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---
title: Function Multiversioning

minutes_to_complete: 30

who_is_this_for: Developers who want to optimize their C/C++ applications across various Arm64 targets.

learning_objectives:
- Take advantage of hardware features for tuning your applications at function level granularity.
- Create multiple versions of C/C++ functions for the targets you intend to run your applications on.
- Assist the compiler in generating better code for those targets, or provide your own optimized versions at the source level.
- Automatically select the most appropriate function version for your host target at runtime.
- Reuse your optimized application binaries across various targets.

prerequisites:
- Basic knowledge of GNU function attributes. Familiarity with indirect functions (ifuncs) is a plus.
- Basic understanding of loop vectorization.
- Familiarity with Arm assembly.
- LLVM 19 compiler with runtime library support or GCC 14.

author_primary: Arm

### Tags
skilllevels: Intermediate
subjects: Tuning
armips:
- Armv8
- Armv9
tools_software_languages:
- C/C++
operatingsystems:
- Linux
- Android
- macOS

# ================================================================================
# FIXED, DO NOT MODIFY
# ================================================================================
weight: 1 # _index.md always has weight of 1 to order correctly
layout: "learningpathall" # All files under learning paths have this same wrapper
learning_path_main_page: "yes" # This should be surfaced when looking for related content. Only set for _index.md of learning path content.
---
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---
next_step_guidance:

recommended_path: /learning-paths/PLACEHOLDER_CATEGORY/PLACEHOLDER_LEARNING_PATH/

further_reading:
- resource:
title: Arm C Language Extensions
link: https://arm-software.github.io/acle/main/acle.html
type: documentation

# ================================================================================
# FIXED, DO NOT MODIFY
# ================================================================================
weight: 21 # set to always be larger than the content in this path, and one more than 'review'
title: "Next Steps" # Always the same
layout: "learningpathall" # All files under learning paths have this same wrapper
---
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---
review:
- questions:
question: >
What is the main benefit of Function Multiversioning?
answers:
- I may reuse my binaries on different targets without sacrificing runtime performance.
- My application binaries will be smaller in size.
correct_answer: 1
explanation: >
The produced binaries can be reused on different targets, but they may be larger in size.

- questions:
question: >
Can I implement versions of a function in separate translation units?
answers:
- Yes, function versions can spread across different translations units.
- No, all of them must be in the same translation unit.

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We did that for clang, but (and I don't know if you care...) did gcc get the same feature?

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We do care about GCC, this learning path is for both toolchains. We made this design decision part of ACLE so GCC will have to adhere to it. Adding @andrewcarlotti for more visibility.

correct_answer: 1
explanation: >
There is no requirement for function versions to be defined in the same translation unit. However, all of them must be declared in the translation unit which contains the definition of the default version.

# ================================================================================
# FIXED, DO NOT MODIFY
# ================================================================================
title: "Review" # Always the same title
weight: 20 # Set to always be larger than the content in this path
layout: "learningpathall" # All files under learning paths have this same wrapper
---
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