syn.directive.expression_balance - 2024.2 English

Description

Sometimes C/C++ code is written with a sequence of operations, resulting in a long chain of operations in RTL. With a small clock period, this can increase the latency in the design. By default, the Vitis HLS tool rearranges the operations using associative and commutative properties. The rearrangement creates a balanced tree that can shorten the chain, potentially reducing latency in the design at the cost of extra hardware.

Expression balancing rearranges operators to construct a balanced tree and reduce latency.

• For integer operations expression balancing is on by default but can be disabled.
• For floating-point operations, expression balancing is off by default but can be enabled.

The syn.directive.expression_balance command allows this expression balancing to be turned off, or on, within a specified scope.

Syntax

syn.directive.expression_balance=[OPTIONS] <location>

• <location> is the location (in the format function[/label]) where expression balancing should be disabled, or enabled.

Options

off

Turns off expression balancing at the specified location.

Specifying the syn.directive.expression_balance command enables expression balancing in the specified scope. Adding the off option disables it.

Examples

Disables expression balancing within function My_Func.

syn.directive.expression_balance=off My_Func

Explicitly enables expression balancing in function My_Func2.

set_directive_expression_balance=My_Func2