[libc++] Add move constructor & assignment to `exception_ptr` #164281

vogelsgesang · 2025-10-20T16:35:15Z

This commit adds move constructor and move assignment and swap
to exception_ptr. Adding those operators allows us to avoid
unnecessary calls to __cxa_{inc,dec}rement_refcount.

Performance results:

TODO: update

github-actions · 2025-10-20T16:37:00Z

✅ With the latest revision this PR passed the C/C++ code formatter.

philnik777

How about we instead add a swap and implement operator= as swap(exception_ptr())? That would avoid having to introduce any new symbols, at least in this patch.

philnik777

Basically LGTM, just some nits.

libcxx/src/support/runtime/exception_pointer_glibcxx.ipp

libcxx/include/__exception/exception_ptr.h

vogelsgesang · 2025-10-21T12:32:19Z

I updated this commit to use implement the move-assignment using swap now:

_LIBCPP_HIDE_FROM_ABI inline exception_ptr& exception_ptr::operator=(exception_ptr&& __other) _NOEXCEPT {
  exception_ptr __tmp(std::move(__other));
  swap(__tmp, *this);
  return *this;
}

Is that what you had in mind?

Benchmark results (original implementation)

Benchmark                                         Baseline    Candidate    Difference    % Difference
----------------------------------------------  ----------  -----------  ------------  --------------
bm_exception_ptr_copy_assign_nonnull                 12.36        12.62          0.26            2.13
bm_exception_ptr_copy_assign_null                     3.97         4.06          0.09            2.23
bm_exception_ptr_copy_ctor_nonnull                   11.67        11.97          0.30            2.59
bm_exception_ptr_copy_ctor_null                       4.77         4.86          0.09            1.80
bm_exception_ptr_move_assign_nonnull                 24.07        14.28         -9.78          -40.65
bm_exception_ptr_move_assign_null                     3.98         2.42         -1.56          -39.17
bm_exception_ptr_move_copy_swap_nonnull              51.74        40.15        -11.59          -22.40
bm_exception_ptr_move_copy_swap_null                 32.63        23.07         -9.56          -29.30
bm_exception_ptr_move_copy_swap_null_optimized       30.50        21.15         -9.35          -30.66
bm_exception_ptr_move_ctor_nonnull                   23.44        14.38         -9.06          -38.65
bm_exception_ptr_move_ctor_null                       4.67         2.63         -2.05          -43.78
bm_exception_ptr_swap_nonnull                        33.51         2.64        -30.88          -92.13
bm_exception_ptr_swap_null                            7.88         2.38         -5.50          -69.82

Benchmark results (swap-based implementation)

Benchmark                                         Baseline    Candidate    Difference    % Difference
----------------------------------------------  ----------  -----------  ------------  --------------
bm_exception_ptr_copy_assign_nonnull                 12.36        12.85          0.49            3.95
bm_exception_ptr_copy_assign_null                     3.97         4.04          0.07            1.70
bm_exception_ptr_copy_ctor_nonnull                   11.67        12.15          0.49            4.17
bm_exception_ptr_copy_ctor_null                       4.77         4.80          0.03            0.61
bm_exception_ptr_move_assign_nonnull                 24.07        15.04         -9.02          -37.49
bm_exception_ptr_move_assign_null                     3.98         4.71          0.73           18.40
bm_exception_ptr_move_copy_swap_nonnull              51.74        39.24        -12.49          -24.15
bm_exception_ptr_move_copy_swap_null                 32.63        21.80        -10.83          -33.19
bm_exception_ptr_move_copy_swap_null_optimized       30.50        19.88        -10.63          -34.84
bm_exception_ptr_move_ctor_nonnull                   23.44        14.46         -8.98          -38.30
bm_exception_ptr_move_ctor_null                       4.67         2.58         -2.09          -44.81
bm_exception_ptr_swap_nonnull                        33.51         1.16        -32.36          -96.55
bm_exception_ptr_swap_null                            7.88         1.18         -6.70          -85.04

Interpretation of benchmark results

I consider the bm_exception_ptr_copy_* regressions to be noise.
For bm_exception_ptr_move_ctor_*, both variants perform the same.
The main difference is in bm_exception_ptr_move_assign_*, as expected.

In particular bm_exception_ptr_move_assign_null even regresses compared to main. This is probably due to the additional __tmp inside operator=. The destructor of the original __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.

The 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. As such, I think we can live with that temporary regression - WDYT?

philnik777 · 2025-10-21T12:39:06Z

I updated this commit to use implement the move-assignment using swap now:
_LIBCPP_HIDE_FROM_ABI inline exception_ptr& exception_ptr::operator=(exception_ptr&& __other) _NOEXCEPT {
  exception_ptr __tmp(std::move(__other));
  swap(__tmp, *this);
  return *this;
}
Is that what you had in mind?

Yes, exactly.

Benchmark results (original implementation)

Benchmark                                         Baseline    Candidate    Difference    % Difference
----------------------------------------------  ----------  -----------  ------------  --------------
bm_exception_ptr_copy_assign_nonnull                 12.36        12.62          0.26            2.13
bm_exception_ptr_copy_assign_null                     3.97         4.06          0.09            2.23
bm_exception_ptr_copy_ctor_nonnull                   11.67        11.97          0.30            2.59
bm_exception_ptr_copy_ctor_null                       4.77         4.86          0.09            1.80
bm_exception_ptr_move_assign_nonnull                 24.07        14.28         -9.78          -40.65
bm_exception_ptr_move_assign_null                     3.98         2.42         -1.56          -39.17
bm_exception_ptr_move_copy_swap_nonnull              51.74        40.15        -11.59          -22.40
bm_exception_ptr_move_copy_swap_null                 32.63        23.07         -9.56          -29.30
bm_exception_ptr_move_copy_swap_null_optimized       30.50        21.15         -9.35          -30.66
bm_exception_ptr_move_ctor_nonnull                   23.44        14.38         -9.06          -38.65
bm_exception_ptr_move_ctor_null                       4.67         2.63         -2.05          -43.78
bm_exception_ptr_swap_nonnull                        33.51         2.64        -30.88          -92.13
bm_exception_ptr_swap_null                            7.88         2.38         -5.50          -69.82

Benchmark results (swap-based implementation)

Benchmark                                         Baseline    Candidate    Difference    % Difference
----------------------------------------------  ----------  -----------  ------------  --------------
bm_exception_ptr_copy_assign_nonnull                 12.36        12.85          0.49            3.95
bm_exception_ptr_copy_assign_null                     3.97         4.04          0.07            1.70
bm_exception_ptr_copy_ctor_nonnull                   11.67        12.15          0.49            4.17
bm_exception_ptr_copy_ctor_null                       4.77         4.80          0.03            0.61
bm_exception_ptr_move_assign_nonnull                 24.07        15.04         -9.02          -37.49
bm_exception_ptr_move_assign_null                     3.98         4.71          0.73           18.40
bm_exception_ptr_move_copy_swap_nonnull              51.74        39.24        -12.49          -24.15
bm_exception_ptr_move_copy_swap_null                 32.63        21.80        -10.83          -33.19
bm_exception_ptr_move_copy_swap_null_optimized       30.50        19.88        -10.63          -34.84
bm_exception_ptr_move_ctor_nonnull                   23.44        14.46         -8.98          -38.30
bm_exception_ptr_move_ctor_null                       4.67         2.58         -2.09          -44.81
bm_exception_ptr_swap_nonnull                        33.51         1.16        -32.36          -96.55
bm_exception_ptr_swap_null                            7.88         1.18         -6.70          -85.04

Interpretation of benchmark results

I consider the bm_exception_ptr_copy_* regressions to be noise. For bm_exception_ptr_move_ctor_*, both variants perform the same. The main difference is in bm_exception_ptr_move_assign_*, as expected.

In particular bm_exception_ptr_move_assign_null even regresses compared to main. This is probably due to the additional __tmp inside operator=. The destructor of the original __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.

The 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. As such, I think we can live with that temporary regression - WDYT?

Yeah, I think that's fine.

Also, please update the commit message to mention that we introduce swap as well.

vogelsgesang · 2025-10-21T13:42:08Z

Yeah, I think that's fine.

👍 then I think we have high-level alignment on this PR. The next step will be to polish it for final review.

Also, please update the commit message to mention that we introduce swap as well.

I will do so after #164278 shipped, so I won't have to repeatedly rebase this PR and redo the measurements anymore

One more question:
The build currently fails due to #define move SYSTEM_RESERVED_NAME.
Should I simply add _LIBCPP_PUSH_MACROS #include <__undef_macros>? Or is there some other recommended solution?

philnik777 · 2025-10-21T14:21:04Z

Yeah, I think that's fine.

👍 then I think we have high-level alignment on this PR. The next step will be to polish it for final review.

Also, please update the commit message to mention that we introduce swap as well.

I will do so after #164278 shipped, so I won't have to repeatedly rebase this PR and redo the measurements anymore

One more question: The build currently fails due to #define move SYSTEM_RESERVED_NAME. Should I simply add _LIBCPP_PUSH_MACROS #include <__undef_macros>? Or is there some other recommended solution?

Yeah, that should fix the CI.

This commit adds move constructor and move assignment to `exception_ptr`. Adding those operators allows us to avoid unnecessary calls to `__cxa_{inc,dec}rement_refcount`. Performance results: ``` Benchmark Baseline Candidate Difference % Difference ------------------------------- ---------- ----------- ------------ -------------- bm_nonnull_exception_ptr 52.22 40.92 -11.31 -21.65 bm_null_exception_ptr 31.41 23.29 -8.12 -25.85 bm_optimized_null_exception_ptr 28.69 20.50 -8.19 -28.55 ``` This commit does not add a `swap` specialization. Thanks to the added move-assignment, we already save a couple of increments/decrements also in the default `swap` implementation. The default `swap` is still not perfect, as it calls the desctructor on `tmp`. As soon as we also inlined the `~exception_ptr` destructor fast-path for `__ptr == nullptr`, the optimizer should be able to optimize the default `swap` just as well as a specialized `swap`, though.

vogelsgesang · 2025-10-27T20:18:33Z

/libcxx-bot benchmark libcxx/test/benchmarks/exception_ptr.bench.cpp

Benchmark results:

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%
bm_make_exception_ptr/threads:1            32.30        32.44          0.14           0.43%
bm_make_exception_ptr/threads:2            16.17        16.34          0.17           1.07%
bm_make_exception_ptr/threads:4             8.32         8.40          0.07           0.90%
bm_make_exception_ptr/threads:8             4.19         4.18         -0.00          -0.11%
Geomean                                    12.00         8.34         -3.65         -30.46%

philnik777

Could you add some basic tests for the new functions? I'm pretty sure everything should be testable portably to some extent.

libcxx/include/__exception/exception_ptr.h

Co-authored-by: Nikolas Klauser <[email protected]>

vogelsgesang force-pushed the avogelsgesang-exceptionptr-move branch from fef6eee to b595542 Compare October 20, 2025 17:11

This was referenced Oct 20, 2025

[libc++] Optimize std::exception_ptr #162773

Open

[libc++] Add benchmark for std::exception_ptr #164278

Merged

philnik777 reviewed Oct 21, 2025

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vogelsgesang force-pushed the avogelsgesang-exceptionptr-move branch from b595542 to 154c286 Compare October 21, 2025 12:18

philnik777 reviewed Oct 21, 2025

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libcxx/src/support/runtime/exception_pointer_glibcxx.ipp Outdated Show resolved Hide resolved

libcxx/include/__exception/exception_ptr.h Outdated Show resolved Hide resolved

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vogelsgesang added 4 commits October 27, 2025 20:00

Implement using swap

36757da

Define functions inline

740129e

Undef move to fix build

8d90812

vogelsgesang force-pushed the avogelsgesang-exceptionptr-move branch from 154c286 to 0fcaeee Compare October 27, 2025 20:10

Make swap a non-hidden friend

b106dc9

vogelsgesang force-pushed the avogelsgesang-exceptionptr-move branch from 0fcaeee to b106dc9 Compare October 27, 2025 20:16

philnik777 reviewed Oct 27, 2025

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libcxx/include/__exception/exception_ptr.h Outdated Show resolved Hide resolved

Update libcxx/include/__exception/exception_ptr.h

1a26712

Co-authored-by: Nikolas Klauser <[email protected]>

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