Intel® Advisor 2016 offers a vectorization analysis tool and a threading design and prototyping tool to help ensure your Fortran, C and C++ applications take full performance advantage of today’s processors. This topic discusses getting started with the Vectorization Advisor GUI on a Linux* platform.
Chosen Platform and Tool/Feature
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Key Features
Key Vectorization Advisor features include the following:
Survey Report - Offers integrated compiler report data and performance data all in one place. Use the Survey Report to help identify:
Where vectorization will pay off the most
If vectorized loops are providing benefit, and if not, why not
Un-vectorized and under-vectorized loops, and the estimated expected performance gain of vectorization or better vectorization
How data accessed by vectorized loops is organized and the estimated expected performance gain of reorganization
The Survey Report also provides:
Friendly, code-specific recommendations for how to fix vectorization issues
Quick visibility into source code and assembly code
Trip Counts analysis - Dynamically identifies the number of times loops are invoked and execute (sometimes called call count/loop count and iteration count respectively). Use this added information in the Survey Report to make better decisions about your vectorization strategy for particular loops, as well as optimize already-parallel loops.
Dependencies Report - For safety purposes, the compiler is often conservative when assuming data dependencies. Use a Dependencies-focused Refinement Report to check for real data dependencies in loops the compiler did not vectorize because of assumed dependencies. If real dependencies are detected, the analysis can provide additional details to help resolve the dependencies. Your objective: Identify and better characterize real data dependencies that could make forced vectorization unsafe.
Memory Access Patterns (MAP) Report - Use a MAP-focused Refinement Report to check for various memory issues, such as non-contiguous memory accesses and unit stride vs. non-unit stride accesses. Your objective: Eliminate issues that could lead to significant vector code execution slowdown or block automatic vectorization by the compiler.
Prerequisites
To build applications that produce the most accurate and complete Vectorization Advisor analysis results, build an optimized binary of your application in release mode using these settings:
To Do This | Optimal C/C++ Settings |
|---|---|
Request full debug information (compiler and linker). | -g |
Request moderate optimization. | -O2 or higher |
Produce compiler diagnostics (necessary for version 15.0 of the Intel compiler; unnecessary for version 16.0 and higher). | -qopt-report=5 |
Enable vectorization | -vec |
Enable SIMD directives | -simd |
Enable generation of multi-threaded code based on OpenMP* directives. | -qopenmp |
To Do This | Optimal Fortran Settings |
|---|---|
Request full debug information (compiler and linker). | -g |
Request moderate optimization. | -O2 or higher |
Produce compiler diagnostics (necessary for version 15.0 of the Intel compiler; unnecessary for version 16.0 and higher). | -qopt-report=5 |
Enable vectorization | -vec |
Enable SIMD directives | -simd |
Enable generation of multi-threaded code based on OpenMP* directives. | -qopenmp |
Verify your application runs before trying to analyze it with the Intel Advisor.
Make sure you run the Intel Advisor in the same environment as your application.
Set Up Environment
Do one of the following to set up your environment.
Run one of the following source commands:
For csh/tcsh users: source <advisor-install-dir>/advixe-vars.csh
For bash users: source <advisor-install-dir>/advixe-vars.sh
The default installation path, <advisor-install-dir>:
For root users: /opt/intel/parallel_studio_xe_201n/advisor_xe_201n/
For non-root users: $HOME/intel/parallel_studio_xe_201n/advisor_xe_201n/
Add <advisor-install-dir>/bin32 or <advisor-install-dir>/bin64 to your path.
Run the <parallel-studio-install-dir>/psxevars.csh or <parallel-studio-install-dir>/psxevars.sh command. The default installation path, <parallel-studio-install-dir>:
For root users: /opt/intel/parallel_studio_xe_201n/
For non-root users: $HOME/intel/parallel_studio_xe_201n/
Get Started
Follow these steps (white blocks are optional) to get started using the Vectorization Advisor in the Intel Advisor.
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Launch the Intel Advisor
Run the advixe-gui command.
Manage Project
Choose File> New> Project… (or click New Project… in the Welcome page) to open the Create a Project dialog box.
Supply a name and location for your project, then click the Create Project button to open the Project Properties dialog box.
On the left side of the Analysis Target tab, ensure the Survey Hotspots/Suitability Analysis type is selected.
Set the appropriate parameters. (Setting the binary/symbol search and source search directories is optional for the Vectorization Advisor.)
Click the OK button to close the Project Properties dialog box and display an empty Survey Report and the VECTORIZATION WORKFLOW.
Tip
If you plan to run other vectorization Analysis Types, set parameters for those Analysis Types now.
The Survey Trip Counts Analysis type has similar parameters to the Survey Hotspots/Suitability Analysis type.
The Dependencies Analysis and Memory Access Patterns Analysis types consume more resources than the Survey Hotspots/Suitability Analysis type. If these Refinement analyses take too long, consider decreasing the workload.
Select Track stack variables in the Dependencies Analysis type to detect all possible dependencies.
When necessary, click the control at the bottom of the WORKFLOW to switch between the VECTORIZATION WORKFLOW and THREADING WORKFLOW.
Run Survey Analysis
Under Survey Target in the VECTORIZATION WORKFLOW, click the Image may be NSFW.
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control to collect Survey data while your application executes.
After the Intel Advisor collects the data, it displays a Survey Report similar to the following:
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There are many controls available to help you focus on the data most important to you, including the following:
1 | Click the various Filter controls (buttons and drop-down lists) to temporarily limit displayed data based on your criteria. |
2 | Click the Search control to search for specific data. |
3 | Click the Expand/Collapse controls to show/hide sets of columns. |
4 | Click a loop data row in the top of the Survey Report to display more data specific to that loop in the bottom of the Survey Report. Double-click a loop data row to display a Survey Source window. |
5 | Click a checkbox to mark a loop for deeper analysis. |
6 | If present, click a light bulb icon to display code-specific how-can-I-fix-this-issue? information in the Recommendations pane. |
7 | If present, click a book icon to display code-specific how-can-I-fix-this-issue? information in the Compiler Diagnostics pane. |
8 | Click the control to show/hide the WORKFLOW pane. |
Run Trip Counts Analysis
This step is optional.
Before running a Trip Counts analysis, make sure you set the appropriate Project Properties for the Survey Trip Counts Analysis type. (Use the same application, but a smaller input data set if possible.)
Under Find Trip Counts in the VECTORIZATION WORKFLOW, click the Image may be NSFW.
Clik here to view. control to collect Trip Counts data while your application executes.
After the Intel Advisor collects the data, it adds a Trip Counts column set to the Survey Report. Median data is shown by default. Min, Max, Call Count, and Iteration Duration data are shown when the column set is expanded.
Investigate Loops
The Survey Report provides a wealth of information, including:
Key information from the Intel compiler vectorization and optimization reports
Source and assembly code for the data row selected at the top of the report
Code-specific how-can-I-fix-this-issue? recommendations for the data row selected at the top of the report, similar to the following:
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Clik here to view.![Recommendations pane]()
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Clik here to view.![Compiler Diagnostics pane]()
Tip
Pay particular attention to the hottest loops in terms of Self Time and Total Time. Optimizing these loops provides the most benefit. Innermost loops and loops near innermost loops are often good candidates for vectorization. Outermost loops with significant Total Time are often good candidates for parallelization with threads.
Check if the best possible Vector Instruction Set is used by your application, or if there are heavy operations required for vectorization that might be a problem, such as masking or gather operations.
Compare the modeled Estimated Achieved Gain with the gain expected from the Vector Instruction Set to ensure you are likely to get the optimal speed-up. For example: AVX2 processing of 32-bit integers should give an 8x performance gain. If the Estimated Achieved Gain is much lower than the expected gain for the Vector Instruction Set, consider optimizing an already vectorized loop by eliminating heavy vector operations, aligning data, or rewriting the loop to remove control-flow clauses.
A vectorized loop may not achieve the best performance when the compiler peels a source loop into peeled and remainder loops. If the peeled or remainder loop takes a significant portion of loop execution time, aligning data or changing the number of loop iterations may help.
After you investigate the data in the Survey Report, you have several choices:
Run Dependencies Analysis
This step is optional.
Before running a Dependencies analysis, make sure you:
Set the appropriate Project Properties for the Dependencies Analysis type. (Use the same application, but a smaller input data set if possible. And select Track stack variables to detect all possible dependencies.)
Mark one or more un-vectorized loops for deeper analysis in the Survey Report.
Under Check Dependences in the VECTORIZATION WORKFLOW, click the Image may be NSFW.
Clik here to view. control to collect Dependencies data while your application executes.
After the Intel Advisor collects the data, it displays a Dependencies-focused Refinement Report similar to the following:
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Clik here to view.
Depending on what the Dependencies Report shows, do one or more of the following:
Rewrite code to remove dependencies.
If there is no real dependency in the loop for the given workload, use one of the following to tell the compiler it is safe to vectorize:
#pragma simdICL/ICC/ICPC directive, or#pragma omp simdOpenMP* 4.0 standard, or!DIR$ SIMDor!$OMP SIMDIFORT directive to ignore all dependencies in the loop#pragma ivdepICL/ICC/ICPC directive or!DIR$ IVDEPIFORT directive to ignore only vector dependencies (which is safest, but less powerful in certain cases)restrictkeyword
If there is an anti-dependency (often called a Write after read dependency or WAR), enable vectorization using the
#pragma simd vectorlength(k)ICL/ICC/ICPC directive or!DIR$ SIMD VECTORLENGTH(k)IFORT directive, where k is smaller than the distance between dependent items in anti-dependency:
After you finish making improvements:
Run a MAP analysis if desired.
Rebuild your modified code.
Run another Survey analysis to verify your application still runs correctly and all test cases pass, all loops are vectorizing properly, and performance is satisfactory.
Run a Memory Access Patterns (MAP) Analysis
This step is optional.
Before running a MAP analysis, make sure you:
Set the appropriate Project Properties for the Memory Access Patterns Analysis type. (Use the same application, but a smaller input data set if possible.)
Mark one or more loops for deeper analysis in the Survey Report.
Under Check Memory Access Patterns in the VECTORIZATION WORKFLOW, click the Image may be NSFW.
Clik here to view. control to collect MAP data while your application executes.
After the Intel Advisor collects the data, it displays a MAP-focused Refinement Report similar to the following:
Image may be NSFW.
Clik here to view.
After you finish making improvements:
Rebuild your modified code.
Run another Survey analysis to verify your application still runs correctly and all test cases pass, all loops are vectorizing properly, and performance is satisfactory.
Tip
Double-click source lines at the bottom of the report to get a more detailed source and assembly access pattern report where stride information is provided at the instruction level.
Troubleshooting/FAQ
Also, see https://software.intel.com/en-us/intel-advisor-xe-support/faq.
To Do This | Optimal C/C++ Settings |
|---|---|
Retrieve better compiler diagnostics. | Disable Interprocedural Optimization (IPO): -no-ipo |
Address any issues with source line matching. |
|
Experiment with generating code for different instructions. | -xHost, -xSSE4.2, -xAVX, -axAVX, -xCORE-AVX2, or -axCORE-AVX2 |
To Do This | Optimal Fortran Settings |
Retrieve better compiler diagnostics. | Disable Interprocedural Optimization (IPO): -no-ipo |
Address any issues with source line matching. |
|
Experiment with generating code for different instructions. | -xHost, -xSSE4.2, -xAVX, -axAVX, -xCORE-AVX2, or -axCORE-AVX2 |