Developer Guide

Intel® oneAPI DPC++/C++ Compiler Handbook for FPGAs

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ID 785441
Date 10/24/2024
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Document Table of Contents
Document Table of Contents x
Intel oneAPI DPC++/C++ Compiler Handbook for FPGAs Overview Introduction To FPGA Design Concepts Intel oneAPI FPGA Development Getting Started with the Intel oneAPI DPC++/C++ Compiler for Intel FPGA Development Defining a Kernel for FPGAs Debugging and Verifying Your Design Analyzing Your Design Optimizing Your Kernel Optimizing Your Host Application Integrating Your Kernel into DSP Builder for Intel FPGAs Integrating Your RTL IP Core Into a System RTL IP Core Kernel Interfaces Loops Pipes Data Types and Arithmetic Operations Parallelism Memories and Memory Operations Libraries Additional FPGA Acceleration Flow Considerations FPGA Optimization Flags, Attributes, Pragmas, and Extensions Quick Reference Additional Information Document Revision History for the Intel oneAPI DPC++/C++ Compiler Handbook for Intel FPGAs Notices and Disclaimers
Introduction To FPGA Design Concepts x
FPGA Architecture Overview Concepts of FPGA Hardware Design How Source Code Becomes a Custom Hardware Datapath
How Source Code Becomes a Custom Hardware Datapath x
Mapping Source Code Instructions to Hardware Scheduling Mapping Parallelism Models to FPGA Hardware Memory Types
Intel oneAPI FPGA Development x
FPGA Flow Terminology Intel oneAPI FPGA Development Flow Types of SYCL* FPGA Compilation FPGA Compilation Flags FPGA Workflows in IDEs
Types of SYCL* FPGA Compilation x
Separating Device and Host Code Compilation
Getting Started with the Intel oneAPI DPC++/C++ Compiler for Intel FPGA Development x
Installing the Intel oneAPI FPGA Development Environment FPGA Development for Intel oneAPI Toolkits with Visual Studio* Code
Installing the Intel oneAPI FPGA Development Environment x
Simulation Prerequisites Installing the Questa*-Intel FPGA Edition Software Set Up the Simulation Environment
FPGA Development for Intel oneAPI Toolkits with Visual Studio* Code x
Set the Environment Variables and Launch Visual Studio* Code Create an FPGA Visual Studio* Code Project Enable Code Completion in a Visual Studio* Code Project Configure Running and Debugging in a Visual Studio* Code Project Debugging Your Kernel in Visual Studio* Code with a Native Debugger Generate and View the FPGA Optimization Report Build and Run the FPGA Hardware Image
Defining a Kernel for FPGAs x
Suggested Kernel Coding Styles Single Work-Item Kernels
Single Work-Item Kernels x
Single Work-item Kernel Design Guidelines Single-Cycle Floating-Point Accumulator for Single Work-Item Kernels
Single-Cycle Floating-Point Accumulator for Single Work-Item Kernels x
Strategies for Inferring the Accumulator
Debugging and Verifying Your Design x
Device Selectors for FPGA printf Command Restrictions Emulate and Debug Your Design Evaluate Your Kernel Through Simulation
Emulate and Debug Your Design x
Emulator Environment Variables Compile and Emulate Your Design Limitations of the Emulator Troubleshooting Discrepancies in Hardware and Emulator Results Emulator Known Issues
Evaluate Your Kernel Through Simulation x
Debug During Verification Compile a Kernel for Simulation Simulate Your Kernel Viewing Simulation Waveforms Troubleshooting Simulator Issues
Analyzing Your Design x
Review the FPGA Optimization Report Quartus (Static) Summary Intel® FPGA Dynamic Profiler for DPC++
Review the FPGA Optimization Report x
Loop Analysis Bottlenecks Viewer Area Estimates System Viewer Kernel Memory Viewer Schedule Viewer Access FPGA Optimization Reports in JSON Format
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 Run Time Reduce Area Resource Use While Profiling Profiler Analyses of Example SYCL* Design Scenarios Limitations
Obtain Profiling Data During Run Time x
Invoke the Profiler Runtime Wrapper to Obtain Profiling Data Use Intel® VTune™ Profiler
Use Intel® VTune™ Profiler x
Interpret Performance Counter Data
Optimizing Your Host Application x
Throughput Resource Use System-level Profiling Using the Intercept Layer for OpenCL™ Applications Multithreaded 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
System-level Profiling Using the Intercept Layer for OpenCL™ Applications x
Set Up the Intercept Layer for OpenCL™ Applications
Double Buffering Host Utilizing Kernel Invocation Queue x
Analyzing Buffering Using the Intercept Layer for OpenCL™ Applications
Integrating Your RTL IP Core Into a System x
Synthesize Your RTL IP Core with Quartus Prime Software Encrypting RTL IP Cores Add an RTL IP Core into a Platform Designer System Add an RTL IP Core into a Quartus Prime Project
Encrypting RTL IP Cores x
Encrypting RTL IP Cores Without Licensing Encrypting RTL IP Core With Licensing
RTL IP Core Kernel Interfaces x
Kernel Argument Interfaces Memory-Mapped (MM) Agent Kernel Invocation Interface Ready/Valid Handshaking Kernel Invocation Interface Memory-Mapped Host Interfaces Structs in RTL IP Core Interfaces IP core Reset Behavior
Memory-Mapped Host Interfaces x
Memory-Mapped Host Interfaces Using Unified Shared Memory Pointers and the annotated_arg Class Memory-Mapped Host Interfaces Using Accessors
Memory-Mapped Host Interfaces Using Unified Shared Memory Pointers and the annotated_arg Class x
Buffer Locations in Memory-Mapped Host Interfaces The annotated_arg Template Class
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 Fuse Loops to Reduce Overhead and Improve Performance Optimize Loops With Loop Speculation Remove Loop Bottlenecks Improve fMAX/II with Shannonization Optimize Inner Loop Throughput Improve Loop Performance by Caching Data in On-Chip Memory
Pipes x
Host Pipes Pipes Extension
Host Pipes x
Host Pipe Declaration Host Pipe API Host Pipes RTL Interfaces
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 The atomic_fence Function and Pipes
I/O Pipes x
Emulate Applications with a Pipe That Reads or Writes to an I/O Pipe
Data Types and Arithmetic Operations x
Optimize Floating-point Operation Minimize the Use of Expensive Functions Variable-Precision Data Type Support
Variable-Precision Data Type Support x
Advantages and Limitations of Variable 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 the ap_float Data Type 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
Parallelism x
Pipelined Kernels NDRange Kernels Asynchronous Parallelism Within Kernels (task_sequence)
Pipelined Kernels x
Stable Arguments
Asynchronous Parallelism Within Kernels (task_sequence) x
Task Functions task_sequence Use Cases
Memories and Memory Operations x
The device_global Extension (Beta) The annotated_ptr Template Class (Beta) Memory Accesses Kernel Variable Accesses
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
Libraries x
Use SYCL Shared Library With Third-Party Applications Use of RTL Libraries for FPGA Object Manifest File Syntax of an RTL Library Restrictions and Limitations in RTL Support Agilex 7 and Stratix 10 Design-Specific Reset Requirements for Stall-Free and Stallable RTL Libraries Stall-Free RTL
Additional FPGA Acceleration Flow Considerations x
FPGA-CPU Interaction FPGA Boards and Board Support Packages (BSPs) Targeting Multiple Identical FPGA Devices Targeting Multiple Platforms Split Kernel into Multiple FPGA Images (Linux only)
FPGA Boards and Board Support Packages (BSPs) x
The FPGA Board Utility Commands Managing an FPGA Board Advanced Board Management The OpenCL™ Installable Client Driver (ICD)
Advanced Board Management x
Extracting the FPGA Device Image (.aocx) File from a Multiarchitecture Binary File
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)
Optimization Flags x
Specify Schedule fMAX Target for Kernels (-Xsclock=<clock target>) Create a 2xclock Interface (-Xsuse-2xclock) Disable Burst-Interleaving of Global Memory (-Xsno-interleaving) 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>) Generate Register Map Wrapper (-Xsregister-map-wrapper-type) Allow Wide Memory Initialization (-Xsallow-wide-device-globals)
Optimization Targets x
Minimum Latency Flow Minimum Area Flow Balanced Throughput-Area Trade-Offs Flow
Kernel Attributes x
Specify Schedule fMAX Target for Kernels (scheduler_target_fmax_mhz) Specify a Workgroup Size (max_work_group_size/reqd_work_group_size) Specify Number of SIMD Work Items (num_simd_work_items) Omit Hardware that Generates and Dispatches Kernel IDs (max_global_work_dim) Omit Hardware that Supports Global Work Offsets (no_global_work_offset) Reduce Kernel Area and Latency (use_stall_enable_clusters)
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)
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
Intel oneAPI DPC++/C++ Compiler Handbook for FPGAs Overview
Introduction To FPGA Design Concepts
Intel oneAPI FPGA Development
Getting Started with the Intel oneAPI DPC++/C++ Compiler for Intel FPGA Development
Defining a Kernel for FPGAs
Debugging and Verifying Your Design
Analyzing Your Design
Optimizing Your Kernel
Optimizing Your Host Application
Integrating Your Kernel into DSP Builder for Intel FPGAs
Integrating Your RTL IP Core Into a System
RTL IP Core Kernel Interfaces
Loops
Pipes
Data Types and Arithmetic Operations
Parallelism
Memories and Memory Operations
Libraries
Additional FPGA Acceleration Flow Considerations
FPGA Optimization Flags, Attributes, Pragmas, and Extensions
Quick Reference
Additional Information
Document Revision History for the Intel oneAPI DPC++/C++ Compiler Handbook for Intel FPGAs
Notices and Disclaimers

N-Way Buffering to Overlap Kernel Execution

N-way buffering is a generalization of the double buffering optimization technique. This system-level optimization enables kernel execution to occur in parallel with host-side processing and buffer transfers between host and device, improving application performance. N-way buffering can achieve this overlap even when the host-processing time exceeds kernel execution time.

In an application where the FPGA kernel is executed multiple times, the host must perform the following processing and buffer transfers before each kernel invocation.

  1. The output data from the previous invocation must be transferred from the device to the host and then processed by the host. Examples of this processing include the following:
    • Copying the data to another location
    • Rearranging the data
    • Verifying it in some way
  2. The input data for the next invocation must be processed by the host and then transferred to the device. Examples of this processing include:
    • Copying the data from another location
    • Rearranging the data for kernel consumption
    • Generating the data in some way

Without the N-way buffering, host processing and buffer transfers occur between kernel executions. Therefore, there is a gap in time between kernel executions, which you can refer as kernel downtime (See Figure 1). If these operations overlap with kernel execution, the kernels can execute back-to-back with minimal downtime, thereby increasing the overall application performance.

Determine When is N-Way Buffering Possible

N-way buffering is frequently referred to as double buffering in the most common case where N=2.

Consider the following illustration:

N-Way Buffering Host

The following are the definitions of the required variables:

  • R: Time to transfer the kernel's output buffer from device to host.
  • Op: Host-side processing time of kernel output data (output processing).
  • Ip: Host-side processing time for kernel input data (input processing).
  • W: Time to transfer the kernel's input buffer from host to device.
  • K: Kernel execution time
  • N: The number of buffer sets used.
  • C: The number of host-side CPU cores.

In general, R, Op, Ip, and W operations must all complete before the next kernel is launched. To maximize performance, while one kernel is executing on the device, these operations must run in parallel and operate on a separate set of buffer locations. They must complete before the current kernel completes, thus allowing the next kernel to be launched immediately with no downtime. In general, to maximize performance, the host must launch a new kernel every K.

If these host-side operations are executed serially, this leads to the following constraint: 

R + Op + Ip + W <= K, to minimize kernel downtime

In the above example, if the constraint is satisfied, the application requires two sets of buffers. In this case, N=2. However, the above constraint may not be satisfied in some applications if host-processing takes longer than the kernel execution time.

NOTE:

A performance improvement may still be observed because kernel downtime may still be reduced (though perhaps not maximally reduced).

In this case, to further improve performance, reduce host-processing time through multithreading. Instead of executing the above operations serially, perform the input and output-processing operations in parallel using two threads, leading to the following constraint: 

Max (R+Op, Ip+W) <= K

R + W <= K, to minimize kernel downtime

If the above constraint is still unsatisfied, the technique can be extended beyond two sets of buffers to N sets of buffers to help improve the degree of overlap. In this case, the constraint becomes: 

Max (R + Op, Ip + W) <= (N-1)*K

R + W <= K, to minimize kernel downtime.

The idea of N-way buffering is to prepare N sets of kernel input buffers, launch N kernels, and when the first kernel completes, begin the subsequent host-side operations. These operations may take a long time (longer than K), but they do not cause kernel downtime because an additional N-1 kernels have already been queued and can launch immediately. By the time these first N kernels complete, the aforementioned host-side operations would have also completed, and the N+1 kernel can be launched with no downtime. As additional kernels complete, corresponding host-side operations are launched on the host, using multiple threads in a parallel fashion. Although the host operations take longer than K, if N is chosen correctly, they complete with a period of K, which is required to ensure you can launch a new kernel every K. To reiterate, this scheme requires multi-threaded host-operations because the host must perform processing for up to N kernels in parallel to keep up.

Use the above formula to calculate the N required to minimize downtime. However, there are some practical limits:

  • N sets of buffers are required on both the host and device. Therefore, both must have the capacity for this many buffers.
  • If the input and output processing operations are launched in separate threads, then (N-1)*2 cores are required so that C can become the limiting factor.

Measure the Impact of N-Way Buffering

You must get a sense of the kernel downtime to identify the degree to which this technique can help improve performance.

You can do this by querying the total kernel execution time from the runtime and comparing it to the overall application execution time. In an application where kernels execute with minimal downtime, these two numbers are close. However, if kernels have a lot of downtime, overall execution time notably exceeds the kernel execution time.

NOTE:

For additional information, refer to the FPGA tutorial sample "N-Way Buffering" on GitHub.

Parent topic: Optimizing Your Host Application
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