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

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ID 683152
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 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

4.2. Control LSUs For Your Variable-Latency MM Host Interfaces

Controlling the type of load-store units (LSUs) that the Intel® HLS Compiler Pro Edition uses to interact with variable-latency Memory Mapped (MM) Host interfaces can help save area in your design. You might also encounter situations where disabling static coalescing of a load/store with other load/store operations benefits the performance of your design.

Review the following tutorial to learn about controlling LSUs: <quartus_installdir>/hls/examples/tutorials/best_practices/lsu_control.

To see if you need to use LSU controls, review the High-Level Design Reports for your component, especially the Function Memory Viewer, to see if the memory access pattern (and its associated LSUs) inferred by the Intel® HLS Compiler Pro Edition match your expected memory access pattern. If they do not match, consider controlling the LSU type, LSU coalescing, or both.

Control the Type of LSU Created

The Intel® HLS Compiler Pro Edition creates either burst-coalesced LSUs or pipelined LSUs.

In general, use burst-coalesced LSUs when an LSU is expected to process many load/store requests to memory words that are consecutive. The burst-coalesced LSU attempts to "dynamically coalesce" the requests into larger bursts in order to utilize memory bandwidth more efficiently.

The pipelined LSU consumes significantly less FPGA area, but processes load/store requests individually without any coalescing. This processing is useful when your design is tight on area or when the accesses to the variable-latency MM Host interface are not necessarily consecutive.

The following code example shows both types of LSU being implemented for a variable-latency MM Host interface: 
component void
dut(mm_host<int, dwidth<128>, awidth<32>, aspace<4>, latency<0>> &Buff1,
    mm_host<int, dwidth<32>, awidth<32>, aspace<5>, latency<0>> &Buff2) {
  int Temp[SIZE];

  using pipelined = lsu<style<PIPELINED>>; 
  using burst_coalesced = lsu<style<BURST_COALESCED>>;

  for (int i = 0; i<SIZE; i++) {
    Temp[i] = burst_coalesced::load(&Buff1[i]); // Burst-Coalesced LSU
  }

  for (int i = 0; i<SIZE; i++) {
    pipelined::store(&Buff2[i], 2*Temp[i]); // Pipelined LSU
  }
}

Disable Static Coalescing

Static coalescing is typically beneficial because it reduces the total number of LSUs in your design by statically combining multiple load/store operations into wider load/store operations

However, there are cases where static coalescing leads to unaligned accesses, which you might not want to occur. There are also cases where multiple loads/stores get coalesced even though you intended for only a subset of them to be operational at a time. In these cases, consider disable static coalescing for the load/store operations that you did not want to be coalesced.

For the following code example, the Intel® HLS Compiler does not statically coalesce the two load operations into one wide load operation: 
component int
dut(mm_host<int, dwidth<256>, awidth<32>, aspace<1>, latency<0>> &Buff1,
    int i, bool Cond1, bool Cond2) {

  using no_coalescing = lsu<style<PIPELINED>, static_coalescing<false>>;
  int Val = 0;
  if (Cond1) {
    Val = no_coalescing::load(&Buff1[i]);
  }
  if (Cond2) {
    Val = no_coalescing::load(&Buff1[i + 1]);
  }
  return Val;
}
If the two load operations were coalesced, an unaligned LSU would be created, which would hurt the throughput of your component.
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