Intel® High Level Synthesis Compiler Pro Edition: Best Practices Guide

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Date 10/04/2021
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Document Table of Contents
Document Table of Contents x
1. Intel® HLS Compiler Pro Edition Best Practices Guide 2. Best Practices for Coding and Compiling Your Component 3. FPGA Concepts 4. Interface Best Practices 5. Loop Best Practices 6. fMAX Bottleneck Best Practices 7. Memory Architecture Best Practices 8. System of Tasks Best Practices 9. Datatype Best Practices 10. Advanced Troubleshooting A. Intel® HLS Compiler Pro Edition Best Practices Guide Archives B. Document Revision History for Intel® HLS Compiler Pro Edition Best Practices Guide
3. FPGA Concepts x
3.1. FPGA Architecture Overview 3.2. Concepts of FPGA Hardware Design 3.3. Methods of Hardware Design
3.1. FPGA Architecture Overview x
3.1.1. Adaptive Logic Module (ALM) 3.1.2. Digital Signal Processing (DSP) Block 3.1.3. Random-Access Memory (RAM) Blocks
3.1.1. Adaptive Logic Module (ALM) x
3.1.1.1. Lookup Table (LUT) 3.1.1.2. Register
3.2. Concepts of FPGA Hardware Design x
3.2.1. Maximum Frequency (fMAX) 3.2.2. Latency 3.2.3. Pipelining 3.2.4. Throughput 3.2.5. Datapath 3.2.6. Control Path 3.2.7. Occupancy
3.3. Methods of Hardware Design x
3.3.1. How Source Code Becomes a Custom Hardware Datapath 3.3.2. Scheduling 3.3.3. Mapping Parallelism Models to FPGA Hardware 3.3.4. Memory Types
3.3.1. How Source Code Becomes a Custom Hardware Datapath x
3.3.1.1. Mapping Source Code Instructions to Hardware 3.3.1.2. Mapping Arrays and Their Accesses to Hardware
3.3.2. Scheduling x
3.3.2.1. Dynamic Scheduling 3.3.2.2. Clustering the Datapath Cluster Types Cluster Characteristics 3.3.2.3. Handshaking Between Clusters
3.3.3. Mapping Parallelism Models to FPGA Hardware x
3.3.3.1. Data Parallelism 3.3.3.2. Task Parallelism
3.3.3.1. Data Parallelism x
3.3.3.1.1. Executing Independent Operations Simultaneously 3.3.3.1.2. Pipelining
3.3.3.1.2. Pipelining x
3.3.3.1.2.1. Pipelining Loops Within A Component 3.3.3.1.2.2. Pipelining Across Component Invocations
3.3.4. Memory Types x
3.3.4.1. Component Memory 3.3.4.2. External Memory
4. Interface Best Practices x
4.1. Choose the Right Interface for Your Component 4.2. Control LSUs For Your Variable-Latency MM Host Interfaces 4.3. Avoid Pointer Aliasing
4.1. Choose the Right Interface for Your Component x
4.1.1. Pointer Interfaces 4.1.2. Avalon® Memory Mapped Host Interfaces 4.1.3. Avalon® Memory Mapped Agent Memories 4.1.4. Avalon® Memory Mapped Agent Registers 4.1.5. Avalon® Streaming Interfaces 4.1.6. Pass-by-Value Interface
5. Loop Best Practices x
5.1. Reuse Hardware By Calling It In a Loop 5.2. Parallelize Loops 5.3. Construct Well-Formed Loops 5.4. Minimize Loop-Carried Dependencies 5.5. Avoid Complex Loop-Exit Conditions 5.6. Convert Nested Loops into a Single Loop 5.7. Place if-Statements in the Lowest Possible Scope in a Loop Nest 5.8. Declare Variables in the Deepest Scope Possible 5.9. Raise Loop II to Increase fMAX 5.10. Control Loop Interleaving
5.2. Parallelize Loops x
5.2.1. Pipeline Loops 5.2.2. Unroll Loops 5.2.3. Example: Loop Pipelining and Unrolling
6. fMAX Bottleneck Best Practices x
6.1. Balancing Target fMAX and Target II
7. Memory Architecture Best Practices x
7.1. Example: Overriding a Coalesced Memory Architecture 7.2. Example: Overriding a Banked Memory Architecture 7.3. Merge Memories to Reduce Area 7.4. Example: Specifying Bank-Selection Bits for Local Memory Addresses
7.3. Merge Memories to Reduce Area x
7.3.1. Example: Merging Memories Depth-Wise 7.3.2. Example: Merging Memories Width-Wise
8. System of Tasks Best Practices x
8.1. Executing Multiple Loops in Parallel 8.2. Sharing an Expensive Compute Block 8.3. Implementing a Hierarchical Design 8.4. Balancing Capacity in a System of Tasks
8.4. Balancing Capacity in a System of Tasks x
8.4.1. Enable the Intel® HLS Compiler to Infer Data Path Buffer Capacity Requirements 8.4.2. Explicitly Add Buffer Capacity to Your Design When Needed
9. Datatype Best Practices x
9.1. Avoid Implicit Data Type Conversions 9.2. Avoid Negative Bit Shifts When Using the ac_int Datatype
10. Advanced Troubleshooting x
10.1. Component Fails Only In Simulation 10.2. Component Gets Poor Quality of Results
1. Intel® HLS Compiler Pro Edition Best Practices Guide
2. Best Practices for Coding and Compiling Your Component
3. FPGA Concepts
4. Interface Best Practices
5. Loop Best Practices
6. fMAX Bottleneck Best Practices
7. Memory Architecture Best Practices
8. System of Tasks Best Practices
9. Datatype Best Practices
10. Advanced Troubleshooting
A. Intel® HLS Compiler Pro Edition Best Practices Guide Archives
B. Document Revision History for Intel® HLS Compiler Pro Edition Best Practices Guide

3.3.2.2. Clustering the Datapath

Dynamically scheduling all operations adds overhead in the form of additional FPGA area needed to implement the required handshaking control logic.

To reduce this overhead, the compiler groups fixed latency operations into clusters. A cluster of fixed latency operations, such as arithmetic operations, needs fewer handshaking interfaces, thereby reducing the area overhead.

Figure 5. Clustered Logic


If A, B, and C from do not contain variable latency operations, the compiler can cluster them together, as illustrated in .

Clustering the logic reduces area by removing the need for signals to stall data flow in addition to other handshaking logic within the cluster.

Cluster Types

The Intel® HLS Compiler can create the following types of clusters:

  • Stall-Enable Cluster (SEC): This cluster type passes the handshaking logic to every pipeline stage in the cluster in parallel. If the cluster is stalled by logic from further down in the datapath, all logic in the SEC stalls at the same time.
    Figure 6. Stall-Enable Cluster


  • Stall-Free Cluster (SFC): This cluster type adds a first in, first out (FIFO) buffer to the end of the cluster that can accommodate at least the entire latency of the pipeline in the cluster. This FIFO is often called an exit FIFO because it is attached to the exit of the cluster datapath.

    Because of this FIFO, the pipeline stages in the cluster do not require any handshaking logic. The stages can run freely and drain into the capacity FIFO, even if the cluster is stalled from logic further down in the datapath.

Figure 7. Stall-Free Cluster


Cluster Characteristics

The exit FIFO of the stall free cluster results in some tradeoffs:
  • Area: Because an SEC does not use an exit FIFO, it can save FPGA area compared to an SFC.

    If you have a design with many small, low-latency clusters, you can save a substantial amount of area by asking the compiler to use SECs instead of SFCs with the hls_use_stall_enable_clusterscomponent attribute. For details, refer to hls_use_stall_enable_clusters Component Attribute in the Intel® HLS Compiler Reference Manual .

  • Latency: Logic that uses SFCs might have a larger latency than logic that uses SECs because of the write-read latency of the exit FIFO.
  • fMAX : In an SFC, the oStall signal has less fanout than in an SEC.

    For a cluster with many pipeline stages, you can improve your design fMAX by using an SFC.

  • Handshaking: The exit FIFO in SFCs allow them to take advantage of hyper-optimized handshaking between clusters. For more information, refer to Hyper Optimized Handshaking.

    SECs do not support this capability.

  • Bubble Handling: SECs remove only leading bubbles. A leading bubble is a bubble that arrives before the first piece of valid data arrives in the cluster. SECs do not remove any arriving afterwards.

    SFCs can use the exit FIFO to remove all bubbles from the pipeline if the SFC gets a downstream stall signal.

  • Stall Behavior: When an SEC receives a downstream stall, it stalls any logic upstream of it within one clock cycle.

    When an SFC receives a downstream stall, the exit FIFO allows it to consume additional valid data depending on how deep the exit FIFO is and how many bubbles are in the cluster datapath.

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