Description
The tool is informing the user that the specified loop nest is an imperfect loop and cannot be flattened.
Explanation
- Imperfect loop nests cannot be flattened by the tool. This results in additional clock cycles to enter and exit the loops.
- Imperfect loops are loops that show the following characteristics:
- The innermost loop has a variable bound.
- The loop body is not exclusively inside the inner loop
- In the small example shown here, because of an imperfect loop nest. it will take one cycles to enter the loop and one cycle to exit the loop copy_A. This creates an additional n_elements * 2 clock cycles.
#define BUFFER_SIZE 256
void foo(int *a, int *b) {
// n_elements: variables
outCopyA: for (int i = 0 ; i < n_elements ; i += BUFFER_SIZE) {
int size = BUFFER_SIZE;
copyA: for (int j = 0 ; j < size/2 ; ++j) // 1 cycle to enter the loop
{
b[j*2] = a[i+j*2]; // even index for b
b[(j*2)|1] = a[i+((j*2)|1)]; // odd index for b
}
}Bursting: At times bursting optimization may result in introducing code in between the loops which may cause this warning. This is a limitation in the current version of the tool and will be addressed in a future releases.
Solution
This message is to inform the user that the loop is an imperfect loop nest and the user needs to convert it to a perfect loop or a semi-perfect loop nest.
HLS provides the loop_flatten constraint to allow labeled perfect and semiperfect nested loops to be flattened, removing the need to re-code for optimal hardware performance and reducing the number of cycles it takes to perform the operations in the loop.
Perfect loop nest: Only the innermost loop has loop body content, there is no logic specified between the loop statements and all the loop bounds are constant. Example is shown below.
void foo_top { a, b, c, d} {
...
Outer: while(j<100)
Inner: while(i<6) // 1 cycle to enter inner
...
LOOP_BODY
...
} // 1 cycle to exit inner
}
...
}Semi-perfect loop nest: Only the innermost loop has loop body content, there is no logic specified between the loop statements but the outermost loop bound can be a variable.