`<algorithm>`: `ranges::minmax` should dereference iterators only once by fsb4000 · Pull Request #3366 · microsoft/STL

Igor Zhukov (fsb4000) · 2023-01-27T16:52:26Z

After #2958 the original repro works, but the issue still exists.

I also found that if we have only one element we also have the issue.

Co-authored-by: Nicole Mazzuca <mazzucan@outlook.com>

Co-authored-by: Alex Guteniev <gutenev@gmail.com> Co-authored-by: timsong-cpp <rs2740@gmail.com>

Stephan T. Lavavej (StephanTLavavej) · 2023-02-01T05:33:45Z

This new pattern is unusual, but I rate it only 2 squirrels out of 5. (The Standard has an example, WG21-N4928 [dcl.init.aggr]/6, where a default member initializer refers to previous members, that's at least 3 squirrels.) I probably would have commented it, but I don't think it's worth resetting testing again. It does seem to be an improvement compared to the previous version, thanks!

Igor Zhukov (fsb4000) · 2023-02-01T05:52:58Z

It does seem to be an improvement compared to the previous version, thanks!

Good, I found it there:
https://github.com/llvm/llvm-project/blob/c4e38fadfa2a615467a8c9071f12edc4cdce5eb8/libcxx/include/__algorithm/ranges_minmax.h#L89

libc++ also has the bug, I created: https://reviews.llvm.org/D142864

Co-authored-by: Nicole Mazzuca <mazzucan@outlook.com>

Approved by Mirror Universe Nicole

Stephan T. Lavavej (StephanTLavavej) · 2023-02-02T01:30:07Z

I'm mirroring this to the MSVC-internal repo - please notify me if any further changes are pushed.

Co-authored-by: Casey Carter <cartec69@gmail.com>

Igor Zhukov (fsb4000) · 2023-02-02T14:54:25Z

Stephan T. Lavavej (@StephanTLavavej)
I pushed some changes after Casey's code review

Casey Carter (CaseyCarter) · 2023-02-02T17:32:40Z

Stephan T. Lavavej (@StephanTLavavej) I pushed some changes after Casey's code review

As did I.

Stephan T. Lavavej (StephanTLavavej) · 2023-02-02T21:26:57Z

Everyone's joining the commit party! (I pushed tiny comment changes and removed parameter names for the defaulted function.)

Stephan T. Lavavej (StephanTLavavej) · 2023-02-03T00:40:13Z

Thanks for minimizing bugs while maximizing performance! 📉 😸 📈

The fix for microsoft#2900 in microsoft#3366 did not take advantage of the fact that the minimum and maximum elements of a range are always distinct except when the range has only one element. It couldn't easily do so due to the way `ranges::minmax` and `ranges::minmax_element` share the common backend `ranges::_Minmax_element_unchecked`. This PR introduces a new backend for `ranges::minmax` (`ranges::_Minmax_fn::_Minmax_fwd_unchecked`) and makes `ranges::_Minmax_element_unchecked` the private member `ranges::_Minmax_element_fn::_Minmax_element_fwd_unchecked`. Since the two are distinct, `_Minmax_fwd_unchecked` can deal in values instead of iterators, and we can unroll the first loop iteration to detect the single-element case naturally. Since no additional branch is needed, we can enable the fix for microsoft#2900 unconditionally. This refactoring is also probably a minor throughput and perf improvement, since `minmax` no longer needs to instantiate the "old" backend's return type and the new backend is tail-called. Drive-by: Fully qualify the names of ugly functions called by `ranges::minmax` and `ranges::minmax_element`.

The fix for microsoft#2900 in microsoft#3366 did not take advantage of the fact that the minimum and maximum elements of a range are always distinct except when the range has only one element. It couldn't easily do so due to the way `ranges::minmax` and `ranges::minmax_element` share the common backend `ranges::_Minmax_element_unchecked`. This PR introduces a new backend for `ranges::minmax` (`ranges::_Minmax_fn::_Minmax_fwd_unchecked`) and makes `ranges::_Minmax_element_unchecked` the private member `ranges::_Minmax_element_fn::_Minmax_element_fwd_unchecked`. Since the two are distinct, `_Minmax_fwd_unchecked` can deal in values instead of iterators, and we can unroll the first loop iteration to detect the single-element case naturally. Since no additional branch is needed, we can enable the fix for microsoft#2900 unconditionally. Drive-by: Fully qualify the names of ugly functions called by `ranges::minmax` and `ranges::minmax_element`.

fix ranges::minmax

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Igor Zhukov (fsb4000) requested a review from a team as a code owner January 27, 2023 16:52

Igor Zhukov (fsb4000) changed the title ~~fix ranges::minmax~~ <algorithm>: ranges::minmax should dereference iterators only once Jan 27, 2023

nicole mazzuca (strega-nil-ms) suggested changes Jan 27, 2023

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Igor Zhukov (fsb4000) and others added 2 commits January 28, 2023 00:23

move last usage of _Temp

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Co-authored-by: Nicole Mazzuca <mazzucan@outlook.com>

Unexpectedly Passed Tests

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Stephan T. Lavavej (StephanTLavavej) added bug Something isn't working ranges C++20/23 ranges labels Jan 28, 2023

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code review suggestions

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Co-authored-by: Alex Guteniev <gutenev@gmail.com> Co-authored-by: timsong-cpp <rs2740@gmail.com>

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Igor Zhukov (fsb4000) and others added 3 commits January 28, 2023 19:44

add a comment about if constexpr

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Merge branch 'main' into fix2900

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Code review feedback.

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Stephan T. Lavavej (StephanTLavavej) reviewed Jan 30, 2023

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add a test coverage for input iterators

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Stephan T. Lavavej (StephanTLavavej) reviewed Feb 1, 2023

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Code review feedback, part 2.

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unneeded moves

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Stephan T. Lavavej (StephanTLavavej) self-assigned this Feb 1, 2023

change if constexpr condition

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Co-authored-by: Nicole Mazzuca <mazzucan@outlook.com>

Stephan T. Lavavej (StephanTLavavej) assigned nicole mazzuca (strega-nil-ms) and unassigned Stephan T. Lavavej (StephanTLavavej) Feb 1, 2023

Stephan T. Lavavej (StephanTLavavej) requested a review from nicole mazzuca (strega-nil-ms) February 1, 2023 23:37

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Comment thread tests/std/tests/P0896R4_ranges_alg_minmax/test.cpp Outdated

code review suggestions

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Co-authored-by: Casey Carter <cartec69@gmail.com>

Igor Zhukov (fsb4000) requested review from Casey Carter (CaseyCarter) and Stephan T. Lavavej (StephanTLavavej) and removed request for Stephan T. Lavavej (StephanTLavavej) and nicole mazzuca (strega-nil-ms) February 2, 2023 14:53

wrong condition

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iterators that yield elements by value are fine

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Nitpicks!

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Stephan T. Lavavej (StephanTLavavej) merged commit 0150312 into microsoft:main Feb 3, 2023

Casey Carter (CaseyCarter) mentioned this pull request Feb 3, 2023

Refactor ranges::minmax and ranges::minmax_element #3384

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Igor Zhukov (fsb4000) commented Jan 27, 2023 •

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