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

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ID 683152
Date 12/04/2023
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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
1. Intel® HLS Compiler Pro Edition Best Practices Guide x
1.1. Pending Deprecation of the Intel® HLS Compiler
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

7.3.1. Example: Merging Memories Depth-Wise

Use the hls_merge("<mem_name>","depth") attribute to force the Intel® HLS Compiler Pro Edition to implement variables in the same memory system, merging their memories by depth.

All variables with the same <mem_name> label set in their hls_merge attributes are merged.

Consider the following component code:

component int depth_manual(bool use_a, int raddr, int waddr, int wdata) {
  int a[128];
  int b[128];

  int rdata;

  // mutually exclusive write
  if (use_a) {
    a[waddr] = wdata;
  } else {
    b[waddr] = wdata;
  }

  // mutually exclusive read
  if (use_a) {
    rdata = a[raddr];
  } else {
    rdata = b[raddr];
  }

  return rdata;
}

The code instructs the Intel® HLS Compiler Pro Edition to implement local memories a and b as two on-chip memory blocks, each with its own load and store instructions.

Figure 36. Implementation of Local Memory for Component depth_manual

Because the load and store instructions for local memories a and b are mutually exclusive, you can merge the accesses, as shown in the example code below. Merging the memory accesses reduces the number of load and store instructions, and the number of on-chip memory blocks, by half. 

component int depth_manual(bool use_a, int raddr, int waddr, int wdata) {
  int a[128] hls_merge("mem","depth");
  int b[128] hls_merge("mem","depth");

  int rdata;

  // mutually exclusive write
  if (use_a) {
    a[waddr] = wdata;
  } else {
    b[waddr] = wdata;
  }

  // mutually exclusive read
  if (use_a) {
    rdata = a[raddr];
  } else {
    rdata = b[raddr];
  }

  return rdata;
}
Figure 37. Depth-Wise Merge of Local Memories for Component depth_manual

There are cases where merging local memories with respect to depth might degrade memory access efficiency. Before you decide whether to merge the local memories with respect to depth, refer to the HLD report ( <result>.prj/reports/report.html) to ensure that they have produced the expected memory configuration with the expected number of loads and stores instructions. In the example below, the Intel® HLS Compiler Pro Edition should not merge the accesses to local memories a and b because the load and store instructions to each memory are not mutually exclusive. 

component int depth_manual(bool use_a, int raddr, int waddr, int wdata) {
  int a[128] hls_merge("mem","depth");
  int b[128] hls_merge("mem","depth");

  int rdata;

  // NOT mutually exclusive write

  a[waddr] = wdata;
  b[waddr] = wdata;
  
  // NOT mutually exclusive read

  rdata = a[raddr];
  rdata += b[raddr];
  
  return rdata;
}

In this case, the Intel® HLS Compiler Pro Edition might double pump the memory system to provide enough ports for all the accesses. Otherwise, the accesses must share ports, which prevent stall-free accesses.

Figure 38. Local Memories for Component depth_manual with Non-Mutually Exclusive Accesses
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