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 Burst-Coalesce Static Coalescing Prefetch Cache Pipeline
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

Load-Store Unit Controls

The Intel® oneAPI DPC++/C++ Compiler allows you to control the LSU that is generated for global memory accesses via a set of templated options in the ext::intel::lsu class. The ext::intel::lsu class has two member functions, load() and store(), which allow loading from and storing to a global pointer, respectively. See also Annotating Unified Shared Memory Pointers.

NOTE:
Not all LSU control options listed below work with both load() and store() member functions. Not all styles and types are supported on every target device. Thus, the use of LSU control options results in a specific type of LSU.

LSU Control Options Available in the ext::intel::lsu Class
Control Syntax Value Default Value Supported Functionality
Burst-Coalesce ext::intel::burst_coalesce<B> B is a boolean False Load and store
Static Coalescing ext::intel::statically_coalesce<B> B is a boolean True Load and store
Prefetch ext::intel::prefetch<B> B is a boolean false Only loads
Cache ext::intel::cache<N> N is an integer greater than or equal to 0. 0 Only loads
Pipeline No option provided - Defaults above result in this. Load and store

Burst-Coalesce

Currently, not every combination of LSU controls is supported by the compiler. The following rules apply:

  • For store, the ext::intel::cache control must be 0 and the ext::intel::prefetch control must be false.
  • For load, if the ext::intel::cache control is set to a value greater than 0, then the ext::intel::burst_coalesce control must be set to true.
  • For load, exactly one of ext::intel::prefetch and ext::intel::burst_coalesce control options are allowed to be true.
  • For load, exactly one of ext::intel::prefetch and ext::intel::cache control options are allowed to be true.

The burst-coalesce control helps the Intel® oneAPI DPC++/C++ Compiler infer a burst-coalesced LSU. It can be combined with other attributes such as cache. For more details about this LSU type, refer to Burst-Coalesced Load-Store Units.

The following is an example of burst-coalesce LSU control: 

using BurstCoalescedLSU =
  ext::intel::lsu<ext::intel::burst_coalesce<true>,
                  ext::intel::statically_coalesce<false>>;
BurstCoalescedLSU::store(output_ptr, X);

Static Coalescing

The static-coalescing control helps the Intel® oneAPI DPC++/C++ Compiler turn on or off static coalescing for global memory accesses. Static coalescing of memory accesses is the default behavior of the compiler, so this feature can be used to turn off static coalescing. It can be combined with other attributes such as burst_coalesce. For more details about this memory optimization modifier, refer to Static Memory Coalescing.

The following is an example that shows how to use the LSU control to turn off static-coalescing LSU control: 

using NonStaticCoalescedLSU =
  ext::intel::lsu<ext::intel::burst_coalesce<true>, 
                  ext::intel::statically_coalesce<false>>;

int X = NonStaticCoalescedLSU::load(input_ptr);

Prefetch

The prefetch control helps the Intel® oneAPI DPC++/C++ Compiler infer a prefetch style LSU. The compiler honors this control only if it is physically possible. If there is a risk that an LSU is not functionally correct, it is not inferred. This control also works only for load accesses. For more details about this LSU type, refer to Prefetching Load-Store Units.

The following is an example of prefetch LSU control: 

using PrefetchingLSU =
  ext::intel::lsu<ext::intel::prefetch<true>,
                  ext::intel::statically_coalesce<false>>;

int X = PrefetchingLSU::load(input_ptr);

Cache

The cache control helps the Intel® oneAPI DPC++/C++ Compiler turn on or off caching behavior and set the size of the cache. This LSU modifier can be useful for parallel_for kernels. The cache control can be combined with other attributes such as burst_coalesce. For more details about this LSU modifier, refer to Cached.

The following is an example of cache LSU control: 

using CachingLSU =
  ext::intel::lsu<ext::intel::burst_coalesce<true>,
		                ext::intel::cache<1024>,
		                ext::intel::statically_coalesce<false>>;

int X = CachingLSU::load(input_ptr);

Pipeline

The pipeline control helps the Intel® oneAPI DPC++/C++ Compiler infer the LSU style of pipeline. It can be combined with other attributes such as static coalesce. For more details about this LSU type, refer to Pipelined Load-Store Units.

For example: 

using PipelinedLSU = ext::intel::lsu<>;

PipelinedLSU::store(output_ptr,X);
Parent topic: Load-Store Units
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