[libcxx-commits] [libcxx] [libcxx] Use generic builtins for popcount, clz and ctz (PR #86563)

[Mark de Wever via libcxx-commits]

================
@@ -47,11 +59,28 @@ inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR int __libcpp_clz(__uint128_t __x)
   // - Any bits set:
   //   - The number of leading zeros of the input is the number of leading
   //     zeros in the high 64-bits.
+#  if __has_builtin(__builtin_clzg)
+  return __builtin_clzg(__x);
+#  else
   return ((__x >> 64) == 0) ? (64 + __builtin_clzll(static_cast<unsigned long long>(__x)))
                             : __builtin_clzll(static_cast<unsigned long long>(__x >> 64));
+#  endif
 }
 #endif // _LIBCPP_HAS_NO_INT128
 
+#if __has_builtin(__builtin_clzg)
+
+template <class _Tp>
+_LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14 int __countl_zero(_Tp __t) _NOEXCEPT {
+  static_assert(__libcpp_is_unsigned_integer<_Tp>::value, "__countl_zero requires an unsigned integer type");
+  if (__t == 0)
+    return numeric_limits<_Tp>::digits;
+
+  return __builtin_clzg(__t) - (numeric_limits<unsigned>::digits - numeric_limits<_Tp>::digits);
----------------
mordante wrote:

This looks wrong
```suggestion
  return __builtin_clzg(__t) - (numeric_limits<unsigned>::digits - numeric_limits<_Tp>::digits);
                                                                              ~~~~~~~~
```

This is the kind of cleanups I would indeed like with this patch.


https://github.com/llvm/llvm-project/pull/86563