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

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ID 683259
Date 12/18/2019
Public
Document Table of Contents
Document Table of Contents x
1. Intel® HLS Compiler Standard Edition Best Practices Guide 2. Best Practices for Coding and Compiling Your Component 3. Interface Best Practices 4. Loop Best Practices 5. Memory Architecture Best Practices 6. Datatype Best Practices 7. Advanced Troubleshooting A. Intel® HLS Compiler Standard Edition Best Practices Guide Archives B. Document Revision History for Intel® HLS Compiler Standard Edition Best Practices Guide
3. Interface Best Practices x
3.1. Choose the Right Interface for Your Component 3.2. Avoid Pointer Aliasing
3.1. Choose the Right Interface for Your Component x
3.1.1. Pointer Interfaces 3.1.2. Avalon® Memory Mapped Master Interfaces 3.1.3. Avalon® Memory Mapped Slave Interfaces 3.1.4. Avalon® Streaming Interfaces 3.1.5. Pass-by-Value Interface
4. Loop Best Practices x
4.1. Reuse Hardware By Calling It In a Loop 4.2. Parallelize Loops 4.3. Construct Well-Formed Loops 4.4. Minimize Loop-Carried Dependencies 4.5. Avoid Complex Loop-Exit Conditions 4.6. Convert Nested Loops into a Single Loop 4.7. Declare Variables in the Deepest Scope Possible
4.2. Parallelize Loops x
4.2.1. Pipeline Loops 4.2.2. Unroll Loops 4.2.3. Example: Loop Pipelining and Unrolling
5. Memory Architecture Best Practices x
5.1. Example: Overriding a Coalesced Memory Architecture 5.2. Example: Overriding a Banked Memory Architecture 5.3. Merge Memories to Reduce Area 5.4. Example: Specifying Bank-Selection Bits for Local Memory Addresses
5.3. Merge Memories to Reduce Area x
5.3.1. Example: Merging Memories Depth-Wise 5.3.2. Example: Merging Memories Width-Wise
6. Datatype Best Practices x
6.1. Avoid Implicit Data Type Conversions 6.2. Avoid Negative Bit Shifts When Using the ac_int Datatype
7. Advanced Troubleshooting x
7.1. Component Fails Only In Cosimulation 7.2. Component Gets Bad Quality of Results
1. Intel® HLS Compiler Standard Edition Best Practices Guide
2. Best Practices for Coding and Compiling Your Component
3. Interface Best Practices
4. Loop Best Practices
5. Memory Architecture Best Practices
6. Datatype Best Practices
7. Advanced Troubleshooting
A. Intel® HLS Compiler Standard Edition Best Practices Guide Archives
B. Document Revision History for Intel® HLS Compiler Standard Edition Best Practices Guide

4.4. Minimize Loop-Carried Dependencies

Loop-carried dependencies occur when the code in a loop iteration depends on the output of previous loop iterations. Loop-carried dependencies in your component increase loop initiation interval (II), which reduces the performance of your component.

The loop structure below has a loop-carried dependency because each loop iteration reads data written by the previous iteration. As a result, each read operation cannot proceed until the write operation from the previous iteration completes. The presence of loop-carried dependencies reduces of pipeline parallelism that the Intel® HLS Compiler Standard Edition can achieve, which reduces component performance. 

for(int i = 1; i < N; i++)
{
    A[i] = A[i - 1] + i;
}

The Intel® HLS Compiler performs a static memory dependency analysis on loops to determine the extent of parallelism that it can achieve. If the Intel® HLS Compiler cannot determine that there are no loop-carried dependencies, it assumes that loop-dependencies exist. The ability of the compiler to test for loop-carried dependencies is impeded by unknown variables at compilation time or if array accesses in your code involve complex addressing.

To avoid unnecessary loop-carried dependencies and help the compiler to better analyze your loops, follow these guidelines:

Avoid Pointer Arithmetic

Compiler output is suboptimal when your component accesses arrays by dereferencing pointer values derived from arithmetic operations. For example, avoid accessing an array as follows: 

for(int i = 0; i < N; i++)
{
    int t = *(A++);
    *A = t;
}

Introduce Simple Array Indexes

Some types of complex array indexes cannot be analyzed effectively, which might lead to suboptimal compiler output. Avoid the following constructs as much as possible:
  • Nonconstants in array indexes.

    For example, A[K + i], where i is the loop index variable and K is an unknown variable.

  • Multiple index variables in the same subscript location.

    For example, A[i + 2 × j], where i and j are loop index variables for a double nested loop.

    The array index A[i][j] can be analyzed effectively because the index variables are in different subscripts.

  • Nonlinear indexing.

    For example, A[i & C], where i is a loop index variable and C is a nonconstant variable.

Use Loops with Constant Bounds Whenever Possible

The compiler can perform range analysis effectively when loops have constant bounds.

Ignore Loop-Carried Dependencies

If there are no implicit memory dependencies across loop iterations, you can use the ivdep pragma to tell the Intel® HLS Compiler Standard Edition to ignore the memory dependency.

For details about how to use the ivdep pragma, see Loop-Carried Dependencies (ivdep Pragma) in the Intel® High Level Synthesis Compiler Standard Edition Reference Manual.

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