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[libc++] Remove unnecessary division and modulo operations in bitset (#121312)

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The PR removes the unnecessary division and modulo operations in the
one-word specialization `__bitset<1, _Size>`. The reason is that for the
one-word specialization, we have `__pos < __bits_per_word` (as
`__bitset<1, _Size>` is an implementation detail only used by the public
`bitset`). So `__pos / __bits_per_word == 0` and `__pos / __pos %
__bits_per_word == __pos`.
This commit is contained in:
Peng Liu 2025-03-26 12:02:03 -04:00 committed by GitHub
parent ac8e18cdce
commit 649cbcc376
No known key found for this signature in database
GPG Key ID: B5690EEEBB952194
2 changed files with 11 additions and 3 deletions
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6
libcxx/include/__bit_reference
View File

@ -329,6 +329,7 @@ public:
}
_LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 reference operator*() const _NOEXCEPT {
_LIBCPP_ASSERT_INTERNAL(__ctz_ < __bits_per_word, "Dereferencing an invalid __bit_iterator.");
return __conditional_t<_IsConst, __bit_const_reference<_Cp>, __bit_reference<_Cp> >(
__seg_, __storage_type(1) << __ctz_);
}
@ -453,7 +454,10 @@ private:
_LIBCPP_HIDE_FROM_ABI
_LIBCPP_CONSTEXPR_SINCE_CXX20 explicit __bit_iterator(__storage_pointer __s, unsigned __ctz) _NOEXCEPT
: __seg_(__s),
__ctz_(__ctz) {}
__ctz_(__ctz) {
_LIBCPP_ASSERT_INTERNAL(
__ctz_ < __bits_per_word, "__bit_iterator initialized with an invalid number of trailing zeros.");
}
friend typename _Cp::__self;

8
libcxx/include/bitset
View File

@ -464,10 +464,14 @@ protected:
return __const_reference(&__first_, __storage_type(1) << __pos);
}
_LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX23 __iterator __make_iter(size_t __pos) _NOEXCEPT {
return __iterator(&__first_ + __pos / __bits_per_word, __pos % __bits_per_word);
// Allow the == case to accommodate the past-the-end iterator.
_LIBCPP_ASSERT_INTERNAL(__pos <= __bits_per_word, "Out of bounds access in the single-word bitset implementation.");
return __pos != __bits_per_word ? __iterator(&__first_, __pos) : __iterator(&__first_ + 1, 0);
}
_LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX23 __const_iterator __make_iter(size_t __pos) const _NOEXCEPT {
return __const_iterator(&__first_ + __pos / __bits_per_word, __pos % __bits_per_word);
// Allow the == case to accommodate the past-the-end iterator.
_LIBCPP_ASSERT_INTERNAL(__pos <= __bits_per_word, "Out of bounds access in the single-word bitset implementation.");
return __pos != __bits_per_word ? __const_iterator(&__first_, __pos) : __const_iterator(&__first_ + 1, 0);
}
_LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX23 void operator&=(const __bitset& __v) _NOEXCEPT;