f02b661054
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
30 lines
959 B
C++
30 lines
959 B
C++
// -*- C++ -*-
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//===----------------------------------------------------------------------===//
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//
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// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
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// See https://llvm.org/LICENSE.txt for license information.
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// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
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//
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//===----------------------------------------------------------------------===//
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export namespace std {
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using std::bad_exception;
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using std::current_exception;
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using std::exception;
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using std::exception_ptr;
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using std::get_terminate;
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using std::make_exception_ptr;
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using std::nested_exception;
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using std::rethrow_exception;
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using std::rethrow_if_nested;
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using std::set_terminate;
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using std::swap;
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using std::terminate;
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using std::terminate_handler;
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using std::throw_with_nested;
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#ifdef _LIBCPP_ENABLE_CXX20_REMOVED_UNCAUGHT_EXCEPTION
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using std::uncaught_exception;
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#endif
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using std::uncaught_exceptions;
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} // namespace std
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