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 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 Example Code for Using the atomic_fence Function and Blocking Inter-Kernel Pipes
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

Pipe and Atomic Fence

ATTENTION:

This topic assumes that you already have an understanding of the atomic_fence function described in the SYCL specification. If you are new to it, then before you proceed, read about the atomic_fence function in the Khronos* SYCL Specification.

When running kernels in parallel, you might want multiple kernels to collaboratively access a shared memory. SYCL* provides the atomic_fence function as a synchronization construct to reason about the order of memory instructions accessing the shared memory. The atomic_fence function controls the reordering of memory load and store operations (subject to the associated memory order and memory scope) when paired with synchronization through an atomic object. Pipe read and write operations behave as if they are SYCL-relaxed atomic load and store operations. When paired with atomic_fence functions to establish a synchronizes-with relationship, pipe operations can provide guarantee on side-effect visibility in memory, as defined by the SYCL memory model. For additional information about the atomic_fence function, refer to the Khronos* SYCL Specification.

CAUTION:

The current atomic_fence function for FPGA uses an overly conservative implementation and is still preliminary.

  • The implementation guarantees only functional correctness and not the maximum performance because the atomic_fence function currently enforces more memory ordering than it requires. If you do not use the atomic_fence function with a correct memory_order parameter, then you might see unexpected behavior in your program when the atomic_fence function handles memory ordering properly in a future release.
  • The implementation does not support the memory_scope::system constraint. The broadest scope supported for FPGA is the memory_scope::device constraint.

Example Code for Using the atomic_fence Function and Blocking Inter-Kernel Pipes

The following code sample shows how to use the atomic_fence function with a blocking inter-kernel pipe to synchronize the load and store to a shared device memory between a producer and a consumer: 

#include <sycl/sycl.hpp>
using namespace sycl;
using my_pipe = ext::intel::pipe<class some_pipe, int>;
constexpr int READY = 1;
 
int produce_data(int data);
int consume_data(int data);
 
event Producer(queue&q, int *shared_ptr, size_t size) {
  return q.submit([&](handler& h) {
    h.single_task<class ProducerKernel>([=]() [[intel::kernel_args_restrict]] {
      // create a device pointer to explicitly inform the compiler the
      // pointer resides in the device's address space
      device_ptr<int> shared_ptr_d(shared_ptr);
 
      // produce data
      for (size_t i = 0; i < size; i++) {
        shared_ptr_d[i] = produce_data(i);
      }
      // use atomic_fence to ensure memory ordering
      atomic_fence(memory_order::seq_cst, memory_scope::device);
      // notify the consumer to start data processing
      my_pipe::write(READY);
    });
  }
}
 
event Consumer(queue & q, int* shared_ptr, size_t size, int *output_ptr) {
  return q.submit([&](handler& h) {
    h.single_task<class ConsumerKernel>([=]() [[intel::kernel_args_restrict]] {
      // create device pointers to explicitly inform the compiler these
      // pointer reside in the device's address space
      device_ptr<int> shared_ptr_d(shared_ptr);
      device_ptr<int> out_ptr_d(output_ptr);
 
      // wait on the blocking pipe_read until notified by the producer
      int ready = my_pipe::read();
 
      // use atomic_fence to ensure memory ordering
      atomic_fence(memory_order::seq_cst, memory_scope::device);
 
      // consume data and write to output memory address
      for(int i = 0; i < size; i++) {
        out_ptr_d[i] = consume_data(shared_ptr_d[i]);
      }
    });
  });
}

In the above example, the consumer loads data produced by the producer. To prevent a scenario where the consumer loads the shared device memory before the producer finishes storing to it, a blocking pipe is used to synchronize between the two kernels. The consumer’s pipe read does not return until it sees the READY written by the producer. In this example, the atomic_fence functions in the producer and consumer prevent the shared memory read and write from being reordered with the pipe instructions. They also form a release-acquire ordering, which ensures that by the time the consumer sees the pipe read returns, the producer's write operation to the shared device memory is also visible to the consumer.

NOTE:

The shared device memory is created using a USM device allocation that allows the two kernels to be running in parallel even though they both access the shared device memory simultaneously.

Parent topic: Pipes Extension
Level Two Title