llvm-project

f02b6610 - [libc++] Add move constructor & assignment to `exception_ptr` (#164281)

[libc++] Add move constructor & assignment to `exception_ptr` (#164281) This commit adds move constructor, move assignment and `swap` to `exception_ptr`. Adding those operators allows us to avoid unnecessary calls to `__cxa_{inc,dec}rement_refcount`. Performance results (from libc++'s CI): ``` Benchmark Baseline Candidate Difference % Difference ------------------------------------ ---------- ----------- ------------ -------------- bm_exception_ptr_copy_assign_nonnull 9.77 9.94 0.18 1.79% bm_exception_ptr_copy_assign_null 10.29 10.65 0.35 3.42% bm_exception_ptr_copy_ctor_nonnull 7.02 7.01 -0.01 -0.13% bm_exception_ptr_copy_ctor_null 10.54 10.60 0.06 0.56% bm_exception_ptr_move_assign_nonnull 16.92 13.76 -3.16 -18.70% bm_exception_ptr_move_assign_null 10.61 10.76 0.14 1.36% bm_exception_ptr_move_ctor_nonnull 13.31 10.25 -3.06 -23.02% bm_exception_ptr_move_ctor_null 10.28 7.30 -2.98 -28.95% bm_exception_ptr_swap_nonnull 19.22 0.63 -18.59 -96.74% bm_exception_ptr_swap_null 20.02 7.79 -12.23 -61.07% ``` As expected, the `bm_exception_ptr_copy_*` benchmarks are not influenced by this change. `bm_exception_ptr_move_*` benefits between 18% and 30%. The `bm_exception_ptr_swap_*` tests show the biggest improvements since multiple calls to the copy constructor are replaced by a simple pointer swap. While `bm_exception_ptr_move_assign_null` did not show a regression in the CI measurements, local measurements showed a regression from 3.98 to 4.71, i.e. by 18%. This is due to the additional `__tmp` inside `operator=`. The destructor of `__other` is a no-op after the move because `__other.__ptr` will be a nullptr. However, the compiler does not realize this, since the destructor is not inlined and is lacking a fast-path. As such, the swap-based implementation leads to an additional destructor call. `bm_exception_ptr_move_assign_nonnull` still benefits because the swap-based move constructor avoids unnecessary __cxa_{in,de}crement_refcount calls. As soon as we inline the destructor, this regression should disappear again. Works towards #44892