Description
Merges consecutive loops into a single loop to reduce overall latency, increase sharing, and improve logic optimization. Merging loops:
- Reduces the number of clock cycles required in the RTL to transition between the loop-body implementations.
- Allows the loops be implemented in parallel (if possible).
The LOOP_MERGE pragma will seek to merge all loops within the scope it is placed. For example, if you apply a LOOP_MERGE pragma in the body of a loop, the Vitis HLS tool applies the pragma to any sub-loops within the loop but not to the loop itself.
The rules for merging loops are:
- If the loop bounds are variables, they must have the same value (number of iterations).
- If the loop bounds are constants, the maximum constant value is used as the bound of the merged loop.
- Loops with both variable bounds and constant bounds cannot be merged.
- The code between loops to be merged cannot have side effects. Multiple execution of this code should generate the same results (a=b is allowed, a=a+1 is not).
- Loops cannot be merged when they contain FIFO reads. Merging changes the order of the reads. Reads from a FIFO or FIFO interface must always be in sequence.
Syntax
Place the pragma in the C source within the required scope or region of code.
#pragma HLS loop_merge forceWhere:
-
force - Optional keyword to force loops to be merged even when the HLS tool issues a warning.
Important: In this case, you must manually ensure that the merged loop will function correctly.
Examples
Merges all consecutive loops in function foo into a single loop.
void foo (num_samples, ...) {
#pragma HLS loop_merge
int i;
...
loop_1: for(i=0;i< num_samples;i++) {
...All loops inside loop_2 (but not loop_2 itself) are merged by using the force option. Place the pragma in the body of loop_2.
loop_2: for(i=0;i< num_samples;i++) {
#pragma HLS loop_merge force
...