Description
syn.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 syn.directive.tripcount.The syn.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 syn.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. 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.
syn.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 syn.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.
syn.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
number of clock cycles from the first transaction of a loop, or nested loop, to the
start of the next transaction 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. 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.
- unit=[sec | cycle]
- Specifies the unit associated with the
target_tiortarget_tlvalues. The unit can either be specified as seconds, or clock cycles. When the unit is specified as seconds, a unit can be specified with the value to indicate nanoseconds (ns), picoseconds (ps), microseconds (us).
Example 1
The loop labeled loop_1 is specified to
have target transaction interval of 4 clock cycles:
syn.directive.performance=loop_1 target_ti=4