[libc++] Use _BitInt and __builtin_popcountg in bitset::count()

[philnik777]

This has multiple benefits:

1. The compiler has to do way less work to figure out things fold into a simple popcount, improving compile times quite a bit
2. The compiler inlines better, since the compile doesn't have to do complicated optimizations to get to the same point. Looking at the pipeline, it seems that without this, LLVM has to go all the way to GVN to get to the same code as there is after the first InstCombine pass with this change.

Currently this applies only to bitsets with at most 64 bits, but that is by far the most common case.

[github-actions]

✅ With the latest revision this PR passed the C/C++ code formatter.

[llvmbot]

@llvm/pr-subscribers-libcxx

Author: Nikolas Klauser (philnik777)

Changes

This has multiple benefits:

1. The compiler has to do way less work to figure out things fold into a simple popcount, improving compile times quite a bit
2. The compiler inlines better, since the compile doesn't have to do complicated optimizations to get to the same point. Looking at the pipeline, it seems that without this, LLVM has to go all the way to GVN to get to the same code as there is after the first InstCombine pass with this change.

Currently this applies only to bitsets with at most 64 bits, but that is by far the most common case.

---

Full diff: https://github.com/llvm/llvm-project/pull/160679.diff

1 Files Affected:

• (modified) libcxx/include/bitset (+8-1)

diff --git a/libcxx/include/bitset b/libcxx/include/bitset
index e2b46154ae730..d8bb938456104 100644
--- a/libcxx/include/bitset
+++ b/libcxx/include/bitset
@@ -867,7 +867,14 @@ bitset<_Size>::to_string(char __zero, char __one) const {
 
 template <size_t _Size>
 inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX23 size_t bitset<_Size>::count() const _NOEXCEPT {
-  return static_cast<size_t>(std::count(__base::__make_iter(0), __base::__make_iter(_Size), true));
+#ifdef _LIBCPP_COMPILER_CLANG_BASED
+  if constexpr (_Size <= __base::__bits_per_word) {
+    return __builtin_popcountg(static_cast<unsigned _BitInt(_Size)>(__base::__first_));
+  } else
+#endif
+  {
+    return static_cast<size_t>(std::count(__base::__make_iter(0), __base::__make_iter(_Size), true));
+  }
 }
 
 template <size_t _Size>

[ldionne]

Do we really need this else? Can't we have

#if defined(CLANG)
if constexpr (...) {
  // A
} else if constexpr (...) {
 // B
}
#endif

return std::count(...);

Seems a bit simpler and equivalent. Are you worried about the mere presence of return std::count(...) preventing inlining?

[philnik777]

We don't need the else, but it improves compile times quite a bit, since we avoid instantiating std::count altogether with it. IMO the tiny simplification isn't worth the compile time increase.