Handling Multiple Implementations of Common Computer Architectures: A Deep Dive into Multi-Versioning

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Introduction

As software developers, we often face the challenge of writing code that can run on a wide variety of hardware configurations. Modern CPUs, such as x86-64 and aarch64, come with numerous micro-architectural features designed to enhance performance. These features can include special acceleration instructions, unique data formats, SIMD capabilities, and more. However, not all CPUs support all features, leading to potential compatibility issues when running software across different systems. This blog post explores the problem of handling multiple CPU implementations and delves into the concept of multi-versioning, specifically focusing on Library Multi-Versioning (LMV), Function Multi-Versioning (FMV), and Automatic Function Multi-Versioning (AFMV).

Understanding Micro-Architectural Features

Micro-architectural features refer to specific enhancements or capabilities present in a CPU beyond its base architecture. These features are usually visible in the "flags" or "features" field in /proc/cpuinfo on Linux systems. Examples include:

  • Acceleration instructions: Such as those for cryptography.
  • Special data formats: Like BF16 (Brain Float 16 bit).
  • SIMD capabilities: Single Instruction, Multiple Data operations.
  • Specific instructions: Like popcnt to count the number of set bits in data.

These features can be referred to in different ways:

  1. Individual feature names: Usually matching the flags in /proc/cpuinfo.
  2. Architectural levels: Such as x86-64-v3 or armv8.5.
  3. Base architectural level plus additional features: For example, armv8.5+sve2.

Executing machine-language code with unsupported instructions on a processor results in a fault (illegal instruction), causing the operating system to terminate the offending process. To avoid this, developers need to strike a balance between broad compatibility and performance optimization.

Multi-Versioning Explained

Multi-versioning allows software to take full advantage of CPU features available on different systems by creating multiple versions of the code for various micro-architectural levels. This ensures compatibility while optimizing performance. There are three types of multi-versioning:

1. Library Multi-Versioning (LMV)

LMV involves building binaries and libraries multiple times, with the operating system selecting the best available version at runtime. This approach requires no source-level changes but significantly increases storage and distribution volumes due to duplicated libraries and executables.

2. Function Multi-Versioning (FMV)

FMV compiles functions multiple times for different micro-architectural targets. There are various levels of FMV support:

  • IFunc System: Fully manual indirect function support where developers write multiple versions of a function and a resolver function to select the implementation at runtime.
  • Compiler FMV Support: Developers write multiple versions of a function with the same name, applying attributes to indicate which features must be present for each version. The compiler generates the resolver function.

Example: Function Multi-Versioning in C

Here is an example of FMV using GCC for an x86-64 target:

#include <stdio.h>

__attribute__((target("default")))

void optimized_function() {

    printf("Default version\n");

}

__attribute__((target("avx2")))

void optimized_function() {

    printf("AVX2 optimized version\n");

}

__attribute__((target("sse4.2")))

void optimized_function() {

    printf("SSE4.2 optimized version\n");

}

int main() {

    optimized_function();

    return 0;

}


In this example, the optimized_function is defined three times with different target attributes. The compiler selects the appropriate version based on the CPU features available at runtime.

3. Automatic Function Multi-Versioning (AFMV)

AFMV aims to automate the FMV process by enabling the compiler to perform automatic FMV cloning of functions that would benefit from it. This approach requires no source code changes, reducing the development and maintenance burden.

Conclusion

Multi-versioning provides a robust solution for handling the diverse capabilities of modern CPUs. By leveraging LMV, FMV, and AFMV, developers can ensure their software runs efficiently on various systems without compromising performance. While LMV offers simplicity at the cost of storage overhead, FMV provides granular control with some manual effort, and AFMV promises an automated, developer-friendly approach. Embracing these techniques allows software to be both compatible and performant, meeting the demands of today's heterogeneous computing environments.



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