Description
Allows nested loops to be flattened into a single loop hierarchy with improved latency.
In the RTL implementation, it requires one clock cycle to move from an outer loop to an inner loop, and from an inner loop to an outer loop. Flattening nested loops allows them to be optimized as a single loop. This saves clock cycles, potentially allowing for greater optimization of the loop body logic.
Apply the LOOP_FLATTEN pragma to the loop body of the inner-most loop in the loop hierarchy. Only perfect and semi-perfect loops can be flattened in this manner:
- Perfect loop nests
-
- Only the innermost loop has loop body content.
- There is no logic specified between the loop statements.
- All loop bounds are constant.
- Semi-perfect loop nests
-
- Only the innermost loop has loop body content.
- There is no logic specified between the loop statements.
- The outermost loop bound can be a variable.
- Imperfect loop nests
- When the inner loop has variable bounds (or the loop body is not exclusively inside the inner loop), try to restructure the code, or unroll the loops in the loop body to create a perfect loop nest.
Syntax
set_directive_loop_flatten [OPTIONS] <location>-
<location>is the location (inner-most loop), in the formatfunction[/label].
Options
-
-offoroff=true - Option to prevent loop flattening from taking place, and can
prevent some loops from being flattened while all others in the specified
location are flattened.Important: The presence of the LOOP_FLATTEN pragma or directive enables the optimization. The addition of
-offdisables it.
Examples
Flattens loop_1 in function
foo and all (perfect or semi-perfect)
loops above it in the loop hierarchy, into a single loop. Place the pragma in the
body of loop_1.
set_directive_loop_flatten foo/loop_1Prevents loop flattening in loop_2 of function foo.
Place the pragma in the body of loop_2.
set_directive_loop_flatten -off foo/loop_2