loop.
Vitis HLS cannot determine the tripcount in some cases. These cases include, for example, those in which the variables used to determine the tripcount are:
- Input arguments, or
- Variables calculated by dynamic operation
In the following example, the maximum iteration of the for-loop is
determined by the value of input num_samples. The
value of num_samples is not defined in the C
function, but comes into the function from the outside.
void foo (num_samples, ...) {
int i;
...
loop_1: for(i=0;i< num_samples;i++) {
...
result = a + b;
}
}In cases where the loop latency is unknown or cannot be calculated,
set_directive_loop_tripcount allows you to
specify minimum, maximum, and average iterations for a loop. This lets the tool
analyze how the loop latency contributes to the total design latency in the reports
and helps you determine appropriate optimizations for the design.
Syntax
set_directive_loop_tripcount [OPTIONS] <location>-
<location>is the location of the loop (in the formatfunction[/label]) at which the tripcount is specified.
Options
-
-avg <integer> - Specifies the average number of iterations.
-
-max <integer> - Specifies the maximum number of iterations.
-
-min <integer> - Specifies the minimum number of iterations.
Examples
loop_1 in function foo is specified to have a minimum tripcount of 12, and
a maximum tripcount of 16:
set_directive_loop_tripcount -min 12 -max 16 -avg 14 foo/loop_1