[libcxx-commits] [libcxx] [libc++] Simplify __bitset::__init (PR #121357)

[via libcxx-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-libcxx

Author: Peng Liu (winner245)

<details>
<summary>Changes</summary>

This PR simplifies `__bitset::__init` into a more compact and readable form, which avoids redundant computations of a `size_t` value and eliminates the overhead of a local array.

---
Full diff: https://github.com/llvm/llvm-project/pull/121357.diff


1 Files Affected:

- (modified) libcxx/include/bitset (+7-16) 


``````````diff
diff --git a/libcxx/include/bitset b/libcxx/include/bitset
index 919d2a0f07e096..57c29f5f85d27c 100644
--- a/libcxx/include/bitset
+++ b/libcxx/include/bitset
@@ -253,26 +253,16 @@ inline _LIBCPP_CONSTEXPR __bitset<_N_words, _Size>::__bitset() _NOEXCEPT
 
 template <size_t _N_words, size_t _Size>
 void __bitset<_N_words, _Size>::__init(unsigned long long __v, false_type) _NOEXCEPT {
-  __storage_type __t[sizeof(unsigned long long) / sizeof(__storage_type)];
-  size_t __sz = _Size;
-  for (size_t __i = 0; __i < sizeof(__t) / sizeof(__t[0]); ++__i, __v >>= __bits_per_word, __sz -= __bits_per_word)
-    if (__sz < __bits_per_word)
-      __t[__i] = static_cast<__storage_type>(__v) & (1ULL << __sz) - 1;
-    else
-      __t[__i] = static_cast<__storage_type>(__v);
-
-  std::copy(__t, __t + sizeof(__t) / sizeof(__t[0]), __first_);
-  std::fill(
-      __first_ + sizeof(__t) / sizeof(__t[0]), __first_ + sizeof(__first_) / sizeof(__first_[0]), __storage_type(0));
+  const size_t __ull_words = sizeof(unsigned long long) / sizeof(__storage_type);
+  for (size_t __i = 0; __i < __ull_words; ++__i, __v >>= __bits_per_word)
+    __first_[__i] = static_cast<__storage_type>(__v);
+  std::fill(__first_ + __ull_words, __first_ + _N_words, __storage_type(0));
 }
 
 template <size_t _N_words, size_t _Size>
 inline _LIBCPP_HIDE_FROM_ABI void __bitset<_N_words, _Size>::__init(unsigned long long __v, true_type) _NOEXCEPT {
   __first_[0] = __v;
-  if (_Size < __bits_per_word)
-    __first_[0] &= (1ULL << _Size) - 1;
-
-  std::fill(__first_ + 1, __first_ + sizeof(__first_) / sizeof(__first_[0]), __storage_type(0));
+  std::fill(__first_ + 1, __first_ + _N_words, __storage_type(0));
 }
 
 #  endif // _LIBCPP_CXX03_LANG
@@ -623,7 +613,8 @@ public:
 
   // 23.3.5.1 constructors:
   _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR bitset() _NOEXCEPT {}
-  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR bitset(unsigned long long __v) _NOEXCEPT : __base(__v) {}
+  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR bitset(unsigned long long __v) _NOEXCEPT
+      : __base(sizeof(unsigned long long) * CHAR_BIT <= _Size ? __v : __v & ((1ULL << _Size) - 1)) {}
   template <class _CharT, __enable_if_t<_IsCharLikeType<_CharT>::value, int> = 0>
   _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX23 explicit bitset(
       const _CharT* __str,

``````````

</details>


https://github.com/llvm/llvm-project/pull/121357