Developer Guide

FPGA Optimization Guide for Intel® oneAPI Toolkits

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ID 767853
Date 3/31/2023
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Document Table of Contents
Document Table of Contents x
FPGA Optimization Guide for Intel® oneAPI Toolkits Introduction To FPGA Design Concepts Analyze Your Design Optimize Your Design FPGA Optimization Flags, Attributes, Pragmas, and Extensions Quick Reference Additional Information Document Revision History for the FPGA Optimization Guide for Intel® oneAPI Toolkits
Introduction To FPGA Design Concepts x
FPGA Architecture Overview Concepts of FPGA Hardware Design Methods of Hardware Design How Source Code Becomes a Custom Hardware Datapath Scheduling Mapping Parallelism Models to FPGA Hardware Memory Types
FPGA Architecture Overview x
Adaptive Logic Module (ALM) Lookup Table (LUT) Register Digital Signal Processing (DSP) Block Random Access Memory (RAM) Blocks
Concepts of FPGA Hardware Design x
Maximum Frequency (fMAX) Latency Pipelining Throughput Datapath Control Path Occupancy
How Source Code Becomes a Custom Hardware Datapath x
Mapping Source Code Instructions to Hardware Mapping Arrays and Their Accesses to Hardware
Scheduling x
Dynamic Scheduling Clustering the Datapath Handshaking Between Clusters
Mapping Parallelism Models to FPGA Hardware x
Data Parallelism Task Parallelism
Data Parallelism x
Executing Independent Operations Simultaneously Pipelining
Memory Types x
Kernel Memory Global Memory
Analyze Your Design x
Analyze the FPGA Early Image Analyze the FPGA Image
Analyze the FPGA Early Image x
Review the FPGA Optimization Report Access HLD FPGA Reports in JSON Format
Review the FPGA Optimization Report x
Loop Analysis Bottlenecks Viewer Area Estimates System Viewer Kernel Memory Viewer Schedule Viewer
Analyze the FPGA Image x
Quartus (Static) Summary Intel® FPGA Dynamic Profiler for DPC++ System-level Profiling Using the Intercept Layer for OpenCL* Applications
Quartus (Static) Summary x
Timing Failures
Intel® FPGA Dynamic Profiler for DPC++ x
Measure Kernel Performance Instrument the Kernel Pipeline with Performance Counters (-Xsprofile) Obtain Profiling Data During Runtime Reduce Area Resource Use While Profiling Profiler Analyses of Example SYCL* Design Scenarios Limitations
Obtain Profiling Data During Runtime x
Invoke the Profiler Runtime Wrapper to Obtain Profiling Data Use Intel® VTune™ Profiler
Use Intel® VTune™ Profiler x
Interpret Performance Counter Data
System-level Profiling Using the Intercept Layer for OpenCL* Applications x
Set Up the Intercept Layer for OpenCL* Applications
Optimize Your Design x
Throughput Resource Use
Throughput x
Single Work-item Kernels NDRange Kernels Memory Accesses Pipes Host
Single Work-item Kernels x
Single Work-item Kernel Design Guidelines Loops Single-Cycle Floating-Point Accumulator for Single Work-Item Kernels
Loops x
Refactor the Loop-Carried Data Dependency Relax Loop-Carried Dependency Transfer Loop-Carried Dependency to Local Memory Minimize the Memory Dependencies for Loop Pipelining Unroll Loops Factors to Consider for Loop Unrolling Fuse Loops to Reduce Overhead and Improve Performance Optimize Loops With Loop Speculation Remove Loop Bottlenecks Shannonization to Improve FMAX/II Optimize Inner Loop Throughput Improve Loop Performance by Caching On-Chip Memory
Single-Cycle Floating-Point Accumulator for Single Work-Item Kernels x
Strategies for Inferring the Accumulator
Memory Accesses x
Load-Store Units Global Memory Accesses Optimization Perform Kernel Computations Using Local or Private Memory Local and Private Memory Accesses Optimization Annotating Unified Shared Memory Pointers Zero-Copy Memory Access Additional Recommendations
Load-Store Units x
Load-Store Unit Styles Load-Store Unit Modifiers Load-Store Unit Controls
Global Memory Accesses Optimization x
Global Memory Bandwidth Use Calculation Manual Partition of Global Memory Partitioning Buffers Across Different Memory Types (Heterogeneous Memory) Partitioning Buffers Across Memory Channels of the Same Memory Type Ignoring Dependencies Between Accessor Arguments Contiguous Memory Accesses Static Memory Coalescing
Host x
Multi-Threaded Host Application Utilizing Hardware Kernel Invocation Queue Double Buffering Host Utilizing Kernel Invocation Queue N-Way Buffering to Overlap Kernel Execution Prepinning Memory Simple Host-Device Streaming Buffered Host-Device Streaming
Double Buffering Host Utilizing Kernel Invocation Queue x
Applying Double-Buffering Using the Intercept Layer for OpenCL* Applications
Resource Use x
Data Types and Operations Kernel Variable Accesses
Data Types and Operations x
Optimize Floating-point Operation Avoid Expensive Functions Variable-Precision Integer and Floating-Point Support
Variable-Precision Integer and Floating-Point Support x
Advantages and Limitations of Arbitrary Precision Data Types Declare and Use the AC Data Types
Declare and Use the AC Data Types x
Declare the ac_int Data Type Declare the ac_fixed Data Type Declare the ac_complex Data Type Declare the ap_float Data Type
Declare the ap_float Data Type x
Conversion Rules for ap_float Operations with Explicit Precision Controls Comparison Operators Additional ap_float Functions Additional Data Types Provided by the ap_float.hpp Header File Quality of Results and the ap_float Data Type
FPGA Optimization Flags, Attributes, Pragmas, and Extensions x
Optimization Flags Optimization Targets Kernel Variables Kernel Attributes Memory Attributes Loop Directives Floating-Point Pragmas Latency Controls (Beta) FPGA Extensions
Optimization Flags x
Specify Schedule FMAX Target for Kernels (-Xsclock=<clock target>) Disable Burst-Interleaving of Global Memory (-Xsno-interleaving=<global_memory_type>) Force Ring Interconnect for Global Memory (-Xsglobal-ring) Force a Single Store Ring to Reduce Area (-Xsforce-single-store-ring) Force Fewer Read Data Reorder Units to Reduce Area (-Xsnum-reorder) Disable Hardware Kernel Invocation Queue (-Xsno-hardware-kernel-invocation-queue) Modify the Handshaking Protocol Between Clusters (-Xshyper-optimized-handshaking) Disable Automatic Fusion of Loops (-Xsdisable-auto-loop-fusion) Fuse Adjacent Loops With Unequal Trip Counts (-Xsenable-unequal-tc-fusion) Pipeline Loops in Non-task Kernels (-Xsauto-pipeline) Control Semantics of Floating-Point Operations (-fp-model=<value>) Modify the Rounding Mode of Floating-point Operations (-Xsrounding=<rounding_type>) Global Control of Exit FIFO Latency of Stall-free Clusters (-Xssfc-exit-fifo-type=<value>) Enable the Read-Only Cache for Read-Only Accessors (-Xsread-only-cache-size=<N>) Control Hardware Implementation of the Supported Data Types and Math Operations (-Xsdsp-mode=<option>)
Optimization Targets x
Minimum Latency Flow
Kernel Attributes x
Specify Schedule FMAX Target for Kernels Specify a Workgroup Size Specify Number of SIMD Work Items Omit Hardware that Generates and Dispatches Kernel IDs Omit Hardware to Support the no_global_work_offset Attribute in parallel_for Kernels Reduce Kernel Area and Latency
Loop Directives x
disable_loop_pipelining Attribute initiation_interval Attribute ivdep Attribute loop_coalesce Attribute max_concurrency Attribute max_interleaving Attribute speculated_iterations Attribute unroll Pragma Loop Fuse Functions and nofusion Attribute max_reinvocation_delay Attribute (Beta)
FPGA Extensions x
Pipes Extension System of Tasks Extension (task_sequence) device_global Extension (Beta)
Pipes Extension x
Key Properties of a Pipe Accessing Pipes The pipe Class and its Use I/O Pipes Characteristics of Pipes Restrictions of Pipes Guidelines for Designing Pipes Pipe and Atomic Fence
System of Tasks Extension (task_sequence) x
Task Functions task_sequence Use Cases
Quick Reference x
Algorithmic C Data Types Floating Point Pragmas FPGA Accessor Properties FPGA Extensions FPGA Kernel Attributes FPGA Local Memory Function Latency Control Properties (Beta) FPGA LSU Controls FPGA Loop Directives FPGA Memory Attributes FPGA Optimization Flags Pipe API
Document Revision History for the FPGA Optimization Guide for Intel® oneAPI Toolkits x
Notices and Disclaimers
FPGA Optimization Guide for Intel® oneAPI Toolkits
Introduction To FPGA Design Concepts
Analyze Your Design
Optimize Your Design
FPGA Optimization Flags, Attributes, Pragmas, and Extensions
Quick Reference
Additional Information
Document Revision History for the FPGA Optimization Guide for Intel® oneAPI Toolkits

Unroll Loops

You can control the way the Intel® oneAPI DPC++/C++ Compiler translates SYCL kernel descriptions to hardware resources by unrolling loops. Loop unrolling decreases the number of iterations that the Intel® oneAPI DPC++/C++ Compiler executes at the expense of increased hardware resource consumption and increases performance. See also Unroll Loops

Consider the SYCL code for a parallel application in which each work-item is responsible for computing the accumulation of four elements in an array: 

queue.submit([&](handler &cgh) {
  accessor x(x_buf, cgh, read_only);
  accessor sum(sum_buf, cgh, write_only);
  cgh.single_task<class unoptimzed>([=]() {
    int accum = 0;
    for (size_t i = 0; i < 4; i++) {
      accum += x[i + get_global_id(0) * 4];
    }
    sum[get_global_id(0)] = accum;
  });
});

Observe the following three main operations that occur in this kernel:

  • Load operations from input x
  • Accumulation
  • Store operations to output sum

The Intel® oneAPI DPC++/C++ Compiler arranges these operations in a pipeline according to the data flow semantics of the SYCL* kernel code. For example, the Intel® oneAPI DPC++/C++ Compiler implements loops by forwarding the results from the end of the pipeline to the top of the pipeline, depending on the loop exit condition.

The SYCL kernel performs one loop iteration of each work-item per clock cycle. With sufficient hardware resources, you can increase kernel performance by unrolling the loop, which decreases the number of iterations that the kernel executes. To unroll a loop, add a #pragma unroll directive to the main loop, as shown in the following code example:

NOTE:

Loop unrolling significantly changes the structure of the compute unit that the Intel® oneAPI DPC++/C++ Compiler creates. 

queue.submit([&](handler &cgh) {
  accessor x(x_buf, cgh, read_only);
  accessor sum(sum_buf, cgh, write_only);
  cgh.single_task<class unoptimzed>([=]() {
    int accum = 0;
   
    #pragma unroll
    for (size_t i = 0; i < 4; i++) {
      accum += x[i + get_global_id(0) * 4];
    }
    sum[get_global_id(0)] = accum;
  });
});

In this example, the #pragma unroll directive causes the Intel® oneAPI DPC++/C++ Compiler to unroll four iterations of the loop completely. To accomplish the unrolling, the Intel® oneAPI DPC++/C++ Compiler expands the pipeline by tripling the number of addition operations and loading four times more data. With the removal of the loop, the compute unit assumes a feed-forward structure. As a result, the compute unit can store the sum elements in every clock cycle after the completion of the initial load operations and additions. The Intel® oneAPI DPC++/C++ Compiler further optimizes this kernel by coalescing the four load operations so that the compute unit can load all necessary input data to calculate a result in one load operation.

Factors to Consider for Loop Unrolling

  • Avoid nested looping structures. Instead, implement a large single loop or unroll inner loops by adding the #pragma unroll directive whenever possible. For example, if you compile a kernel that has a heavily nested loop structure, wherein each loop includes a #pragma unroll directive, you might experience a long compilation time. The Intel® oneAPI DPC++/C++ Compiler might fail to meet scheduling because it cannot unroll this nested loop structure easily, resulting in a high II. In this case, the Intel® oneAPI DPC++/C++ Compiler issues the following error message along with the line number of the outermost loop: 

    Kernel <function> exceeded the Max II. The Kernel's resource usage is estimated to be much larger than FPGA capacity. It will perform poorly 
even if it fits. Reduce resource utilization of the kernel by reducing loop unroll factors within it (if any) or otherwise reduce amount of 
computation within the kernel.

  • Unrolling the loop and coalescing load operations from global memory allow the hardware implementation of the kernel to perform more operations per clock cycle.
  • The Intel® oneAPI DPC++/C++ Compiler might not be able to unroll a loop completely under the following circumstances:
    • You specify complete unrolling of a data-dependent loop with a very large number of iterations. Consequently, the hardware implementation of your kernel might not fit into the FPGA.
    • You specify complete unrolling and the loop bounds are not constants.
    • The loop consists of complex control flows (for example, a loop containing complex array indexes or exit conditions that are unknown at compilation time).

    For the last two cases listed above, the Intel® oneAPI DPC++/C++ Compiler issues the following warning: 

    Full unrolling of the loop is requested but the loop bounds cannot be determined. The loop is not unrolled.

    To enable loop unrolling in these situations, specify the #pragma unroll <N> directive, where <N> is the unroll factor. The unroll factor limits the number of iterations that the Intel® oneAPI DPC++/C++ Compiler unrolls. Refer to Single Work-item Kernel Design Guidelines for tips on constructing well-structured loops.

TIP:

For additional information, refer to the FPGA tutorial sample "Loop Unroll" listed in the Intel® oneAPI Samples Browser on Linux* or Windows*, or access the code sample on GitHub.

Parent topic: Loops
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