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
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

max_interleaving Attribute

Use the max_interleaving attribute to maximize the throughput and hardware resource occupancy of pipelined inner loops in a loop nest by issuing new inner loop iterations as frequently as possible (minimizing the loop initiation interval). When the compiler cannot achieve a loop II of 1 for an inner loop, the compiler configures the loop nest to interleave iterations of one invocation of the inner loop with iterations of other invocations of the inner loop.

Syntax

[[intel::max_interleaving(n)]]

The Intel® oneAPI DPC++/C++ Compiler restricts the annotated (inner) loop to be invoked at most n times per outer loop iteration. When this attribute is specified with n=0, the compiler allows the pipeline to contain a number of simultaneous invocations of the annotated loop equal to the loop initiation interval (II) of that loop. For example, an annotated inner loop with an II of 2 can have iterations from two invocations in the pipeline at a time. This behavior is the default behavior for the compiler if you do not specify the max_interleaving attribute.

As an example, consider the loop nest in the following code snippet: 

int a[N];
// ...
// Loop j is pipelined with ii=1
for (int j = 0; j < M; j++) {
  // Loop i is pipelined with ii=2
  for (int i = 0; i < N; i++) {
    a[i] += foo(i);
  }
}

In this example, the inner i loop is pipelined with a loop II of 2. Under normal pipelining, this means that the inner loop hardware only achieves 50% utilization since one i iteration is initiated every other cycle. To take advantage of these idle cycles, the compiler interleaves a second invocation of the i loop from the next iteration of the outer j loop. Here, a loop invocation means to start pipelined execution of a loop body. In this example, since the i loop resides inside the j loop, and the j loop has a trip count of M, the i loop is invoked M times. Since the j loop is an outermost loop, it is invoked once. The following table illustrates the difference between normal pipelined execution of the i loop and interleaved execution for this example where N=5:

Difference Between Normal Pipelined Execution and Interleaved Execution
Cycle Pipelined Interleaved
0 (0,0) (0,0)
1 --- (1,0)
2 (0,1) (0,1)
3 --- (1,1)
4 (0,2) (0,2)
5 --- (1,2)
6 (0,3) (0,3)
7 --- (1,3)
8 (0,4) (0,4)
9 --- (1,4)
10 (1,0) (2,0)
11 --- (3,0)
12 (1,1) (2,1)
13 --- (3,1)
14 (1,2) (2,2)
15 --- (3,2)
16 (1,3) (2,3)
17 --- (3,3)
18 (1,4) (2,4)
19 --- (3,4)

The table shows the values (j,i) for each inner loop iteration that is initiated at each cycle. At cycle 0, both modes of execution initiate the (0,0)th iteration of the i loop. Under normal pipelined execution, no i loop iteration is initiated at cycle 1. Under interleaved execution, the (1,0)th iteration of the innermost loop, that is, the first iteration of the next (j=1) invocation of the i loop is initiated. By cycle 10, interleaved execution has initiated all of the iterations of both the j=0 invocation of the i loop and the j=1 invocation of the i loop. This represents twice the efficiency of the normal pipelined execution.

In some cases, you may decide that the performance benefit from interleaving is not equal to the area cost associated with enabling interleaving. In these cases, you may want to limit or restrict the amount of interleaving to reduce FPGA area utilization. To limit the number of interleaved invocations of an inner loop that can be executed simultaneously, annotate the inner loop with the [[intel::max_interleaving(n)]] attribute. The annotated loop must be contained inside another pipelined loop. The required parameter (n) specifies an upper bound on the degree of interleaving allowed, that is, how many invocations of the containing loop can execute the annotated loop at a given time.

Specify the [[intel::max_interleaving(n)]] attribute in one of the following ways:

  • [[intel::max_interleaving(1)]]

    The compiler restricts the annotated (inner) loop to be invoked only once per outer loop iteration. That is, all iterations of the inner loop travel the pipeline before the next invocation of the inner loop can occur.

  • [[intel::max_interleaving(0)]]

    The compiler allows the pipeline to contain a number of simultaneous invocations of the inner loop equal to the loop initiation interval (II) of the inner loop. For example, an inner loop with an II of 2 can have iterations from two invocations in the pipeline at a time. This behavior is the default behavior for the compiler if you do not specify the [[intel::max_interleaving(n)]] attribute.

Example

In the following code snippet, the compiler restricts the pipelined execution of the i loop. A new invocation of the i loop corresponds only to the subsequent iteration of the j loop. 

// Loop j is pipelined with ii=1
for (int j = 0; j < M; j++) {
  int a[N];
  // Loop i is pipelined with ii=2
 [[intel::max_interleaving(1)]]  
  for (int i = 1; i < N; i++) {
    a[i] = foo(i)
  }
  …
}
NOTE:

For additional information, refer to the FPGA tutorial sample max_interleaving listed in the Intel® oneAPI Samples Browser on Linux* or Windows*, or access the code sample in GitHub.

Parent topic: Loop Directives
Level Two Title