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Implement SSO for constexpr string #1735

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StephanTLavavej merged 5 commits into from
Jun 19, 2022
Merged

Implement SSO for constexpr string #1735

StephanTLavavej merged 5 commits into from
Jun 19, 2022

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@miscco miscco commented Mar 13, 2021

In #1502 we found that it should be possible to enable SSO, but we wanted to get constexpr string in.

It turns out that we were not that far off. I verified that all test compile fine with clang-12. That said, it seems I uncovered 2 distinct MSVC bugs:

  1. The test for basic_string::operator-> is failing. MSVC complains about an uninitialized symbol. Clang is happy. I have no idea what is wrong there.
  2. Clang complained heavily about switching back to SSO form. It turns out that previously we were directly constructing the sentinel in the first slot of the SSO array. However, we never switched from the _Ptr to the _Buf member. So I believe clang is correct here and MSVC should recognize that _Bufs lifetime did not yet start

@miscco miscco requested a review from a team as a code owner March 13, 2021 13:15
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miscco commented Mar 13, 2021
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@StephanTLavavej StephanTLavavej added the enhancement Something can be improved label Mar 14, 2021
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mnatsuhara reviewed Jul 21, 2021
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Thanks for going through and working on this! Sorry for the wait on a review here. Things look pretty good to me, just a few questions / nitpicks and request for more extensive test coverage :)

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@miscco miscco force-pushed the SSO_string branch 2 times, most recently from c164242 to de327ee Compare August 27, 2021 09:54
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miscco commented Aug 27, 2021
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mnatsuhara suggested changes Sep 9, 2021
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The only real "request changes" here is the missing _STD, otherwise looks great! Thanks for all of the bug fixes and extensive added test coverage 😻

@StephanTLavavej StephanTLavavej mentioned this pull request Sep 15, 2021
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@miscco miscco force-pushed the SSO_string branch 2 times, most recently from 4e114ce to 31c97c9 Compare June 16, 2022 07:45
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StephanTLavavej reviewed Jun 17, 2022
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StephanTLavavej commented Jun 17, 2022

Thanks, this is so much easier to understand now! 😻

I pushed changes to remove one more workaround (didn't affect correctness).

@CaseyCarter
Centralize code to activate SSO elements
806301f 
... and do so without ending the lifetime of the enclosing `basic_string` by reusing its storage.
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CaseyCarter approved these changes Jun 18, 2022
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I'm going to push a tiny change to address the pre-existing issue and approve.

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miscco commented Jun 18, 2022
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_Activate_SSO_buffer can/does not throw
3d2e9cd 
Co-authored-by: Michael Schellenberger Costa <[email protected]>
@StephanTLavavej StephanTLavavej self-assigned this Jun 19, 2022
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StephanTLavavej commented Jun 19, 2022

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

@StephanTLavavej StephanTLavavej merged commit 1ea95df into microsoft:main Jun 19, 2022
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StephanTLavavej commented Jun 19, 2022

Thanks for simplifying these codepaths and making basic_string easier to maintain! 🎉 ✅ 😺

@frederick-vs-ja frederick-vs-ja mentioned this pull request Jul 26, 2022
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@frederick-vs-ja frederick-vs-ja mentioned this pull request Mar 9, 2023
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@achabense achabense mentioned this pull request Sep 23, 2023
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@StephanTLavavej StephanTLavavej StephanTLavavej approved these changes

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@mnatsuhara mnatsuhara mnatsuhara requested changes

@CaseyCarter CaseyCarter CaseyCarter approved these changes

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@miscco @mnatsuhara @StephanTLavavej @AlexGuteniev @CaseyCarter