Use C++20 function std::popcount

[a-noni-mousse]

High Level Overview of Change

Remove the custom code and special compiler functions and use C++20 function std::popcount and make inline for optimization of the code. The changed code is good when compiled and produces same or even better assembly code which you can see on the compiler explorer

Context of Change

Type of Change

• Refactor (non-breaking change that only restructures code)

[nbougalis]

Looks OK, modulo the MacOS compilation issues, and I like that it uses std instead of intrinsics or hand-rolled code. I think someone with MacOS can check if newer versions of XCode support std::popcount and if so, whether it makes sense to upgrade the CI.

One option might be to leave the popcnt16 function in place for now (perhaps inlining it in the header file?) as a thin wrapper around std::popcount for non MacOS platforms, and a call to __builtin_popcount for MacOS.

[a-noni-mousse]

OK i can makethe change

[intelliot]

CI has reported failures; please review them.

[HowardHinnant]

This change is required to compile on macOS: HowardHinnant@ab5e386

Explanation: The expression isBranch_ | (1 << m) promotes to int which is then sent to std::popcount. But popcount has a constraint that its argument be an unsigned integral type. Adding the u suffix makes the expression unsigned.

[intelliot]

@a-noni-mousse please review the comments above.

[a-noni-mousse]

I have fixed the comments from Hooward and Nic.

[intelliot]

@a-noni-mousse for future reference - there's no need to force push, as this PR will be squashed when it is merged.

Keeping historical commits may help reviewers to more easily see how the PR has changed over time.

[nbougalis]

Looks fine.

[HowardHinnant]

I'm getting several errors on a non-unity build that look like this:

In file included from /Users/howardhinnant/Development/rippled/src/ripple/app/consensus/RCLConsensus.cpp:20:
In file included from /Users/howardhinnant/Development/rippled/src/ripple/app/consensus/RCLConsensus.h:23:
In file included from /Users/howardhinnant/Development/rippled/src/ripple/app/consensus/RCLCensorshipDetector.h:24:
In file included from /Users/howardhinnant/Development/rippled/src/ripple/shamap/SHAMap.h:30:
/Users/howardhinnant/Development/rippled/src/ripple/shamap/SHAMapInnerNode.h:213:12: error: use of undeclared identifier 'popcnt16'
    return popcnt16(isBranch_);
           ^

Not positive, but I suspect we're looking at a cyclic include dependency which is preventing the declaration of popcnt16 appearing before its use.

[a-noni-mousse]

Ah I edit the code on the github and that's what I get! Thank you i will fix this @HowardHinnant

[intelliot]

Suggested commit message for squashed commit:

refactor: use C++20 function std::popcount (#4389)

- Replace custom popcnt16 implementation with std::popcount from C++20
- Maintain compatibility with older compilers and MacOS by providing a
  conditional compilation fallback to __builtin_popcount and a lookup
  table method
- Move and inline related functions within SHAMapInnerNode for
  performance and readability

@manojsdoshi - be sure to squash this PR when merging, as requested above.