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 Loop Fuse Functions nofusion Attribute See Also 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

Loop Fuse Functions and nofusion Attribute

This topic describes the loop fuse functions and nofusion attribute that the Intel® oneAPI DPC++/C++ Compiler supports.

Loop Fuse Functions

The loop fuse functions are declared in the sycl/ext/intel/fpga_loop_fuse.hpp header file, which is invoked by the sycl/ext/intel/fpga_extensions.hpp header file. Apply these loop functions to a block of code to indicate to the compiler that it must fuse adjacent loops in the code block overriding the compiler profitability or safety analysis of the fusion. Fusing adjacent loops reduces the amount of loop control overhead in your kernel that reduces the FPGA area used and increases the performance by executing both loops as one (fused) loop. For additional information, see Fuse Loops to Reduce Overhead and Improve Performance

The compiler supports the following loop fuse functions:

  • sycl::ext::intel::fpga_loop_fuse<v>(f): Directs the compiler to fuse loops within the function f and up to a depth of v >= 1 without affecting the functionality of either loop, overriding the compiler profitability analysis of fusing the loops. By default, v = 1, which is equivalent to indicating that the compiler should consider only the adjacent top-level loops for fusing. For example: 
    [=]() { //Kernel
  sycl::ext::intel::fpga_loop_fuse<1>([&] {
    L1: for(...) {}
    L2: for(...) {
      L3: for(...) {}
      L4: for(...) {
        L5: for(...) {}
        L6: for(...) {}
      }
    }
  });
}

    By default (v = 1), only loops L1 and L2 are initially considered for fusing. At a depth of v = 2, the compiler considers L1-L2 and L3-L4 loop pairs for fusing.

    The compiler automatically considers fusing adjacent loops with equal trip counts when the loops meet the Automatic Loop Fusion criteria or when fusion is deemed safe and profitable. You can use the sycl::ext::intel::fpga_loop_fuse<v>(f) function to inform the compiler to consider fusing adjacent loops with different trip counts, which is considered to be unprofitable by default.. With the loop fuse function applied to a block of code, the compiler always attempts to fuse adjacent loops (with equal or different trip counts) in the block whenever the compiler determines that it is safe to fuse the loops. Two loops are considered safe to merge if they meet the Fusion Criteria. The following example shows the effects of fusing loops with unequal trip counts:

    Unfused Loops Fused Loops 
    [=]() { //Kernel
  sycl::ext::intel::fpga_loop_fuse([&] {
    for (int i = 0; i < N; i++) {
      // Loop Body 1
    }
    for (int j = 0; j < M; j++) {
      // Loop Body 2
    }
  });
}
    		 
    for (int f = 0; f < max(M,N); f++) {
  if (f < N) {
    // Loop Body 1
  }
  if (f < M) {
    // Loop Body 2
  }
}

    A fused loop can itself be considered for fusing with other loops. For example, in the following code, L1 and L2 are initially considered for fusing. That resulting fused loop can then be considered for fusing with L4. 

    [=]() { //Kernel
  sycl::ext::intel::fpga_loop_fuse([&] {
    L1: for(...) {}
    L2: for(...) {
      L3: for(...) {}
    }
    L4: for(...) {
    }
  });
}
  • sycl::ext::intel::fpga_loop_fuse_independent<v>(f): Directs the compiler to fuse loops within the function f up to a depth v >= 1 while overriding fusion-safety checks. Here, v = 1 by default. When you use this function, you are guaranteeing to the compiler that fusing pairs of loops affected by the loop fuse function is safe. That is, there are no negative distance dependencies between the pairs of loops. If it is not safe, you might get functional errors in your kernel. Besides this difference, the sycl::ext::intel::fpga_loop_fuse<v>(f) and sycl::ext::intel::fpga_loop_fuse_independent<v>(f) functions behave identically.

Function Calls in Loop Fuse Code Blocks

If a function call occurs in a code block annotated with a loop fuse function and inlining that function call contains a loop, the resulting loop can be a candidate for loop fusion.

Nested Loop Fusion Functions

When you nest loop fusion functions, you might create overlapping sets of candidate loops. Consider the following example: 

[=]() { //Kernel
  sycl::ext::intel::fpga_loop_fuse_independent<2>([&] {
    L1: for(...) {}
    L2: for(...) {
      sycl::ext::intel::fpga_loop_fuse<2>([&] {
        L3: for(...) {}
        L4: for(...) {
          L5: for(...) {}
          L6: for(...) {}
        }
      });
    }
  });
}

In this example, the compiler considers the following loop pairs for fusion: L1-L2, L3-L4, and L5-L6. In addition, the compiler overrides the compiler negative distance dependency analysis of L1-L2 and L3-L4 loop pairs.

nofusion Attribute

You can exempt a loop from being fused with an adjacent loop by annotating the loop with the nofusion loop attribute. This attribute prevents the annotated loop from being automatically or fused when it is subject to the loop fusion functions.

Syntax

[[intel::nofusion]]

Example

For example, the following code samples have the same effect. If one loop in a pair is annotated with the nofusion attribute, the other loop has no other loop to fuse with. 

[[intel::nofusion]]
L1: for (int j = 0; j < N; ++j){
  data[j] += Q;
}
L2: for (int i = 0; i < N; ++l) {
  output[i] = Q * data[i];
}
 
L1: for (int j = 0; j < N; ++j){
  data[j] += Q;
}
[[intel::nofusion]]
L2: for (int i = 0; i < N; ++l) {
  output[i] = Q * data[i];
}

In the following example, the compiler does not apply the loop fusion transformation to the loops: 

for (int x = 0; x < N; x++) { // loop 1
  arr1_acc[x] = x; 
}

[[intel::nofusion]] 
for (int x = 0; x < N; x++) { // loop 2
  arr2_acc[x] = x; 
}

Because you have applied the nofusion attribute to loop 2, the compiler cannot fuse loop 1 with loop 2.

TIP:

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

Parent topic: Loop Directives
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