Description
set_directive_performance applies to loops and loop nests, and requires a known
loop tripcount to determine the performance. If your loop has a variable tripcount then you
must also specify set_directive_loop_tripcount.The set_directive_performance lets you
specify a high-level constraint, target_ti or target_tl, defining the number of clock cycles between
successive starts of a loop, and lets the tool infer lower-level UNROLL, PIPELINE,
ARRAY_PARTITION, and INLINE directives needed to achieve the desired result. The set_directive_performance does not guarantee the specified value
will be achieved, and so it is only a target.
The target_ti is the interval between
successive starts of the loop, or between the start of the first iteration of the loop, and
the next start of the first iteration of the loop. In the following code example, a target_ti=T would mean the target interval for the start of loop
L2 between two consecutive iterations of L1 should be 100 cycles.
const int T = 100;
L1: for (int i=0; i<N; i++)
L2: for (int j=0; j<M; j++){
pragma HLS performance target_ti=T
...
}The target_tl is the interval between
start of the loop and end of the loop, or between the start of the first iteration of the
loop and the completion of the last iteration of the loop. For example, in the preceding
code example a target_tl=T means the target completion of
loop L2 for a single iteration of L1 should be 100 cycles.
set_directive_inline is applied automatically to functions inside any pipelined
loop that has II=1 to improve throughput. If you apply the PERFORMANCE pragma or directive
that infers a pipeline with II=1, it will also trigger the auto-inline optimization. You can
disable this for specific functions by using set_directive_inline
off.The transaction interval is the initiation interval (II) of the loop times
the number of iterations, or tripcount: target_ti = II *
loop tripcount. Conversely, target_ti
= FreqHz / Operations per second.
For example, assuming an image processing function that processes a single
frame per invocation with a throughput goal of 60 fps, then the target throughput for the
function is 60 invocations per second. If the clock frequency is 180 MHz, then target_ti is 180M/60, or 3 million clock cycles per function
invocation.
Syntax
Specify the directive for a labeled loop.
set_directive_performance <location> [OPTIONS]Where:
<location> specifies the loop in the format function/loop_label.
Options
-
-target_ti=<value> - Specifies a target transaction interval defined as the number of clock
cycles for the loop to complete an iteration. The transaction interval refers to the
time the loop starts processing a transaction and the time it begins processing the next
transaction. The <value> can be specified as an integer, floating point, or
constant expression that is resolved by the tool as an integer. Note: A warning will be returned if truncation occurs.
-
-target_tl=<value> -
Specifies a target latency defined as the number of clock cycles for the loop to complete all iterations. The transaction latency is defined as the interval between the start of the first iteration of the loop , and the completion of the last iteration of the loop. The <value> can be specified as an integer, floating point, or constant expression that is resolved by the tool as an integer.
Example 1
The loop labeled loop_1 in the function
func is specified to have target transaction interval of 4 clock cycles:
set_directive_performance func/loop_1 -target_ti=4