3.6.2.4.2 LATENCY_REDUCTION

The LATENCY_REDUCTION settings control the SmartHLS's expression balancing optimization, of which the objective is to reduce the circuit latency. Below are the related settings:

Parameter NameDefault ValueDescription
LATENCY_REDUCTION1The main switch that enables or disables expression balancing. Setting to 0 disables all expression balancing optimizations.
LATENCY_REDUCTION_ALLOW_FP_REORDERING0By default, expression balancing does not re-order floating-point operations to prevent loss of precisions.

Setting to 1 allows to re-order floating-point operations if the circuit latency can be reduced.

Because the floating point operations may lose their commutativity and associativity with finite precision, this optimization may result in the failure of the co-simulation.
CAUTION: This is an unsafe optimization. Users should only enable this option on error tolerant designs.
LATENCY_REDUCTION_REDUCE_FP_CONVERSIONS 0

Setting to 1 will allow SmartHLS™ to cancel out back-and-forth conversion between floating-point and integer, with potential variations in numerical values.

For example, the following conversions can be cancelled when this setting is 1.

int a = (int)(float)(3); // a == 3.

float b = (float)(int)(1.2); // b == 1.2 instead of 1.

Level of accuracy loss caused by this optimization is run-time value dependent. Therefore, carefully examine the potential value sets of the associated operations as well as your design tolerance before enabling this option.
CAUTION: This is an unsafe optimization. Users should only enable this option on error tolerant designs.
LATENCY_REDUCTION_BALANCE_MULTI_USE_NODE0

By default expression balancing does not optimize the intermediate operations that have multiple uses, to avoid potential increase of resource usage.

Setting to 1 allows to re-order intermediate operations that have multiple uses and more latency reduction could be achieved.

Category
HLS Constraints
Value Type
Integer
Valid Values
0, 1
Default Value
As listed in the table above.
Location Where Default is Specified
examples/legup.tcl
Dependencies
None
Applicable Flows
All devices and flows
Test Status
Actively in-use
Examples
set_parameter LATENCY_REDUCTION 1
set_parameter LATENCY_REDUCTION_ALLOW_FP_REORDERING 0
set_parameter LATENCY_REDUCTION_REDUCE_FP_CONVERSIONS 0
set_parameter LATENCY_REDUCTION_BALANCE_MULTI_USE_NODE 0