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Author: Sophomore42

_code/CppCon/2024/hidden_overhead_of_a_function_api.md

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+---
+title: Hidden Overhead of a Function API by Oleksandr Bacherikov
+date: 2025-03-25
+tags:
+  - Cpp
+  - CppCon
+---
+
+## Introduction
+
+ref: C++ Now 2023 Tony Van Eerd Value Oriented Programming
+
+also C++ Now 2021 and cppcon 2017
+
+> People are not writing enough functions
+
+When people finally start writing more functions, we'd prefer to get only the well designed ones!
+
+Well designed function api can have a large impact on performance.
+
+Compare performance by compare the number of instructions generated by a compiler.
+
+ref: Accelerate large-scale applications with BOLT
+
+Disclaimer: only for non-inlined functions.
+
+- System V ABI
+- Microsoft ABI
+
+C++ Core Guidelines seem like a good candidate
+
+ref: C++ Core Guidelines
+
+## Return Value
+
+> For "out" output values, prefer return values to output parameters.
+
+Reason: A return value is self-documenting, whereas a & could be either in-out or out-only and is liable to be misused.
+
+### Returning std::unique_ptr
+
+Output value is shorter.
+
+Call site.
+
+> Always initialize an object
+
+Reason: Avoid used-before-set errors and their associated undefined behavior.
+
+Any C++ compiler checks that every execution path in a function ends with a return statement. We just need to return by value.
+
+> Use a `unique_ptr<T>` to transfer ownership where a pointer is needed.
+
+Reason: Using unique_ptr is the cheapest way to pass a pointer safely.
+
+unique_ptr add some overhead
+
+memory is about 3 times slower than register.
+
+### Wrapper over int
+
+Class return the memory.
+
+ref: Itanimum C++ ABI
+
+Non-trivial Return Value: callee constructs the return value into this address.
+
+For trivial class it returns register. And also size limit.
+
+> If you can avoid defining default operations, do.
+
+reason: It's simplest and gives the cleanest semantics. It's knows as the rule of zero.
+
+Don't use std::pair and especially `std::tuple`. Named struct is better for both readability and performance.
+
+For `std::unique_ptr`, using `[[always_inline]]` and wrapper the raw pointer.