Developer Guide

FPGA Optimization Guide for Intel® oneAPI Toolkits

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Date 12/16/2022
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Document Table of Contents
Document Table of Contents x
FPGA Optimization Guide for Intel® oneAPI Toolkits
FPGA Optimization Guide for Intel® oneAPI Toolkits x
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 Notices and Disclaimers
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 Kernel Memory List Kernel Memory Viewer Code View Details 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 (<span class='codeph'>-Xsprofile</span>) 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 <span class='codeph'>ac_int</span> Data Type Declare the <span class='codeph'>ac_fixed</span> Data Type Declare the <span class='codeph'>ac_complex</span> Data Type Declare the <span class='codeph'>ap_float</span> Data Type
Declare the <span class='codeph'>ap_float</span> Data Type x
Conversion Rules for <span class='codeph'>ap_float</span> Operations with Explicit Precision Controls Comparison Operators Additional <span class='codeph'>ap_float</span> Functions Additional Data Types Provided by the <span class='codeph'>ap_float.hpp</span> Header File Quality of Results and the ap_float Data Type
FPGA Optimization Flags, Attributes, Pragmas, and Extensions x
Optimization Flags Kernel Attributes Kernel Controls Kernel Variables Memory Attributes Loop Directives Floating-Point Pragmas Latency Controls (Beta) System of Tasks Extension (<span class='codeph'>task_sequence</span>)
Optimization Flags x
Specify Schedule FMAX Target for Kernels (<span class='codeph'>-Xsclock=<clock target>) Disable Burst-Interleaving of Global Memory (<span class='codeph'>-Xsno-interleaving=<global_memory_type></span>) Force Ring Interconnect for Global Memory (<span class='codeph'>-Xsglobal-ring</span>) Force a Single Store Ring to Reduce Area (<span class='codeph'>-Xsforce-single-store-ring</span>) Force Fewer Read Data Reorder Units to Reduce Area (<span class='codeph'>-Xsnum-reorder</span>) Disable Hardware Kernel Invocation Queue (<span class='codeph'>-Xsno-hardware-kernel-invocation-queue</span>) Modify the Handshaking Protocol Between Clusters (<span class='codeph'>-Xshyper-optimized-handshaking</span>) Disable Automatic Fusion of Loops (<span class='codeph'>-Xsdisable-auto-loop-fusion</span>) Fuse Adjacent Loops With Unequal Trip Counts (<span class='codeph'>-Xsenable-unequal-tc-fusion</span>) Pipeline Loops in Non-task Kernels (<span class='codeph'>-Xsauto-pipeline</span>) Control Semantics of Floating-Point Operations (<span class='codeph'>-fp-model=<var><value></var> </span>) Modify the Rounding Mode of Floating-point Operations (<span class='codeph'>-Xsrounding=<rounding_type></span>) Global Control of Exit FIFO Latency of Stall-free Clusters (<span class='codeph'>-Xssfc-exit-fifo-type=<var><value></var> </span>) Enable the Read-Only Cache for Read-Only Accessors (<span class='codeph'>-Xsread-only-cache-size=<var><N></var>)</span> Control Hardware Implementation of the Supported Data Types and Math Operations (<span class='codeph'>-Xsdsp-mode=<var><option></var> </span>)
Kernel Attributes x
Specify Schedule FMAX Target for Kernels Specify a Workgroup Size Specify Number of SIMD WorkItems Omit Hardware that Generates and Dispatches Kernel IDs Omit Hardware to Support the <span class='codeph'>no_global_work_offset</span> Attribute in <span class='codeph'>parallel_for</span> Kernels Reduce Kernel Area and Latency
Kernel Controls x
Pipes Extension
Pipes Extension x
Key Properties of a Pipe Accessing Pipes The <span class='codeph'>pipe</span> Class and its Use I/O Pipes Characteristics of Pipes Restrictions of Pipes Guidelines for Designing Pipes Pipe and Atomic Fence
Loop Directives x
<span class='codeph'>disable_loop_pipelining</span> Attribute <span class='codeph'>initiation_interval</span> Attribute <span class='codeph'>ivdep</span> Attribute <span class='codeph'>loop_coalesce</span> Attribute <span class='codeph'>max_concurrency</span> Attribute <span class='codeph'>max_interleaving</span> Attribute <span class='codeph'>speculated_iterations</span> Attribute <span class='codeph'>unroll</span> Pragma Loop Fuse Functions and <span class='codeph'>nofusion</span> Attribute
System of Tasks Extension (<span class='codeph'>task_sequence</span>) x
Task Functions <span class='codeph'>task_sequence</span> 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 <span class='codeph'>task_sequence</span> Template Parameters and Function APIs
FPGA Optimization Guide for Intel® oneAPI Toolkits

Kernel Memory Viewer

Data movement is a bottleneck in many algorithms. The Kernel Memory Viewer shows you how the Intel® oneAPI DPC++/C++ Compiler interprets the data connections and synthesizes memory for your kernel. Use the Kernel Memory Viewer to help identify data movement bottlenecks in your kernel design.

Some patterns in memory accesses can cause undesired arbitration in the load-store units (LSUs), affecting the throughput performance of your kernel. Use the Kernel Memory Viewer to identify unwanted arbitration in the LSUs.

To analyze your SYCL* system, click Views > Kernel Memory Viewer.

The following image illustrates the layout of the Kernel Memory Viewer:

Kernel Memory Viewer Layout

The Kernel Memory Viewer has the following panes:

Kernel Memory Viewer Panes

Pane

Description

Kernel Memory List

Lists all memories present in your design. When you select a memory name, you can view its graphical representation in the Kernel Memory Viewer pane.

Kernel Memory Viewer

Shows a graphical representation of the memory system or memory bank selected in the Kernel Memory List pane.

Code View

Shows the source code file for which the reports are generated.

Details

Shows the details of the memory system or memory bank selected in the Kernel Memory List pane.

Kernel Memory List

The Kernel Memory List pane displays a kernel hierarchy with memories synthesized (RAMs, ROMs, and registers) and optimized away in that kernel.

Features and Details of the Kernel Memory List Pane

The following table describes each numbered feature highlighted in the above image:

Kernel Memory List Pane Icons and Labels

No.

Icon or Label

Name

Description

1



Kernel name

You can expand or collapse the list of memories in your kernel. Memories that do not belong to any kernel are displayed under (Other).

2



RAM

A RAM is a memory that has at least one write to it. The name of the RAM memory is the same as its name in your design.

When you select a memory name, you can view a logical representation of the RAM in the Kernel Memory Viewer pane. By default, only the first bank of the memory system is displayed.

To select banks that you want the Kernel Memory Viewer pane to display:

  • Expand the memory name.
  • Clear the memory name check box to collapse all memory banks in the view.
  • Select the memory name check box to show all memory banks in the view.

3



ROM

A ROM is a read-only memory. The name of the ROM memory is same as its name in your design.

When you select a memory name, you can view a logical representation of the ROM in the Kernel Memory Viewer pane. By default, only the first bank of the memory system is displayed.

To select banks that you want the Kernel Memory Viewer pane to display:

  • Expand the memory name.
  • Clear the memory name check box to collapse all memory banks in the view.
  • Select the memory name check box to show all memory banks in the view.

4

Bank #num

Bank

A memory bank is always associated with a RAM or a ROM. Each bank is named Bank #num, where #num is the memory bank's ID starting from 0.

  • Click on the bank name to display the bank view in the Kernel Memory Viewer pane, which displays a graphical representation of the bank with all its replicates and private copies. This view can help you focus on specific memory banks of a complex memory design.
  • Clear the memory bank name check-box to collapse the bank in the logical representation of the memory.
  • Select the memory bank name check-box to display the bank in the logical representation of the memory.

5



Register

A register is a kernel variable carried through the pipeline in registers (rather than being stored in a RAM or ROM). The name of the register is the same as its name in your design.

A register variable is implemented either exclusively in FFs or in a combination of FFs and RAM-based FIFOs.

6

Text label

Optimized Away

A kernel variable can be optimized away because it is unused in your design, or compiler optimizations have transformed all uses of the variable such that it is unnecessary. The name of the optimized away variable is the same as its name in your design.

7



Filter

Use the Kernel Memory List filter to selectively view the list of RAMs, ROMs, registers, and optimized away variables in your design.

When you clear the checkbox associated with an item in the filter, you hide all occurrences of that kind of item in the Kernel Memory List. Filter your Memory List to help you focus on a specific type of memory in your design.

Kernel Memory Viewer

In the Kernel Memory Viewer pane, you can view connections between loads and stores to specific logical ports on the banks in a memory system. You can also view the number of replicates and private copies created per bank for your memory system. You can see the following types of nodes in the Kernel Memory Viewer pane, depending on the kernel memory system and your selection in the Kernel Memory List pane:

Node Types Observed in the Kernel Memory Viewer Pane

Node Type

Description

Memory node

The memory system for a given variable in your design.

Bank node

A bank in the memory system. A memory system contains at least one bank. A memory bank can connect to one or more port nodes.

Replication node

A replication node shows memory bank replicates created to support multiple accesses to a local memory efficiently. A bank contains at least one replicate. You can view replicate nodes when you view a memory bank by clicking its name in the Kernel Memory List pane.

Private-copy node

A private-copy node shows private copies within a replicate created to allow simultaneous execution of multiple loop iterations. A replicate contains at least one private copy. You can view private-copy nodes when you view a memory bank by clicking its name in the Kernel Memory List pane.

Port node

Each read or write access to local memory is mapped to a port. The logical port for a bank. There are three types of ports:

  • R: A read-only port
  • W: A write-only port
  • RW: A read and write port

LSU node

A store (ST) or load (LD) node connected to the memory through port nodes.

Arbitration node

An arbitration (ARB) node shows that LSUs compete for access to a shared port node, which can lead to stalls.

Port-sharing node

A port-sharing node (SHARE) shows that LSUs have mutually exclusive access to a shared port node, so the load-store units are free from stalls.

Within the graphical representation of a memory in the Kernel Memory Viewer pane, you can perform the following:

  • Hover over any node to view the attributes of that node.
  • Hover over an LSU node to highlight the path from the LSU node to all ports to which the LSU connects.
  • Hover over a port node to highlight the path from the port node to all LSUs that read or write to the port node.
  • Click a node to select it and display the node attributes in the Details pane.

The following images illustrate examples of what you see in the Kernel Memory Viewer:

Logical Representation of a Memory in the Kernel Memory Viewer Pane

Bank View of a Memory Bank in the Kernel Memory Viewer Pane

Code View

The Code View pane displays your source code. When you click on a memory or a bank in the Kernel Memory Viewer pane, the code view pane highlights the line of your code where you declared the memory.

Details

The Details pane shows the attributes of the node selected in the Kernel Memory Viewer pane. For example, when you select a memory in a kernel, the Details pane displays the following information:

  • Width and depths of memory banks
  • Memory layout
  • Address-bit mapping
  • Memory attributes that you specified in your source code

The content of the Details pane persists until you select a different node in the Kernel Memory Viewer pane.

Parent topic: Review the FPGA Optimization Report
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