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

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Date 9/23/2022
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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

10.2. Component Gets Poor Quality of Results

While there are many reasons why your design achieves a poor quality of results (QoR), bad memory configurations are often an important factor. Review the Function Memory Viewer report in the High Level Design Reports, and look for stallable arbitration nodes and unexpected RAM utilization.

The information in this section describes some common sources of stallable arbitration nodes or excess RAM utilization.

Component Uses More FPGA Resource Than Expected

By default, the Intel® HLS Compiler Pro Edition tries to optimize your component for the best throughput by trying to maximize the maximum operating frequency (fMAX).

A way to reduce area consumption is to relax the fMAX requirements by setting a target fMAX value with the --clock i++ command option or the hls_scheduler_target_fmax_mhz component attribute. The HLS compiler can often achieve a higher fMAX than you specify, so when you set a target fMAX to a lower value than you need, your design might still achieve an acceptable fMAX value, and a design that consumes less area.

To learn more about the behavior of fMAX target value control see the following tutorial: <quartus_installdir>/hls/examples/tutorials/best_practices/set_component_target_fmax

Loops Do Not Achieve II=1

If you specify a target fMAX , the compiler might conservatively increase II in order to achieve your target fMAX .

If you specify a target fMAX and require II=1, you should use #pragma ii 1 on your loops that require II=1. For more details, refer to Balancing Target fMAX and Target II.

Incorrect Bank Bits

If you access parts of an array in parallel (either a single- or multidimensional array), you might need to configure the memory bank selection bits.

See Memory Architecture Best Practices for details about how to configure efficient memory systems.

Conditional Operator Accessing Two Different Arrays of struct Variables

In some cases, if you try to access different arrays of struct variables with a conditional operator, the Intel® HLS Compiler Pro Edition merges the arrays into the same RAM block. You might see stallable arbitration in the Function Memory Viewer because there are not enough Load/Store site on the memory system.

For example, the following code examples show an array of struct variables, a conditional operator that results in stallable arbitration, and a workaround that avoids stallable arbitration. 
struct MyStruct {
  float a;
  float b;
}

MyStruct array1[64];
MyStruct array2[64];
The following conditional operator that uses these arrays of struct variables causes stallable arbitration: 
MyStruct value = (shouldChooseArray1) ? array1[idx] : array2[idx];
You can avoid the stallable arbitration that the conditional operator causes here by removing the operator and using an explicit if statement instead. 
MyStruct value;
if (shouldChooseArray1)
{
    value = array1[idx];
} else
{
    value = array2[idx];
}

Cluster Logic

Your design might consume more RAM blocks than you expect, especially if you store many array variables in large registers.

You can use the hls_use_stall_enable_clusters component attribute to prevent the compiler from inserting stall-free cluster exit FIFOs.

The Area Analysis of System report in the high-level design report (report.html) can help find this issue.

The three matrices are stored intentionally in RAM blocks, but the RAM blocks for the matrices account for less than half of the RAM blocks consumed by the component.

If you look further down the report, you might see that many RAM blocks are consumed by Cluster logic or State variable. You might also see that some of your array values that you intended to be stored in registers were instead stored in large numbers of RAM blocks.

Notice the number of RAM blocks that are consumed by Cluster Logic and State.

In some cases, you can reduce this RAM block usage by with the following techniques:
  • Pipeline loops instead of unrolling them.
  • Storing local variables in local RAM blocks (hls_memory memory attribute) instead of large registers (hls_register memory attribute).

Component with a System of Tasks Hangs or has Poor Throughput

If your component contains a system of tasks, you might need to add launch/collect capacity.

Incorrectly specifying launch/collect capacity can result in hangs or poor throughput.

For details, refer to Balancing Capacity in a System of Tasks.

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