Developer Guide

FPGA Optimization Guide for Intel® oneAPI Toolkits

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ID 767853
Date 7/13/2023
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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 Speculated Iterations Dynamic Trip Counts 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
Pipes x
Host Pipes
Host Pipes x
Host Pipe Declaration Host Pipe API Host Pipes RTL Interfaces
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>) Create a 2xclock Interface (-Xsuse-2xclock) Disable Burst-Interleaving of Global Memory (-Xsno-interleaving=<global_memory_name>) 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>) Generate Register Map Wrapper (-Xsregister-map-wrapper-type)
Optimization Targets x
Minimum Latency Flow Maximum Throughput Without Area Optimization Heuristics 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 Asynchronous Parallelism Within Kernels (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
Asynchronous Parallelism Within Kernels (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

Optimize Inner Loop Throughput

In this topic, you learn how to optimize throughput of an inner loop with a low trip count. A low trip count is relative. For understanding the concept, consider low to be on the order of 100 or fewer iterations.

NOTE:

Prior to reading the rest of this topic, you must review Maximum Frequency (fMAX) and Optimize Loops With Loop Speculation.

Consider the following example code snippet: 

for (int i; i < kOuterLoopBound; i++) {
  int inner_loop_iterations = rand() % kInnerLoopBound;
  for (int j; j < inner_loop_iterations; j++) {
    /* ... */
  }
}

Before you optimize the inner loops with low trip counts, assume the following:

  • kOuterLoopBound is a number greater than one million.
  • kInnerLoopBound is 3. This means that the value of inner_loop_iterations is dynamic, but you know that it is in the range (0,3).
  • II of the inner loop is 1, which means that a new inner loop iteration can start every cycle. This means that the outer loop II is dynamic and depends on how many inner loop iterations the previous outer loop iteration must start.

A possible timing diagram for this loop structure is shown in the following figure, where the numbers in the squares are the values of i and j, respectively:

Timing Diagram for the Loop

In general, the Intel® oneAPI DPC++/C++ Compiler optimizes loops for throughput with the assumption that the loop has a high trip count. These optimizations include (but are not limited to) speculating iterations and inserting pipeline registers in the circuit that starts loops. The next two sections describe how these optimizations can substantially decrease throughput and how you can disable them to improve your design when applied to inner loops with low trip counts.

Speculated Iterations

Loop Speculation enables loop iterations to be initiated before determining whether they should have been initiated. Speculated iterations are the iterations of a loop that launch before the exit condition computation has been completed. This is beneficial when the computation of the exit condition is preventing effective loop pipelining. However, when an inner loop has a low trip count, speculating iterations result in a relatively high proportion of invalid loop iterations.

For example, consider that the compiler speculated two inner loop iterations for the code above. In that case, the timing diagram appears as shown in the following diagram, where the orange blocks with the S denote an invalid speculated iteration:

Timing Diagram with Invalid Speculated Iteration

In this case where the inner loop iteration count is in the range (0,3), speculating two iterations can cause up to a 3x reduction in the design's throughput. This happens when each outer loop iteration launches one inner loop iteration (inner_loop_iterations is always 1), but two iterations are speculated. For this reason, Intel® advises to force the compiler to not speculate iterations for inner loops with known small trip counts using the [[intel::speculated_iterations(0)]] attribute.

Dynamic Trip Counts

As mentioned earlier, the compiler's default behavior is to optimize loops for throughput. However, as you saw in the previous section, loops with low trip counts have unique throughput characteristics that lead to the compiler making different optimizations. The compiler attempts its best to determine if a loop has a high or low trip count and optimizes accordingly. However, in some circumstances, you may need to provide it with more information to make a better decision.

In the previous section, this additional information was the speculated_iterations attribute. However, it is not just speculated iterations that cause delays in the launching of inner loops. The compiler uses other heuristics. For example, the compiler may attempt to improve the fMAX of a loop circuit by adding a pipeline register on the circuit path that starts a loop, which results in a one-cycle delay in starting the loop. For outer loops with large trip counts, this one cycle delay is negligible. However, for inner loops with small trip counts, this one cycle delay can cause throughput degradation. Like the speculated iteration case discussed in the previous section, this one cycle delay can result in up to a 2x reduction in the design's throughput.

If the inner loop bounds are known to the compiler, it decides whether to turn on/off this delay register depending on the (known) trip count. However, in the code snippet above, the inner loop's trip count is not a constant (inner_loop_iterations is a random number at runtime). In cases like this, Intel recommends explicitly bounding the trip count of the inner loop. This is illustrated in the example code snippet in the following, where j < kInnerLoopBound exit condition is added to the inner loop. This gives the compiler more explicit information about the loop's trip count and allows it to optimize accordingly. 

for (int i; i < kOuterLoopBound; i++) {
  int inner_loop_iterations = rand() % kInnerLoopBound;
  for (int j; j < inner_loop_iterations && j < kInnerLoopBound; j++) {
    /* ... */
  }
}
NOTE:

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

Parent topic: Loops
Level Two Title