Inline selection of __zero and __one inside __to_string

Author: winner245

libcxx/docs/ReleaseNotes/21.rst

@@ -70,6 +70,9 @@ Improvements and New Features
 - The segmented iterator optimization for ``std::for_each`` has been backported to C++11. Previously it was only available
   in C++23 and later.
 
+- The ``bitset::to_string`` function has been optimized, resulting in a performance improvement of up to 8.3x for bitsets
+  with uniformly distributed zeros and ones, and up to 13.5x and 16.1x for sparse and dense bitsets, respectively.
+
 Deprecations and Removals
 -------------------------
 

libcxx/include/bitset

@@ -226,14 +226,25 @@ protected:
     return to_ullong(integral_constant < bool, _Size< sizeof(unsigned long long) * CHAR_BIT>());
   }
 
-  template <bool _Spare, class _CharT, class _Traits, class _Allocator>
-  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX23 basic_string<_CharT, _Traits, _Allocator>
-  __to_string(_CharT __zero, _CharT __one) const;
-
   _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX23 bool all() const _NOEXCEPT;
   _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX23 bool any() const _NOEXCEPT;
   _LIBCPP_HIDE_FROM_ABI size_t __hash_code() const _NOEXCEPT;
 
+  template <bool _Sparse, class _CharT, class _Traits, class _Allocator>
+  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX23 basic_string<_CharT, _Traits, _Allocator>
+  __to_string(_CharT __zero, _CharT __one) const {
+    basic_string<_CharT, _Traits, _Allocator> __r(_Size, _Sparse ? __zero : __one);
+    for (size_t __i = 0, __bits = 0; __i < _N_words; ++__i, __bits += __bits_per_word) {
+      __storage_type __word = std::__invert_if<!_Sparse>(__first_[__i]);
+      if (__i == _N_words - 1 && _Size - __bits < __bits_per_word)
+        __word &= (__storage_type(1) << (_Size - __bits)) - 1;
+      for (; __word; __word &= (__word - 1))
+        __r[_Size - 1 - (__bits + std::__countr_zero(__word))] = _Sparse ? __one : __zero;
+    }
+
+    return __r;
+  }
+
 private:
 #  ifdef _LIBCPP_CXX03_LANG
   void __init(unsigned long long __v, false_type) _NOEXCEPT;
@@ -394,22 +405,6 @@ __bitset<_N_words, _Size>::to_ullong(true_type, true_type) const {
   return __r;
 }
 
-template <size_t _N_words, size_t _Size>
-template <bool _Spare, class _CharT, class _Traits, class _Allocator>
-_LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX23 basic_string<_CharT, _Traits, _Allocator>
-__bitset<_N_words, _Size>::__to_string(_CharT __zero, _CharT __one) const {
-  basic_string<_CharT, _Traits, _Allocator> __r(_Size, __zero);
-  for (size_t __i = 0, __bits = 0; __i < _N_words; ++__i, __bits += __bits_per_word) {
-    __storage_type __word = std::__invert_if<!_Spare>(__first_[__i]);
-    if (__i == _N_words - 1 && _Size - __bits < __bits_per_word)
-      __word &= (__storage_type(1) << (_Size - __bits)) - 1;
-    for (; __word; __word &= (__word - 1))
-      __r[_Size - 1 - (__bits + std::__countr_zero(__word))] = __one;
-  }
-
-  return __r;
-}
-
 template <size_t _N_words, size_t _Size>
 _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX23 bool __bitset<_N_words, _Size>::all() const _NOEXCEPT {
   // do middle whole words
@@ -507,7 +502,17 @@ protected:
 
   template <bool _Sparse, class _CharT, class _Traits, class _Allocator>
   _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX23 basic_string<_CharT, _Traits, _Allocator>
-  __to_string(_CharT __zero, _CharT __one) const;
+  __to_string(_CharT __zero, _CharT __one) const {
+    basic_string<_CharT, _Traits, _Allocator> __r(_Size, _Sparse ? __zero : __one);
+    __storage_type __word = std::__invert_if<!_Sparse>(__first_);
+    if (_Size < __bits_per_word)
+      __word &= (__storage_type(1) << _Size) - 1;
+    for (; __word; __word &= (__word - 1)) {
+      size_t __pos           = std::__countr_zero(__word);
+      __r[_Size - 1 - __pos] = _Sparse ? __one : __zero;
+    }
+    return __r;
+  }
 
   _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX23 bool all() const _NOEXCEPT;
   _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX23 bool any() const _NOEXCEPT;
@@ -556,21 +561,6 @@ inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX23 unsigned long long __
   return __first_;
 }
 
-template <size_t _Size>
-template <bool _Spare, class _CharT, class _Traits, class _Allocator>
-_LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX23 basic_string<_CharT, _Traits, _Allocator>
-__bitset<1, _Size>::__to_string(_CharT __zero, _CharT __one) const {
-  basic_string<_CharT, _Traits, _Allocator> __r(_Size, __zero);
-  __storage_type __word = std::__invert_if<!_Spare>(__first_);
-  if (_Size < __bits_per_word)
-    __word &= (__storage_type(1) << _Size) - 1;
-  for (; __word; __word &= (__word - 1)) {
-    size_t __pos           = std::__countr_zero(__word);
-    __r[_Size - 1 - __pos] = __one;
-  }
-  return __r;
-}
-
 template <size_t _Size>
 inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX23 bool __bitset<1, _Size>::all() const _NOEXCEPT {
   __storage_type __m = ~__storage_type(0) >> (__bits_per_word - _Size);
@@ -635,7 +625,7 @@ protected:
   _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX23 unsigned long to_ulong() const { return 0; }
   _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX23 unsigned long long to_ullong() const { return 0; }
 
-  template <bool _Spare, class _CharT, class _Traits, class _Allocator>
+  template <bool _Sparse, class _CharT, class _Traits, class _Allocator>
   _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX23 basic_string<_CharT, _Traits, _Allocator>
   __to_string(_CharT, _CharT) const {
     return basic_string<_CharT, _Traits, _Allocator>();
@@ -898,7 +888,7 @@ bitset<_Size>::to_string(_CharT __zero, _CharT __one) const {
   if (__sparse)
     return __base::template __to_string<true, _CharT, _Traits, _Allocator>(__zero, __one);
   else
-    return __base::template __to_string<false, _CharT, _Traits, _Allocator>(__one, __zero);
+    return __base::template __to_string<false, _CharT, _Traits, _Allocator>(__zero, __one);
 }
 
 template <size_t _Size>