Project - Stage 2 (Part 2)

In this post, I will continue to work on SPO600 Project - Stage 2.

This post will include the part 2 of the project, which is about the design.

You can see Stage 2 - Part 1 here.


1. Design how an automatic ifunc implementation within the GCC compiler could work from the user's point of view.

1-1) What options should the user specify on the command line to indicate that they want automatic ifunc capability to be applied during the compilation?

-fauto-ifunc

It would instruct the GCC compiler to enable automatic ifunc generation during the compilation process. 
 

1-2) How should the user specify the list of architecture variants they want to target? (e.g., a base architecture of "armv8-a" and extended architectures of "armv8-a+sve" and "armv8-a+sve2", or a similar list of architecture variants for other platforms).

User can specify the list of architecture variants by using -march-variants. For example, -march-variants=base selects the base architecture. To extend the list of architecture, add the architectures like below:

-march-variants=armv8-a,armv8-a+sve,armv8-a+sve2

2) What constraints need to be applied to the list of architecture variants? (i.e., how do we figure out if the architecture variants can be combined into a sane executable)?

  • Compatibility
    • Ensure that the selected architecture variants are compatible with each other. Some variants may have dependencies or conflicts with others. For example, combining conflicting vector extensions might lead to issues.
  • Feature Availability
    • Verify that the selected architecture variants are supported by the target hardware. Not all variants may be available or supported on a specific platform.
  • Compiler Support
    • Check if the compiler used supports the specified architecture variants. Some variants might require specific compiler versions or flags.
  • Library Support
    • Ensure that the libraries and runtime environment support the chosen architecture variants. If the required runtime support is not available, it might lead to runtime errors.
  • Linker Compatibility
    • Verify that the linker can handle the specified architecture variants. Some combinations may not be supported by certain linkers.
  • Consistency
    • Aim for a consistent set of architecture variants. Mixing conflicting variants might result in unpredictable behavior or errors.
  • Code Generation
    • Understand how the compiler generates code for the specified variants. Certain combinations might lead to suboptimal code or unintended side effects.
  • Testing
    • Thoroughly test the executable on the target platform to ensure that the selected architecture variants behave as expected and do not introduce compatibility issues.

3-1) To which functions should the automatic ifunc capability be added? Should the user specify the functions to be affected? If so, how? (What should the source code syntax look like?) 

The user can specify the functions to be affected by using attribute.
The source code utilizes the attribute as shown below:

void func() __attribute__((ifunc("resolver_function")));

This line sets the ifunc capability to the func function to be resolved by resolver_function.

3-2) If the functions should be selected automatically, what should the selection criteria be? 

The selection criteria could be below:
  • Function Size
    • Criteria based on the size of functions, selecting small functions or those below a certain size threshold. Applying ifunc to small functions can minimize overhead.
  • Call Frequency
    • Criteria based on the frequency of function calls. Selecting functions that are frequently called can improve performance by applying ifunc to core functions.
  • Preferred Platform
    • Criteria based on the preferred platform, selecting functions that perform better on a specific platform. For example, choosing functions that leverage supported extensions on a particular platform.
  • Optimization Potential
    • Criteria based on the potential for optimization. Selecting functions that the compiler deems suitable for optimization, as applying ifunc to such functions may yield benefits.
  • User-defined Criteria
    • Criteria based on user-defined factors. Providing users with the flexibility to define their own criteria, possibly through explicit options, allows for customized function selection.

3-3) And should the user be able to tune the selection criteria, and if so, how (what should the command-line option syntax look like)?

When users want to set the automatic ifunc performance to meet specific requirements, offering options to tune various selection criteria is valuable.
For example, the command-line option syntax could be designed as follows:

# Adjust size threshold for function selection
-ftune-ifunc-size=size_threshold      
# Adjust criteria based on call frequency
-ftune-ifunc-call-frequency=frequency_threshold
# Set a preferred platform for optimization
-ftune-ifunc-preferred-platform=platform_name   
# Adjust criteria based on optimization potential
-ftune-ifunc-optimization-potential=potential_threshold 
# Define and set user-defined criteria 
-ftune-ifunc-user-defined-criteria=criteria_name=value  


4) What diagnostics should be produced during the compilation process?

  • ifunc Resolver Selection Diagnostic
    • The compiler should output a message indicating how the ifunc resolver function (adjust_channels__resolver) was selected. This helps users understand the behavior of ifunc and aids in debugging.
  • ifunc Application Diagnostic
    • There should be a message indicating whether ifunc has been successfully applied to the function. Successful ifunc application means that the function has been dynamically linked based on the selected resolver.
  • Architecture Selection Diagnostic
    • A message is needed to confirm that the correct function has been selected based on the explicitly set architecture. This verifies that the function compiled with -march-variants and #pragma GCC target aligns with the specified architecture.
  • Hardware Support Verification Diagnostic
    • A message indicating whether the function used by ifunc supports specific hardware features is necessary. This is related to checking hardware features using getauxval and selecting the appropriate function accordingly.
  • ifunc-Related Warning and Error Messages
    • Warning and error messages are needed for cases where the function intended for ifunc application is not successfully selected or if the resolver function is not working correctly. Additionally, warning messages related to ifunc application may be necessary.
  • Compile Success/Failure Messages
    • Clear messages indicating whether the compilation was successful or failed are essential. This informs users about the compilation status.

2. Make sure that these options are consistent with the current design of GCC (i.e., they will make sense to existing GCC users)

The proposed options such as -fauto-ifunc for enabling automatic ifunc and -march-variants for specifying architecture variants follow the established GCC convention of using -f for feature-related options and providing intuitive syntax for architecture selection.

The usage of __attribute__((ifunc("resolver_function"))) aligns with existing GCC practices for specifying attributes. This consistency ensures that users familiar with GCC's attribute-based features will find the syntax intuitive.

The introduced tuning options, such as -ftune-ifunc-size, are consistent with GCC's approach to tuning compiler behavior using -ftune options.


3. Explain your design decisions in detail, clearly presenting the case for each decision and why you feel it's the best option.

-fauto-ifunc
  • This option is chosen to enable automatic ifunc generation during the compilation process.
  • Enabling ifunc automatically simplifies the user experience. Instead of manually specifying ifunc for each function, users can opt-in for automatic application, reducing the burden on developers. This aligns with the principle of providing convenient and intuitive options for users.
-march-variants
  • Allowing users to specify a list of architecture variants using this option.
  • Different platforms may have varying architectures and feature sets. Allowing users to explicitly define architecture variants ensures that the compiled code is optimized for the targeted platform. This option provides flexibility and precision, crucial for performance optimization on diverse hardware.
attribute((ifunc("resolver_function")))
  • Using the __attribute__((ifunc("resolver_function"))) attribute to specify ifunc resolver functions.
  • This attribute provides a clear and concise syntax for associating ifunc resolver functions with target functions. It enhances readability and maintains a consistent style with other attributes in the C language. The attribute approach aligns with established conventions, making it an intuitive choice for users familiar with C programming.
-ftune-ifunc-optionParameter
  • The introduction of specific tuning options for ifunc based on different criteria such as function size, call frequency, preferred platform, optimization potential, and user-defined criteria.
  • Recognizing that users may have diverse optimization requirements, providing a set of tuning options allows for fine-grained control over ifunc behavior. Each tuning option addresses a specific aspect that can influence the performance and effectiveness of ifunc. This modular approach enables users to tailor ifunc usage to their application's unique characteristics, promoting flexibility and customization in the optimization process. The design decision aligns with the broader goal of accommodating various use cases and optimizing for specific scenarios without compromising simplicity.

Comments

Post a Comment

Popular posts from this blog

Project - Stage 2 (Part 1)

In  SPO600 project - Stage 2 , i t is divided into 2 parts: (1) examine the intermediate code output of the compiler (2) propose a design for how the automatic ifunc capability could operate from the user's point of view I will work on the part 1 in this post. 1. Unpack the file First of all, run  cd ; tar xvf /public/spo600-autoifunc-preprocessor-pos.tgz  to make sure you have the unpacked file in your directory. 2. Build the code What does the scripts/autoifunc script or program do? It utilizes the GCC compiler to automatically build functions as Indirect Functions (ifunc). ifunc is a mechanism used to selectively support specific features at runtime. Certainly, let's take a brief look at a few lines of code related to the functionality of the script: The script uses the makeheaders command to generate a .h file from a  .c  file for the purpose of obtaining function prototypes.      ...     makeheaders "${TEMP_C}.c"     ... The generated resolver function utilize
Read more

Understanding Docker: A Comprehensive Guide

Image
Introduction In recent years, containerization has become a pivotal technology in the world of software development and deployment. Among the various containerization tools available, Docker stands out as a powerful and widely adopted solution. In this blog post, we will delve into the fundamentals of Docker, exploring its key concepts, benefits, and practical applications. What is Docker? Docker is an open-source platform designed to automate the deployment, scaling, and management of applications within lightweight, portable containers. Containers encapsulate an application and its dependencies, ensuring consistency across different environments. Docker provides a standardized runtime environment, enabling seamless execution of applications across diverse systems. Key Concepts Containers Containers are lightweight, standalone, and executable software packages that include everything needed to run a piece of software, including the code, runtime, libraries, and system tools. Docker
Read more

Project - Stage 1

Image
In this post, I will build GCC compiler on servers located in SPO600 server Portugal and  Israel, and then verify the results. You can see the detailed information about the SPO600 project here . Build GCC in x86_64 System Before getting started, execute the following command to connect to  portugal.cdot.systems  using SSH with the private SSH key and username. When prompted for a passphrase, enter the passphrase you set previously to establish the connection: ssh -i private_ssh_key username @portugal.cdot.systems If the connection is successful, you can confirm the presence of the password.txt  file using the ls  command. This file contains a randomly generated string password that can be used for login instead of the passphrase. First, execute the following command to fetch the current development version of GCC from the Git repository: git clone git://gcc.gnu.org/git/gcc.git To create an object tree outside the source tree, create a build directory outside of the gcc  directory and
Read more