eden/docs/policies/Coding.md

# Coding guidelines

These are **not** stylistic guidelines, they're, for the most part, suggestions on how to architecture new systems or improve upon the existing codebase.

## Foreword

Keep your code simple, efficient, and readable.

## C++ guidelines

Everyone has their own way of viewing good/bad C++ practices, my general outline:

- At your disposal you may use `boost::container::static_vector<>` (beware it has a ctor/initialization cost which goes up the more elements you add).
  - Or you may use `boost::container::small_vector<>` (which has an initialization cost as well, and will use extra book-keeping for heap, try to keep a balance).
- Don't use `[[likely]]` or `[[unlikely]]`; PGO builds exist for that.
- Don't use inline assembly to try to outsmart the compiler unless you're 100% sure the assembly you're writing is actually good.
  - And if so, try to restructure your C++ code so the compiler vectorizes it/makes it better
  - Or if that fails, use intrinsics instead of raw `asm volatile`.
- Use `std::optional<>` instead of `std::unique_ptr<>` if possible.
  - `std::unique_ptr<>` carries indirection cost due to it being memory allocated on the heap.
  - It isn't often that objects that contain `std::unique_ptr<>`, are allocated on the heap themselves, allocating even more things on the heap seems redundant.
- Avoid `std::recursive_mutex` at all costs.
  - It's basically implemented as a linked list most of the time and has HEAVY performance penalties.
- Exploit the fact `std::atomic<uint32_t>/std::atomic<int32_t>` is basically free on most arches that matter.
  - In x86_64, an atomic `uint32_t` is basically `mov [m32], r32`, which is essentially free/cheap.
- Avoid template parameters unless you really need them.
  - For small inlineable functions this is fine, for more complex ones, please consider the generated assembly.
- Dont make your own memcpy/memset/strcpy/strncpy/etc.
  - Seriously DON'T DO THIS. You will NOT beat the compiler.
  - Nor 30 years of writing optimized `mem*`.
  - If your code is slow, don't blame `mem*`, blame your code.
- Try to avoid using `virtual` since vtable indirection has a cost
- Avoid `dynamic_cast` and `typeid` at all costs.
  - The reason is because the project has `-fno-rtti` disabled by default, due to the costs of dynamic polymorphism.
- Always copy-on-value for objects with `sizeof(void *) >= sizeof(T) * 2`, i.e objects sized as 2 pointers or less, for bigger objects you can use ref/pointer as usual.
- Try using move semantics instead of references, whenever possible.
- Function parameters are cheap. Don't be afraid to use as many as needed (API usability permitting).
- Don't save a reference in structures of a parent object, i.e:

    ```c++
    struct Child {
        Parent& parent;
        void Mehod() {
            parent.Something();
        }
    };
    ```

  - Instead you can do the following:

    ```c++
    struct Child {
        void Mehod(Parent& parent) {
            parent.Something();
        }
    };
    ```

  - This reduces the amount of pointers you have lying around, and also works better because of the aforementioned cheapness of parameter functions.

## Engineering guidelines

Programming, alongside the physical act of writing code, also consists of architecting the code you write into a coherent, maintainable system.

- Try to reduce your usage of dependencies
  - Dependencies that are legitimately useful to have are few and far between.
  - At the same time, NIHing your own implementations of widely adopted algorithms or standards can be quite subpar.
  - For dependencies that are very large but contain something you need, consider cherry-picking the individual files it needs (or writing a smaller version of it)
- Try to rely less on indirection for architecturing systems
  - If the underlying HLE kernel emulation requires it, try making a solution that keeps things local
    - For example, there isn't a need for file descriptors to each be a pointer, when they could be a fixed table size with elements that may be emplaced at will.