syn.directive.loop_merge - 2025.2 English - UG1702

Description

Merges all loops into a single loop. Merging loops:

• Reduces the number of clock cycles required in the RTL to transition between the loop-body implementations.
• Allows implementation of loops in parallel (if possible).

The rules for loop merging are:

• If the loop bounds are variables, they must have the same value (number of iterations).
• If loops bounds are constants, the tool uses the maximum constant value as the bound of the loop being merged.
• Loops with both variable bounds and constant bounds cannot be merged.
• The code between merging loops cannot have side effects. Multiple executions of this code need to generate the same results.
  • a=b is allowed
  • a=a+1 is not allowed.
• 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

syn.directive.loop_merge=[options] <location>

• <location> is the location (in the format function[/label]) at which the loops reside.

Options

force

Forces loops to be merged even when Vitis HLS issues a warning. Ensures that the merged loop functions correctly.

Examples

Merges all consecutive loops in function foo into a single loop.

syn.directive.loop_merge=foo

All loops inside loop_2 of function foo (but not loop_2 itself) are merged by using the force option.

syn.directive.loop_merge=force foo/loop_2

See Also

• syn.directive.loop_flatten
• syn.directive.unroll