[libcxx-commits] [libcxx] [libc++] Optimize bitset shift operations (PR #106225)

[Louis Dionne via libcxx-commits]

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@@ -290,6 +290,15 @@ _LIBCPP_CONSTEXPR_SINCE_CXX20 _LIBCPP_HIDE_FROM_ABI __bit_iterator<_Cp, false> _
 template <class _Cp, bool _IsConst>
 inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 __bit_iterator<_Cp, false>
 copy(__bit_iterator<_Cp, _IsConst> __first, __bit_iterator<_Cp, _IsConst> __last, __bit_iterator<_Cp, false> __result) {
+  if (__builtin_constant_p(
+          __result.__ctz_ == 0 && __first.__seg_ == __last.__seg_ && __last.__seg_ == __result.__seg_) &&
+      __result.__ctz_ == 0 && __first.__seg_ == __last.__seg_ && __last.__seg_ == __result.__seg_) {
+    if (__first == __last)
+      return __result;
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ldionne wrote:

We could move this outside the `__builtin_constant_p` block and then assume that `__n != 0` from `__copy_aligned` & `__copy_unaligned`. I'd add `_LIBCPP_ASSERT_INTERNAL` inside those functions just to bake in that assumption safely.

https://github.com/llvm/llvm-project/pull/106225