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 Local Memory Banks Optimize the Geometric Configuration of Local Memory Banks Based on Array Index 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

Local and Private Memory Accesses Optimization

To optimize local memory access efficiency, consider the following guidelines:

  • Implementing certain optimization techniques, such as loop unrolling, might lead to more concurrent memory accesses.
    • Increasing the number of memory accesses can complicate memory systems and degrade performance.
  • If you have function scope local data (defined in the body of a parallel_for_work_group lambda or by using group_local_memory_for_overwrite), the Intel® oneAPI DPC++/C++ Compiler statically sizes the local data that you define within a function body at compilation time.
  • For accessors in the local space, the host assigns their memory sizes dynamically at runtime. However, the compiler must set these physical memory sizes at compilation time. By default, that size is 16 kB.
    • The host can request a smaller data size than has been physically allocated at compilation time, but never a larger size.
  • When accessing local memory, use the simplest address calculations possible and avoid pointer math operations that are not mandatory.
    • Intel® recommends this coding style to reduce FPGA resource utilization and increase local memory efficiency by allowing the Intel® oneAPI DPC++/C++ Compiler to make better inferences about access patterns through static code analysis. For information about the benefits of static coalescing, refer to Static Memory Coalescing. Complex address calculations and pointer math operations can prevent the Intel® oneAPI DPC++/C++ Compiler from identifying the memory system accessed by a given load or store, leading to increased area use and decreased runtime performance.
  • Avoid storing pointers to memory whenever possible. Stored pointers often prevent static compiler analysis from determining the data sets accessed, when pointers are subsequently retrieved from memory. Storing pointers to memory usually leads to suboptimal area and performance results.
  • Create local array elements that are a power of two bytes to allow the Intel® oneAPI DPC++/C++ Compiler to provide an efficient memory configuration.
    • Whenever possible, the Intel® oneAPI DPC++/C++ Compiler automatically pads the elements of the local memory to be a power of two to provide a more efficient memory configuration. For example, if you have a struct containing three chars, the Intel® oneAPI DPC++/C++ Compiler pads it to four bytes, instead of creating a narrower and deeper memory with multiple accesses (that is, a 1-byte wide memory configuration). However, there are cases where the Intel® oneAPI DPC++/C++ Compiler might not pad the memory, such as when the kernel accesses local memory indirectly through pointer arithmetic.
    • To determine if the Intel® oneAPI DPC++/C++ Compiler has padded the local memory, review the memory dimensions in the Memory Viewer. If the Intel® oneAPI DPC++/C++ Compiler fails to pad the local memory, it prints a warning message in the Area Analysis of System report.

Local Memory Banks

Specifying the numbanks(N) and bankwidth(M) memory attributes allow you to configure the local memory banks for parallel memory accesses. The banking geometry described by these attributes determines which elements of the local memory system your kernel can access in parallel.

The following code example depicts an 8 x 4 local memory system that is implemented in a single bank. As a result, no two elements in the system can be accessed in parallel. 

[[intel::fpga_memory]] int lmem[8][4];
#pragma unroll
for(int i = 0; i<4; i+=2) {
  lmem[i][x] = ...; 
}
Serial Accesses to an 8 x 4 Local Memory System

To improve performance, you can add numbanks(N) and bankwidth(M) in your code to define the number of memory banks and the bank widths in bytes. The following code implements eight memory banks, each 16-bytes wide. This memory bank configuration enables parallel memory accesses to the 8 x 4 array. 

[[intel::numbanks(8), intel::bankwidth(16)]] int lmem[8][4];
#pragma unroll
for (int i = 0; i < 4; i+=2) {
  lmem[i][x & 0x3] = ...; 
}
NOTE:

To enable parallel access, you must mask the dynamic access on the lower array index. Masking the dynamic access on the lower array index informs the Intel® oneAPI DPC++/C++ Compiler that x does not exceed the lower index bounds.

Parallel Access to an 8 x 4 Local Memory System with Eight 16-Byte-Wide Memory Banks Parallel access to an 8 x 4 local memory system with eight 16-byte-wide memory banks

By specifying different values for the numbanks(N) and bankwidth(M) attributes, you can change the parallel access pattern. The following code implements four memory banks, each being four bytes wide:

NOTE:

This memory bank configuration enables parallel memory accesses to the 8 x 4 array. 

[[intel::numbanks(4), intel::bankwidth(4)]] int lmem[8][4];
#pragma unroll
for (int i = 0; i < 4; i+=2) {
  lmem[x][i] = ...; 
}
Parallel Access to an 8 x 4 Local Memory System with Four 4-Byte-Wide Memory Banks Parallel access to an 8 x 4 local memory system with four 4-byte-wide memory banks

Optimize the Geometric Configuration of Local Memory Banks Based on Array Index

By default, the Intel® oneAPI DPC++/C++ Compiler attempts to improve performance by automatically banking a local memory system. The compiler includes advanced features that allow you to customize the banking geometry of your local memory system. To configure the geometry of local memory banks, include numbanks(N) and bankwidth(M) kernel attributes in your SYCL* kernel.

The table provides code examples illustrating how the bank geometry changes based on the values you assign to numbanks and bankwidth. The first and last rows of this table illustrate how to bank memory on the upper and lower indexes of a 2D array, respectively.

Effects of numbanks and bankwidth on the Bank Geometry of 2 x 4 Local Memory System

Code Example

Bank Geometry 

[[intel::numbanks(2), 
intel::bankwidth(16)]] 
int lmem[2][4];
 

[[intel::numbanks(2), 
intel::bankwidth(8)]] 
int lmem[2][4];
 

[[intel::numbanks(2), 
intel::bankwidth(4)]] 
int lmem[2][4];
 

[[intel::numbanks(4), 
intel::bankwidth(8)]] 
int lmem[2][4];
 

[[intel::numbanks(4), 
intel::bankwidth(4)]] 
int lmem[2][4];

Parent topic: Memory Accesses
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