3.6.2.2.6 set_operation_latency
(Ask a Question)This Tcl command sets the latency of a given operation. Latency refers to the number of clock cycles required to complete the computation; an operation with latency one requires one cycle, while zero-latency operations are completely combinational, meaning multiple such operations can be chained together in a single clock cycle. This command is used to schedule each type operation to take the specified number of cycles.
This Tcl command should only be used by advanced SmartHLS™ users.
- Category
- HLS Constraints
- Value Type
set_operation_latency <operation> <constraint>, in which<operation>is a string and<constraint>is an integer- Valid Values
- See Default and Examples.
- Default Values
fp_add 14 fp_subtract 14 fp_multiply 11 fp_divide_32 33 fp_divide_64 61 fp_truncate_64 3 fp_extend_32 2 fp_fptosi 6 fp_sitofp 6 signed_comp_o 1 signed_comp_u 1 reg 2
- Location Where Default is Specified
examples/legup.tcl- Dependencies
- None
- Applicable Flows
- All devices and flows
- Test Status
- Actively in-use
- Examples
# set memory operations to take 3 cycles
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 does not support a user-configurable latency. When set_operation_latency multiply X is specified by user, SmartHLS will turn off the split-multiply feature and use a generic multiplier that will adapt to user-specified latency. SmartHLS will also print a warnign as follows, Warning: Detected set_operation_latency setting for integer multiplier. However, configurable latency is incompatible with the multiply splitting feature. Disabling multiply splitting feature and the normal multiplier modules will be used.
Similarly, set_resource_constraint multiply X will trigger the same behaviour because the split-multiply does not support sharing the multipliers. :::