Developer Guide

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

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Date 5/08/2024
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Document Table of Contents
Document Table of Contents x
Intel oneAPI DPC++/C++ Compiler Handbook for Intel FPGAs 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 Additional SYCL* HLS 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 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
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
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 Memory-Mapped Interface Unified-Shared-Memory Virtual Address Space Mixing Annotated and Unannotated Pointers in Your Kernel Buffer Locations and Unannotated Pointers Buffer Location Virtual Address Spaces Determining Virtual Address Space Information 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 Emulate Applications with a Pipe That Reads or Writes to an I/O Pipe
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
Data Types and Arithmetic Operations x
Optimize Floating-point Operation Avoid 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 BSPs and Boards Targeting Multiple Homogeneous FPGA Devices Targeting Multiple Platforms Split Kernel into Multiple FPGA Images (Linux only)
FPGA BSPs and Boards x
FPGA Board Initialization Obtain FPGA Hardware Image Information Extracting the FPGA Hardware Configuration (.aocx) File from a Multiarchitecture Binary File
Additional SYCL* HLS Flow Considerations x
IP core Reset Behavior
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=<global_memory_name>) 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-mif)
Optimization Targets x
Minimum Latency Flow Minimum Area Flow Balanced Throughput-Area Trade-Offs 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)
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 Intel FPGAs
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
Additional SYCL* HLS 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

Buffer Locations in Memory-Mapped Host Interfaces

Each unique identifier specified by the buffer_location<id> property of a pointer kernel argument represents a unique Avalon® host interface for the kerne. Ensure that you avoid using the same buffer location id for interfaces with different properties.

The caller is responsible for ensuring the correct buffer location is specified; otherwise, functional failures might occur. If you specify a buffer_location property on your kernel argument, specify the same buffer location on the USM allocation API call that allocates memory on the test bench (that is, the host code).

Similarly, the caller is responsible for aligning the data to the set value for the align argument; otherwise, functional failures might occur. If you specify the alignment<value> property, ensure that you specify the same alignment value the following locations:

  • The sycl::ext::intel::experimental::alignment<value> property
  • The alignment argument of the aligned_malloc_shared template function in your host code. For example, 
    aligned_malloc_shared<int>(value, 1, q );

Memory-Mapped Interface Unified-Shared-Memory Virtual Address Space

The compiler encodes certain information regarding the virtual address space in the top bits of a 64-bit pointer address as follows:

Pointer-Address Bit-Range Descriptions

Bit Range

Description

40:0

Used for addressing within the memory system

63:41

Stores the virtual address space information that is derived from the buffer location

In some cases, the compiler cannot determine which buffer location a pointer corresponds to and it creates logic in the generated RTL that inspects the top bits of the pointer at runtime to detect the buffer location and route the memory transaction to the correct external memory interface.

You do not need to encode the buffer location information yourself in most cases. Exceptions are outlined in Buffer Location Virtual Address Spaces.

The compiler automatically generates logic to embed this information from the buffer location specified on the pointer kernel argument in the source file.

Mixing Annotated and Unannotated Pointers in Your Kernel

The compiler infers Avalon host interfaces as follows:

  • If annotated kernel arguments are present, one interface is inferred for each unique buffer location.
  • If no annotated pointer kernel arguments are present, a single Avalon Host interface is inferred for all pointer arguments.

If your design has a mix of annotated kernel arguments and unannotated pointer arguments, the unannotated pointers can access any of the inferred Avalon host interfaces depending on the addresses passed to the kernel via the pointer arguments.

When your designs mix annotated and unannotated pointers, there are situations where the compiler is unable to determine if an error condition exists. In these situations, functional errors or hangs can appear when you try to simulate your design or use the generated RTL.

If you have a design that mixes annotated and unannotated pointers and your design fails in simulation, check for the following conditions:

Buffer Locations and Unannotated Pointers

Ensure that your unannotated pointer accesses are to interfaces that support the access. For example, an unannotated pointer attempting a read access to a write-only interface is an error that the compiler cannot always catch.

Consider the following design with two kernels:

// Kernel1: Write-only MM Host Interface
// Kernel1 defines a write-only memory-mapped host interface
struct Kernel1 {
  annotated_arg<int*, decltype(properties{buffer_location<1>, read_write_mode_write})> arg_a;
  annotated_arg<int*, decltype(properties{buffer_location<2>, read_write_mode_read})> arg_b;

  ...
  void operator()() const {
    ...
  }
};

//Kernel2: Unannotated Pointer
// Kernel2 uses an unannotated pointer and does a read access
struct Kernel2 {
  int* arg_a;
  ...
  void operator()() const {
    ... = *arg_a;
  }
};

The kernel Kernel1 specifies two memory-mapped host interfaces. The compiler infers the two interfaces, and the unannotated pointer argument in kernel Kernel2 can access either of them, depending on the memory addresses that are passed to it.

Because the buffer_location<1> memory has a write-only interface, passing an address for the buffer_location<1> memory to the unannotated pointer in kernel Kernel2 is an illegal operation.

If your program has such mixed usage, ensure that correct addresses are passed via kernel arguments.

Buffer Location Virtual Address Spaces

If the kernel has at least one kernel argument where the buffer location is specified (annotated argument) and at least one argument where the buffer location is not specified (unannotated argument), then you must embed the buffer location information in the top bits of any unannotated kernel arguments.

Consider the following code example:

// This struct defines the IP that is generated
struct MyIPComponent{
  // struct members are kernel arguments 
  int* a; // no buffer location specified
  annotated_arg<int*, decltype(properties{buffer_location<1>})> b; // buffer location 1
  annotated_arg<int*, decltype(properties{buffer_location<2>})> c; // buffer location 2
  annotated_arg<int*, decltype(properties{buffer_location<3>})> d; // buffer location 3

// operator()() defines the device/IP code 
void operator()() const {
    *a *= 2;
    *b *= 2;
    *c *= 2;
    *d *= 2;
   }
};

In this example, the compiler does not know what external memory pointer a will point to, so the compiler creates logic to check the top bits of the pointer to determine, at run time, which buffer location to access. Therefore, you must set those top bits for argument a (but not for the other kernel arguments).

In such cases, if the unannotated pointer argument has a conduit interface then the port is 64 bits wide. And, if the interface is register-map based, all 64 bits are passed to the kernel.

Simulation Exception:
When you simulate your kernel you do not need to write any host code (even in this use case) to embed information in the pointer bits. The buffer locations are all taken care of by the runtime stack that allocates the pointers in the host code.

When the compiler can deduce which buffer location that a pointer argument points to (for example, when there is only one mm_host interface), the compiler embeds the buffer location automatically. Buffer locations need to be specified manually only when your kernels have a mix of annotated and unannotated kernel arguments.

The compiler infers one global memory (with inferred buffer location 0) when there are no annotated pointer kernel arguments in your entire design (that is, across all kernels). In the following code example, because there are only unannotated pointer arguments, the compiler infers only one global memory and so it can embed the correct information in the top bits of the pointer kernel arguments.

// This struct defines the IP that will be generated
struct MyIPComponent{
    // struct members are kernel arguments
    int* a; // no buffer location specified
    int* b; // no buffer location specified
    // no other annotated kernel argument is present

    // operator()() defines the device/IP code
    void operator()() const {
        *a = ...
        *b = ...
    }
};

Determining Virtual Address Space Information

If you need to embed the virtual address space information in the top bits of the pointer kernel arguments because the compiler cannot do so, get the information to embed from the HTML reports:

  1. If you have not already done so, compile your kernel to obtain the HTML reports
  2. Open the HTML reports.
  3. Go to Views > System Viewer
  4. In the left pane, expand System and then expand Global memory

    Under Global memory, you see entries for all external memories for your kernel.

  5. Click on a memory to display that memory in the System Viewer pane.
  6. In the System Viewer pane, find the box that represents the memory and click the node inside the box.

    This node represents the “interface” of that global memory.

  7. In the Details pane, find the Start Address of the memory.

    The top bits of your unannotated pointer argument must match the top bits of this start address if you want the pointer to access this buffer location.

The following screen capture shows an example of determining the top bit of the start address needed to address a buffer location 1.
Parent topic: Memory-Mapped Host Interfaces Using Unified Shared Memory Pointers and the annotated_arg Class
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