Make bit counting operations constexpr on Windows using C++20 features
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- Dominant language
- Python
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Description
For Windows build support in https://github.com/Point72/csp/pull/229, we remove the constexpr specifier from the clz/ffs bit counting operations as MSVC's _BitScanForward and _BitScanReverse compiler intrinsics are not constexpr, unlike gcc's __builtin_clz and __builtin_ffs. It also leads to a macro being defined to control whether the functions are marked constexpr or not in DynamicBitset.
However, using a combination of std::bit_width in the new <bits> header and std::is_constant_evaluation we can make these functions constexpr for Windows too by only using the (faster) compiler intrinsics at runtime, not compile-time. It should make things a bit simpler and also ensures the same functions are constexpr across all platforms.
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First steps
- Read the whole issue, then the project's contributing guide.
- Comment on the issue to say you are picking it up — it saves two people doing the same work.
- Fork the repository and make your change on a branch.
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Research direction
Start by locating the clz/ffs bit-counting operations and DynamicBitset, then inspect how the Windows MSVC intrinsics and the existing constexpr-control macro are selected. Check the C++20 support for std::bit_width and std::is_constant_evaluation. Done means the same functions remain constexpr on Windows while compiler intrinsics are used at runtime.
Written by the indexing model from the issue text.
Assessment
- Tech stack
- cpp
- Domain
- operating-systems
- Issue type
- Feature
- Difficulty
- 3/5
- Estimated time
- 1-2 days
- Activity status
- Stale
- Clarity
- Mostly clear
- Newbie friendliness
- 45/100