`<bit>`: Should emit rbit, clz on ARM64 for `countr_zero`
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ARM64
performance
- Dominant language
- C++
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Description
Right now countr_zero falls back to software on arm, this could be improved to emit rbit followed by clz. (alternatively countr_zero_fallback could call countl_zero instead of _Countl_zero_fallback see https://github.com/microsoft/STL/pull/795#discussion_r445676597 for more details.
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First steps
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Research direction
Start in the implementation for countr_zero and trace its ARM fallback path. Compare the existing fallback with the linked discussion about calling countl_zero, and verify the intended ARM64 instruction sequence is rbit followed by clz. Done means countr_zero no longer uses the software fallback on ARM64 and the relevant tests pass.
Written by the indexing model from the issue text.
Assessment
- Tech stack
- cpp
- Domain
- performance
- Issue type
- Feature
- Difficulty
- 4/5
- Estimated time
- 3-5 days
- Activity status
- Stale
- Clarity
- Mostly clear
- Newbie friendliness
- 42/100