3.6.2.2.8 set_resource_constraint

This Tcl command constrains the resource allocated by SmartHLS™. For instance, to only have a single divider in the entire circuit, user can specify: set_resource_constraint divide 1. This makes SmartHLS instantiate a maximum of 1 divider in the circuit, and if there are multiple division operations required, they will share the same divider.

Important: A constraint on "divide" will apply to:
  • signed_divide_8
  • signed_divide_16
  • signed_divide_32
  • signed_divide_64
  • unsigned_divide_8
  • unsigned_divide_16
  • unsigned_divide_32
  • unsigned_divide_64

It can also be used to constrain the number of memory ports. To make all memories single-ported: set_resource_constraint memory_port 1 For memory ports, only 1 and 2 are valid values, as FPGA RAMs have up to 2 ports.

This Tcl command should only be used by advanced SmartHLS users.

Category
HLS Constraints
Value Type
set_resource_constraint <operation> <constraint>, in which <operation> is a string and <constraint> is an integer
Valid Values
See Default and Examples
Important: Operator name should match the device family operation database file: boards/PolarFire/PolarFire.tcl
Default Values
memory_port 2
divide 1
modulus 1
multiply 2
fp_add 1
fp_subtract 1
fp_multiply 1
fp_divide 1
fp 1
Location Where Default is Specified
examples/legup.tcl
Dependencies
None
Applicable Flows
All devices and flows
Test Status
Actively in-use
Examples
set_resource_constraint signed_divide_16 3
set_resource_constraint signed_divide 2
set_resource_constraint divide 1
Important:

When implementing an integer multiply operation, SmartHLS's default behaviour is to optimize for timing (Fmax) by mapping a multiply operation to target DSPs. When a multiply operation is wider than the DSP can support, SmartHLS splits the multiply into multiple DSPs and automatically inserts registers to help RTL synthesis tool to utilize registers in DSPs better, and hence achieve better timing.

However, this split-multiply feature cannot support sharing multipliers. When set_resource_constraint multiply X is specified by user, SmartHLS will turn off the split-multiply feature and use a generic multiplier that can be shared by more than one multiply operations. SmartHLS will also print a warning as follows, Warning: Detected set_resource_constraint setting for integer multiplier. However, multiplier sharing is incompatible with the multiply splitting feature. Disabling multiply splitting feature and the normal multiplier modules will be used.

Similarly, set_operation_latency multiply X will trigger the same behaviour because the split-multiply does not support user-configured multiply latency.