pragma HLS performance - 2022.2 English

Vitis High-Level Synthesis User Guide (UG1399)

Document ID
Release Date
2022.2 English


Note: The PERFORMANCE pragma 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 the TRIPCOUNT pragma.

The PERFORMANCE pragma lets you specify a high-level constraint (target_ti) 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 pragmas needed to achieve the desired result. The PERFORMANCE pragma does not guarantee the specified value will be achieved, and so it is only a target.

Note: The INLINE pragma is applied automatically to functions inside any pipelined loop that has II=1 to improve throughput. If you apply the PERFORMANCE pragma that infers a pipeline with II=1, it will also trigger the auto-inline optimization. You can disable this for specific functions by using #pragma HLS INLINE OFF

The target transaction interval (target_ti) specifies a performance target for loops, where a transaction is a complete set of loop iterations (tripcount) and the interval is the time between when the first transaction starts and the second transaction starts.

Target Transaction Interval (target_ti)
Specifies the number of clock cycles between successive starts of the loop. In other words, the clock cycles from the start of the first transaction of a loop, or nested loop, and the start of the next transaction of the loop.

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.


Place the pragma within the boundary a loop, or the outer loop of a loop nest.

#pragma HLS performance target_ti=<value>


Specifies a target transaction interval defined as the number of clock cycles for the function, loop, or region of code to complete an iteration. 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.

Example 1

The outer loop is specified to have target transaction interval of 1000 clock cycles.

  for (int i =0; i < 1000; ++i) {
#pragma HLS performance target_ti=1000
    for (int j = 0; j < 8; ++j) {
      int tmp = b_buf[j].read();
      b[i * 8 + j] = tmp + 2;