Understanding High-Level Synthesis Scheduling and Binding - 2023.2 English

Vitis High-Level Synthesis User Guide (UG1399)

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2023.2 English

High-Level Synthesis tools transform an untimed high-level specification into a fully timed implementation. During this transformation, a custom architecture is implemented to meet the specification requirements. The architecture generated contains the data path, control logic, memory interfaces, and how the RTL communicates with the external world. A data path consists of a set of storage elements such as (registers, register files, or memories), a set of functional units (such as ALUs, multipliers, shifters, and other custom functions), and interconnect elements (such as tristate drivers, multiplexers, and buses). Each component can take one or more clock cycles to execute, can be pipelined, and can have input or output registers. In addition, the entire data path and controller can be pipelined in several stages.

The designers should invest the early part of the project in redefining the architecture of the algorithm to meet the performance while keeping the algorithm at a higher level. For any specific HLS tool, there are design principles and best practices that are required to be followed to generate the optimized RTL that meets the expected performance.

The HLS Tool executes the following tasks as shown in the diagram below.

Figure 1. HLS Tasks
  1. Compile the algorithm written to meet specifications: This step includes several code optimizations such as dead-code elimination, constant folding, reporting unsupported constructs, etc.
  2. Schedule the operations for given clock cycles:
    1. The "Schedule" phase determines which operations occur during each clock cycle based on:

      • When an operation’s dependencies have been satisfied or are available.
      • The length of the clock cycle or clock frequency.
      • The time it takes for the operation to complete, as defined by the target device. More operations can be completed in a single clock cycle for longer clock periods. Some operations might need to be implemented as multi-cycle resources. HLS automatically schedules operations over more clock cycles
      • The available resources.
      • Incorporation of any user-specified optimization directives.
    2. During the "Schedule" phase, the tool determines what operator will execute in a given cycle and how many of these components are needed. The next step determines what operation binds to what resource.
  3. Bind the operations to the functional components and variables to the storage elements
    1. The binding task assigns hardware resources to implement each scheduled operation and maps operators (such as addition, multiplication, and shift) to specific RTL implementations. For example, a mult operation can be implemented in RTL as a combinational or pipelined multiplier.
    2. The binding task assigns memories, registers, or combinations of these to the array variables inside the function to meet the desired performance.
    3. If multiple operations use the same resource, this step can perform the resource sharing if not used in the same cycle.
  4. Control logic extraction creates a finite state machine (FSM) that sequences the operations in the RTL design according to the defined schedule.
  5. Creates the logic to communicate with the external world: The RTL generated will be communicating with the external world like streaming data from the external port or start/stop logic or accessing external memory.
  6. Finally, generate the RTL architecture

The next section walks through conceptually, how an HLS tool in general schedules the operators based on input constraints like a clock cycle and binds them to available hardware resources.