[libc++] Replaces std::sort by Bitset sorting algorithm.

[nilayvaish]

Summary

Bitset Sort

Bitset Sort is a variant of quick sort, specifically BlockQuickSort. Bitset Sort uses a carefully written partition function to let the compiler generates SIMD instructions without actually writing SIMD intrinsics in the loop.

Bitset Sort is interface compatible with std::sort and meant to replace std::sort in libc++.

Bitset Sort is 3.4x faster (or spends 71% less time) than libc++ std::sort when sorting uint64s and 1.58x faster (or spends 37% less time) when sorting std::string.

Bitset Sort uses multiple techniques to improve runtime performance of sort. This includes sorting networks, a variant of merge sort called Bitonic Order Merge Sort that is faster for small N, and pattern recognitions.

Please see Github page for more detailed documentations and full results.

[MaskRay]

Did you upload the right diff? The code does not show sort.

[minjaehwang]

Made the incorrect patch previously.

[minjaehwang]

In D93233#2452523, @MaskRay wrote:

Did you upload the right diff? The code does not show sort.

I am very sorry. Just updated the review with the sort.

[lebedev.ri]

What are the perf changes on CPU's without support for those instructions?

[MaskRay]

IMHO bitset sort is not a standard term, so it needs some clarification. From a glance this uses a bitset partition and needs temporary storage (heap memory allocation?). Does the original algorithm have heap memory allocation?

[mclow.lists]

In D93233#2452718, @MaskRay wrote:

IMHO bitset sort is not a standard term, so it needs some clarification. From a glance this uses a bitset partition and needs temporary storage (heap memory allocation?). Does the original algorithm have heap memory allocation?

There's a list of standard algorithms that are allowed to use temporary heap storage.
That list does not include std::sort (it consists of stable_sort, stable_partition and inplace_merge)

[ldionne]

This is an interesting contribution. I'm trying to wrap my head around it.

In D93233#2452718, @MaskRay wrote:

From a glance this uses a bitset partition and needs temporary storage (heap memory allocation?). Does the original algorithm have heap memory allocation?

Unless I missed a subtlety, it doesn't require heap allocation. The "bitsets" used are actually 64 (or 32) bit integers. The current algorithm also doesn't have heap allocation, so keeping that property is something we want (and something this patch does AFAICT).

@minjaehwang Is there documentation for this algorithm? I believe it would help us (at least myself) understand the algorithm and the potential tradeoffs involved. Also, are you aware of Timsort? IIRC there had been an analysis conducted that concluded that we should be using Timsort (as it was faster than our sort and respected the complexity requirements of the Standard).

libcxx/test/libcxx/algorithms/sort.pass.cpp

21 ↗

(On Diff #311640)

Shouldn't this test work with all implementations? Why not?

[minjaehwang]

In D93233#2452760, @ldionne wrote:

This is an interesting contribution. I'm trying to wrap my head around it.

Thanks!

In D93233#2452718, @MaskRay wrote:

From a glance this uses a bitset partition and needs temporary storage (heap memory allocation?). Does the original algorithm have heap memory allocation?

Unless I missed a subtlety, it doesn't require heap allocation. The "bitsets" used are actually 64 (or 32) bit integers. The current algorithm also doesn't have heap allocation, so keeping that property is something we want (and something this patch does AFAICT).

You are right. It only requires two 64 (or 32) bit integers to represent bitsets on left partition and right partition.

@minjaehwang Is there documentation for this algorithm? I believe it would help us (at least myself) understand the algorithm and the potential tradeoffs involved. Also, are you aware of Timsort? IIRC there had been an analysis conducted that concluded that we should be using Timsort (as it was faster than our sort and respected the complexity requirements of the Standard).

I have the documentation which was intended to present in my employer. I will work with the company to publish the document.

I am aware of Timsort. Timsort is probably good for replacing std::stable_sort. It is a variant of merge sort and could be generally slower compared to unstable sorts.

[minjaehwang]

In D93233#2452579, @lebedev.ri wrote:

What are the perf changes on CPU's without support for those instructions?

This is definitely something that I would like to do. Could you suggest which platforms or architectures should I try?

This implementation benefits from bsr or lzcnt instructions. I am guessing bitset will speed up even without specialized instructions because lzcnt and bsr are not terribly slow to implement with general instructions.

[minjaehwang]

In D93233#2452718, @MaskRay wrote:

IMHO bitset sort is not a standard term, so it needs some clarification. From a glance this uses a bitset partition and needs temporary storage (heap memory allocation?). Does the original algorithm have heap memory allocation?

You are right. I coined bitset sort terminology and have not published anything publicly yet. I will work on that.

Bitset partition works with stack variables without heap memory allocation.

[minjaehwang]

libcxx/test/libcxx/algorithms/sort.pass.cpp

21 ↗

(On Diff #311640)

You are right. This is a general sort test. Will fix it.

[Morwenn]

Hi, @ldionne asked me whether it was possible to review the algorithm, and so did I over the last three days. I eventually decided to create an account here and share again what I already sent him by mail, with additional information.

First, it looks like the main new trick is an implementation of the idea described in *BlockQuicksort: How Branch Mispredictions don't affect Quicksort* by Edelkamp and Weiß: basically, the partitioning algorithm performs comparisons over blocks of 32~64 elements, stores their results as booleans (here in a bitset), then performs the swaps to put the misplaced elements into place. It nicely avoids issues linked to branch misprediction, and thus is kind of a silver bullet for quicksort as long as the comparisons and projections passed to the algorithm are themselves branchless. As we can see for the strings benchmark, this partitioning scheme can be slower than a more traditional one when there are branches in the comparisons, which is likely due to the overhead from the additional logic involved.

On the good side, I tweaked bitset_sort so that I could inject it in my cpp-sort library, and ran the full test suite: the algorithm doesn't seem to have obvious bugs, and ubsan and asan didn't fid any issue either. Also the benchmarks show good results (here for a std::vector<double>: https://i.imgur.com/Nu6l8lI.png

On the not-so-good side, bitset_sort seems to be O(n²), just like the current implementation of std::sort in C++. I reran the quicksort killer implementation of Orson Peters from this issue, and got the same results. The quicksort killer is based on *A Killer Adversary for Quicksort* by M. D. McIlroy. Here is a graph showing the O(n log n) vs. O(n²) behaviour of the different algorithms: https://i.imgur.com/IWa2teO.png

So far my overall conclusion is that the idea of reimplementing the BlockQuicksort logic over bitsets is nice, but pdqsort otherwise seems better in every regard: it is truly O(n log n) - and even O(n log k) for k unique elements when k is small -, is efficient at breaking common quicksort-adverse patterns such as the pipe organ pattern in my first benchmark, and it actually switches over to a simpler non-branchless partitioning algorithm when it can't prove at compile time that the comparison will be branchless, avoiding the regression on types such as std::string.

If std::sort has to be changed, then I'd definitely reconsider using pdqsort instead. It should be possible to reimplement it over bitsets à la bitset_sort if you care about the reduced impact on stack memory.

[minjaehwang]

Thank @Morwenn very much for looking into this code.

@Morwenn, you are right that the idea is largely based on Block Quicksort except that Bitset sort uses a 32/64 bit integer instead of an array. A bitset will be kept within CPU registers and won't require store instructions like Block Quicksort does.

As you know, Pdqsort is a variation of Block Quicksort with a pattern recognition and a different pivot selection. It recognizes ascending ranges, descending ranges, and O(n ^ 2) cases. Pdqsort shows O(n) performance on these known patterns and guarantees O(n lg n) in the worse case.

The bitset trick can be applied to Pdqsort's partition function just like this code does for the existing std::sort's partition function. I have implemented Bitset trick on top of pdqsort as well but chose the current implementation over pdqsort-based Bitset sort. The current implementation introduces the minimal change to std::sort. It replaces the partition function but keeps most of the outer layer of std::sort.

The reason for keeping the outer layer of std::sort is that it will keep most of performance characteristics unchanged and adds a little overhead when there is a branch in the comparison function.

@Morwenn mentioned the O(n ^ 2) case for quicksort which also happens for the existing std::sort. As you might know, a simple change can avoid O(n ^ 2). Introsort avoids O(n ^ 2) by calling heap sort when a quicksort depth goes above O(lg n). There has been efforts to introduce introsort into libc++ (Kumar's introsort). For unknown reason to me, it has not been submitted. Pdqsort avoids O(n ^ 2) by calling heap sort when partitions are highly unbalanced.

I understand that Pdqsort is faster in many known patterns over std::sort. If we are not afraid of changing the entire sort implementation, I can certainly bring Bitset-on-pdqsort as another code review. @Morwenn and @ldionne, could you give your thoughts on the future direction?

@Morwenn Bitset sort is faster for std::strings than pdqsort which turns off Block sort technique for non-arithmetic types. See the following comparison. Block sort technique can be faster even with comparison functions with branches.

BM_PdqSort_string_Random_1                                                   4.65 ns         4.64 ns    150470656
BM_PdqSort_string_Random_4                                                   19.0 ns         19.0 ns     36175872
BM_PdqSort_string_Random_16                                                  46.4 ns         46.4 ns     14942208
BM_PdqSort_string_Random_64                                                  64.7 ns         64.7 ns     10485760
BM_PdqSort_string_Random_256                                                 84.1 ns         84.1 ns      8126464
BM_PdqSort_string_Random_1024                                                 103 ns          103 ns      6553600
BM_PdqSort_string_Random_16384                                                160 ns          160 ns      4456448
BM_PdqSort_string_Random_262144                                               242 ns          242 ns      2359296
BM_BitsetSort_string_Random_1                                                3.85 ns         3.85 ns    181665792
BM_BitsetSort_string_Random_4                                                18.1 ns         18.1 ns     38535168
BM_BitsetSort_string_Random_16                                               45.5 ns         45.5 ns     14942208
BM_BitsetSort_string_Random_64                                               62.6 ns         62.6 ns     10747904
BM_BitsetSort_string_Random_256                                              74.4 ns         74.4 ns      9175040
BM_BitsetSort_string_Random_1024                                             85.9 ns         85.9 ns      7864320
BM_BitsetSort_string_Random_16384                                             121 ns          121 ns      5767168
BM_BitsetSort_string_Random_262144                                            179 ns          179 ns      3407872
BM_Sort_string_Random_1                                                      4.05 ns         4.05 ns    173277184
BM_Sort_string_Random_4                                                      17.9 ns         17.9 ns     38010880
BM_Sort_string_Random_16                                                     50.4 ns         50.3 ns     13369344
BM_Sort_string_Random_64                                                     73.3 ns         73.3 ns      9437184
BM_Sort_string_Random_256                                                    94.6 ns         94.4 ns      7077888
BM_Sort_string_Random_1024                                                    115 ns          115 ns      6029312
BM_Sort_string_Random_16384                                                   177 ns          177 ns      3932160
BM_Sort_string_Random_262144                                                  279 ns          279 ns      2097152

[Morwenn]

In D93233#2466828, @minjaehwang wrote:

The bitset trick can be applied to Pdqsort's partition function just like this code does for the existing std::sort's partition function. I have implemented Bitset trick on top of pdqsort as well but chose the current implementation over pdqsort-based Bitset sort. The current implementation introduces the minimal change to std::sort. It replaces the partition function but keeps most of the outer layer of std::sort.

The reason for keeping the outer layer of std::sort is that it will keep most of performance characteristics unchanged and adds a little overhead when there is a branch in the comparison function.

That's surely a fair concern.

@Morwenn mentioned the O(n ^ 2) case for quicksort which also happens for the existing std::sort. As you might know, a simple change can avoid O(n ^ 2). Introsort avoids O(n ^ 2) by calling heap sort when a quicksort depth goes above O(lg n). There has been efforts to introduce introsort into libc++ (Kumar's introsort). For unknown reason to me, it has not been submitted. Pdqsort avoids O(n ^ 2) by calling heap sort when partitions are highly unbalanced.

Yeah, I'm still rather surprised that the libc++ implementation of std::sort isn't some flavour of introsort yet. It's not excessively hard to implement, and it would make the algorithm standard-compliant. I know that a few years ago they wanted proper sorting benchmarks before giving the "go" to a new sort implementation, but it seems like something that could have been fixed regardless.

I understand that Pdqsort is faster in many known patterns over std::sort. If we are not afraid of changing the entire sort implementation, I can certainly bring Bitset-on-pdqsort as another code review. @Morwenn and @ldionne, could you give your thoughts on the future direction?

@Morwenn Bitset sort is faster for std::strings than pdqsort which turns off Block sort technique for non-arithmetic types. See the following comparison. Block sort technique can be faster even with comparison functions with branches.

Oh, I thought that deactivating the branchless partitioning algorithm was done because it introduced regressions for non-arithmetic types. I guess I blindly believed that it had been compared against std::string of all things to make this decision, but never ran benchmarks to back that claim. That's new info to me.

As for the future direction I do believe that pdqsort has some interesting ideas that would be nice to have, but I'm not part of the libc++ project in the first place, so it's not really my call to make. I'm glad to see that things are moving in this area nonetheless, and bitset_sort would be a step in the right direction compared to the status quo anyway :)

[minjaehwang]

I worked on performance improvements and wrote a documentation what bitset sort is and why it is faster. Thanks to LLVM's ability to generate SIMD instructions, bitset sort lets LLVM to generate SIMD instructions for the critical path (bitset partition).

The documentation and the newer implementation is here - https://github.com/minjaehwang/bitsetsort. This implementation is faster than pdqsort for all randomized inputs. For example, in the case of uint64 256k set, bitset sort only takes 15ns per element while pdqsort takes 24ns per element.

Bitset sort shows a little regression on patterns such as ascending or descending. This is something I can work on but I do believe that speed-ups in randomized inputs triumphs the regression in these patterns because these are so small in difference.

[tschuett]

Also, are you aware of Timsort? IIRC there had been an analysis conducted that concluded that we should be using Timsort (as it was faster than our sort and respected the complexity requirements of the Standard).

Swift went for the standard library from introsort to timsort: https://github.com/apple/swift/pull/19717

[minjaehwang]

Improved randomized sort performance for all types.
Added introsort feature.

[ldionne]

I stumbled upon this again while looking at the review queue and realized I had never followed up. Please rest assured that the lack of response here is only a measure of how backed up we are with reviews and not how interested we are with this change. I also want to give special thanks to @Morwenn for responding to my request to review this and for the detailed explanation and interesting discussion that followed. For a non-sort-expert like me, that is essential in shedding light on the whole thing.

So my take away from the discussion is that Bitsetsort and pdqsort are comparable in efficiency, however Bitsetsort still suffers from O(n^2) worst case. Is that correct? If so, is it possible to modify Bitsetsort to avoid this O(n^2) worst case? That's a pretty bad offender in our conformance right now.

I'm going to commandeer this so I can rebase it onto main and apply a few changes necessary for libc++. I'm sorry this slept for so long. In particular, this patch as-is breaks the ABI because we stop exporting the internal __sort_whatever symbols from the dylib. We can't do that -- we're stuck with these symbols forever unfortunately. However, nothing prevents us from introducing new ones (even though for the time being I would try to avoid baking parts of the sort implementation in the dylib at all).

[ldionne]

• Rebase onto main
• Keep the old std::sort implementation around for ABI compatibility
• Do not explicitly instantiate bitset sort in the library -- that ties our hands too much

Still left to do:

• Some tests are failing, they need to be investigated
• Do basic code size testing since we're not instantiating the sorts in the dylib anymore -- does it cause the size of user programs to blow up badly?
• Make sure I applied the diff correctly - I had to do it manually but the diff was generated weirdly, so I'm not sure the diff I applied is correct.
• We need to decide on the visibility of implementation detail functions. We probably want to sprinkle _LIBCPP_HIDE_FROM_ABI all around.

Ideally @minjaehwang if you could commandeer this again and make those fixes, that would be awesome. From here on, it should be fairly straightforward to make changes until we can ship this, since I should have handled most of the annoying libc++ specific things.

[nilayvaish]

In D93233#3046799, @ldionne wrote:

• Rebase onto main
• Keep the old std::sort implementation around for ABI compatibility
• Do not explicitly instantiate bitset sort in the library -- that ties our hands too much

Still left to do:

• Some tests are failing, they need to be investigated
• Do basic code size testing since we're not instantiating the sorts in the dylib anymore -- does it cause the size of user programs to blow up badly?
• Make sure I applied the diff correctly - I had to do it manually but the diff was generated weirdly, so I'm not sure the diff I applied is correct.
• We need to decide on the visibility of implementation detail functions. We probably want to sprinkle _LIBCPP_HIDE_FROM_ABI all around.

Ideally @minjaehwang if you could commandeer this again and make those fixes, that would be awesome. From here on, it should be fairly straightforward to make changes until we can ship this, since I should have handled most of the annoying libc++ specific things.

ldionne@, I'll work on this on MinJae's behalf. Would post a new diff sometime soon.

[ldionne]

In D93233#3047865, @nilayvaish wrote:

In D93233#3046799, @ldionne wrote:

• Rebase onto main
• Keep the old std::sort implementation around for ABI compatibility
• Do not explicitly instantiate bitset sort in the library -- that ties our hands too much

Still left to do:

• Some tests are failing, they need to be investigated
• Do basic code size testing since we're not instantiating the sorts in the dylib anymore -- does it cause the size of user programs to blow up badly?
• Make sure I applied the diff correctly - I had to do it manually but the diff was generated weirdly, so I'm not sure the diff I applied is correct.
• We need to decide on the visibility of implementation detail functions. We probably want to sprinkle _LIBCPP_HIDE_FROM_ABI all around.

Ideally @minjaehwang if you could commandeer this again and make those fixes, that would be awesome. From here on, it should be fairly straightforward to make changes until we can ship this, since I should have handled most of the annoying libc++ specific things.

ldionne@, I'll work on this on MinJae's behalf. Would post a new diff sometime soon.

Awesome, thanks! I saw your post on libcxx-dev just now. To reproduce CI failures locally, you can use libcxx/utils/ci/run-buildbot-container to spawn a Docker container matching our CI nodes, and then libcxx/utils/ci/run-buildbot <config you want> to run the actual build/test. There's documentation in run-buildbot-container and also the Dockerfile in the same directory.

[nilayvaish]

• Fixed the implementation. No tests fail now: https://buildkite.com/llvm-project/libcxx-ci/builds/5917.
• Added _LIBCPP_HIDE_FROM_ABI to most functions.

Questions for ldionne@

• How do we test code size?
• Do we need to hide member functions in classes from the ABI?

[nilayvaish]

It seems that we can use _LIBCPP_HIDE_FROM_ABI with __bitsetsort_loop. Causes the compiler to report errors like the following:

589 | bitsetsort_loop(_RandomAccessIterator first, _RandomAccessIterator last, _Compare comp,

| ^~~~~~~~~~~~~~~~~

/home/libcxx-builder/.buildkite-agent/builds/75f0779fb064-1/llvm-project/libcxx-ci/build/generic-gcc/include/c++/v1/__algorithm/sort.h:642:34: note: called from here

642 |       __bitsetsort_loop<_Compare>(__first, __ret.first, __comp, __buff, __limit);
    |       ~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

/home/libcxx-builder/.buildkite-agent/builds/75f0779fb064-1/llvm-project/libcxx-ci/build/generic-gcc/include/c++/v1/__algorithm/sort.h:589:1: error: inlining failed in call to 'always_inline' 'void std::1::bitsetsort::bitsetsort_loop(_RandomAccessIterator, _RandomAccessIterator, _Compare, typename std::1::iterator_traits<_RandomAccessIterator1>::value_type*, typename std::1::iterator_traits<_RandomAccessIterator1>::difference_type) [with _Compare = std::1::__less<double, double>; _RandomAccessIterator = double*]': recursive inlining

589 | __bitsetsort_loop(_RandomAccessIterator __first, _RandomAccessIterator __last, _Compare __comp,
    | ^~~~~~~~~~~~~~~~~

/home/libcxx-builder/.buildkite-agent/builds/75f0779fb064-1/llvm-project/libcxx-ci/build/generic-gcc/include/c++/v1/__algorithm/sort.h:645:34: note: called from here

645 |       __bitsetsort_loop<_Compare>(__ret.first + 1, __last, __comp, __buff, __limit);
    |       ~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

[nilayvaish]

libcxx/src/legacy-sort.cpp

339–393

ldionne@, I am wondering if these symbols need to be in this file. Can we continue with the setup from before i.e. these symbols have extern declarations in sort.h and are defined in algorithm.cpp? Further is there a need for retaining the existing sorting algorithm? It seems to me we need to retain __insertion_sort_incomplete only. What do you think?

[ldionne]

Sorry for the delay, see my answers below.

In D93233#3060110, @nilayvaish wrote:

• Fixed the implementation. No tests fail now: https://buildkite.com/llvm-project/libcxx-ci/builds/5917.
• Added _LIBCPP_HIDE_FROM_ABI to most functions.

Questions for ldionne@

• How do we test code size?

I think we have benchmarks for std::sort -- one thing we could do is look at the size of that executable before and after the change.

Can you also provide the runtime output of these benchmarks before and after the change?

• Do we need to hide member functions in classes from the ABI?

Yes, but I think you have figured that out already.

libcxx/include/__algorithm/sort.h

34

Why is this called __sorting_network? Sounds like a weird name.

libcxx/src/legacy-sort.cpp

339–393

I think we do need to retain all those functions since they were previously exported from the shared library. Removing these functions would be an ABI break.

I've put it in legacy-sort.cpp because we don't ever want to deal with these functions again anymore -- they are only there for ABI compatibility purpose. I thought it was better to separate them into their own little file than keeping them around in the headers that we actually use. If you have a strong reason to keep them around in sort.h, let me know and we can discuss.

340

This should be guarded by _LIBCPP_HAS_NO_WIDE_CHARACTERS. I think this must have happened when you (or I) rebased the patch.

363–364

Same.

[nilayvaish]

Hi ldionne@, I have question for you. Do the tests we have for libc++ ensure that the new implementation works well across all versions of the C++ standard? I want to make sure that I am not going to unintentionally break someone's standard-compliant code out there.

[nilayvaish]

Fixed some compilation issues and reduced the number of changes in ordering of equivalent elements

[nilayvaish]

ldionne@, I am going to break this change into few parts and provide the benchmark results for each of the part separately. To begin with, I have posted: https://reviews.llvm.org/D113413 that makes the introsort change to the std::sort implementation.

libcxx/include/__algorithm/sort.h

34

I think the name has been prevalent in the theory community for a long time. More info here: https://en.wikipedia.org/wiki/Sorting_network. The code in functions __sort[3...8] mimics what the sorting networks for those lengths would look like. The name is strange from namespace perspective. Willing to name it something else that you prefer.

[nilayvaish]

libcxx/src/legacy-sort.cpp

339–393

I do not have a preference for where these symbols are kept. Is there a way to avoid having sort implementation in legacy-sort.cpp file?

[ldionne]

In D93233#3101571, @nilayvaish wrote:

Hi ldionne@, I have question for you. Do the tests we have for libc++ ensure that the new implementation works well across all versions of the C++ standard? I want to make sure that I am not going to unintentionally break someone's standard-compliant code out there.

They should, however you're welcome to look at them - our tests are sometimes not up to our quality standards, so you should make sure they will catch all issues you can think about.

We do run those tests in C++03, C++11, C++14, C++17, C++20 and C++23 modes, if that was the sole intent of your question.

In D93233#3116956, @nilayvaish wrote:

ldionne@, I am going to break this change into few parts and provide the benchmark results for each of the part separately. To begin with, I have posted: https://reviews.llvm.org/D113413 that makes the introsort change to the std::sort implementation.

That's fine by me if you prefer doing it that way, however at the moment I am failing to understand where https://reviews.llvm.org/D113413 fits into the picture. Can you explain what's the plan? Basically introduce introsort in D113413 and then replace it by this bitset sort? Sorry if that's a stupid question.

libcxx/include/__algorithm/sort.h

34

Oh, that's all good. If there's prior art with this name, ignore my comment -- I was not aware of it.

[nilayvaish]

In D93233#3118424, @ldionne wrote:

In D93233#3101571, @nilayvaish wrote:

Hi ldionne@, I have question for you. Do the tests we have for libc++ ensure that the new implementation works well across all versions of the C++ standard? I want to make sure that I am not going to unintentionally break someone's standard-compliant code out there.

They should, however you're welcome to look at them - our tests are sometimes not up to our quality standards, so you should make sure they will catch all issues you can think about.

We do run those tests in C++03, C++11, C++14, C++17, C++20 and C++23 modes, if that was the sole intent of your question.

I think the current tests do not cover some of the ways in which folks use sort. I did some internal testing and that revealed few compilation issues with one of the previous versions of the diff. I'll try to reproduce those issues and add those to tests.

In D93233#3116956, @nilayvaish wrote:

ldionne@, I am going to break this change into few parts and provide the benchmark results for each of the part separately. To begin with, I have posted: https://reviews.llvm.org/D113413 that makes the introsort change to the std::sort implementation.

That's fine by me if you prefer doing it that way, however at the moment I am failing to understand where https://reviews.llvm.org/D113413 fits into the picture. Can you explain what's the plan? Basically introduce introsort in D113413 and then replace it by this bitset sort? Sorry if that's a stupid question.

D93233 has three different changes in it which I would like to break into parts. So the plan is to do the following:

• D113413: introduce introsort (improves worst case from O(n^2) to O(n log n).
• D93233 or another commit: introduce bitset sort (improves the core of the quicksort implementation to avoid branch mispredictions)
• D93233 or another commit: improve small sized sort (improves small sized sorting so that it is possibly vectorized by the compiler and the number of comparisons in use is optimal).

[ldionne]

Gentle ping here -- we have now fixed the O(N^2) behavior with introsort, but do we still want to make these improvements? (I suspect we do)

libcxx/include/__algorithm/nth_element.h

 //===----------------------------------------------------------------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 
 #ifndef _LIBCPP___ALGORITHM_NTH_ELEMENT_H
 #define _LIBCPP___ALGORITHM_NTH_ELEMENT_H
 
-#include <__config>
-#include <__algorithm/comp.h>
 #include <__algorithm/comp_ref_type.h>
+#include <__algorithm/comp.h>
+#include <__algorithm/min_element.h>
 #include <__algorithm/sort.h>
+#include <__config>
 #include <__iterator/iterator_traits.h>
 #include <__utility/swap.h>
 
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #pragma GCC system_header
 #endif
 
 _LIBCPP_BEGIN_NAMESPACE_STD
 
+// Assumes size > 0
+template <class _Compare, class _BidirectionalIterator>
+_LIBCPP_CONSTEXPR_AFTER_CXX11 void
+__selection_sort(_BidirectionalIterator __first, _BidirectionalIterator __last, _Compare __comp)
+{
+    _BidirectionalIterator __lm1 = __last;
+    for (--__lm1; __first != __lm1; ++__first)
+    {
+        _BidirectionalIterator __i = _VSTD::min_element<_BidirectionalIterator, _Compare&>(__first, __last, __comp);
+        if (__i != __first)
+            swap(*__first, *__i);
+    }
+}
+
 template<class _Compare, class _RandomAccessIterator>
 _LIBCPP_CONSTEXPR_AFTER_CXX11 bool
 __nth_element_find_guard(_RandomAccessIterator& __i, _RandomAccessIterator& __j,
                          _RandomAccessIterator __m, _Compare __comp)
 {
     // manually guard downward moving __j against __i
     while (true) {
         if (__i == --__j) {
while (true)
             return;
         case 2:
             if (__comp(*--__last, *__first))
                 swap(*__first, *__last);
             return;
         case 3:
             {
             _RandomAccessIterator __m = __first;
-            _VSTD::__sort3<_Compare>(__first, ++__m, --__last, __comp);
+            __sorting_network::__sort3<_Compare>(__first, ++__m, --__last, __comp);
             return;
             }
         }
         if (__len <= __limit)
         {
             _VSTD::__selection_sort<_Compare>(__first, __last, __comp);
             return;
         }
         // __len > __limit >= 3
         _RandomAccessIterator __m = __first + __len/2;
         _RandomAccessIterator __lm1 = __last;
-        unsigned __n_swaps = _VSTD::__sort3<_Compare>(__first, __m, --__lm1, __comp);
+        unsigned __n_swaps = __sorting_network::__sort3<_Compare>(__first, __m, --__lm1, __comp);
         // *__m is median
         // partition [__first, __m) < *__m and *__m <= [__m, __last)
         // (this inhibits tossing elements equivalent to __m around unnecessarily)
         _RandomAccessIterator __i = __first;
         _RandomAccessIterator __j = __lm1;
         // j points beyond range to be tested, *__lm1 is known to be <= *__m
         // The search going up is known to be guarded but the search coming down isn't.
         // Prime the downward search with a guard.

libcxx/include/__algorithm/sort.h

 //===----------------------------------------------------------------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 
 #ifndef _LIBCPP___ALGORITHM_SORT_H
 #define _LIBCPP___ALGORITHM_SORT_H
 
-#include <__config>
-#include <__algorithm/comp.h>
 #include <__algorithm/comp_ref_type.h>
-#include <__algorithm/min_element.h>
+#include <__algorithm/comp.h>
+#include <__algorithm/iter_swap.h>
 #include <__algorithm/partial_sort.h>
 #include <__algorithm/unwrap_iter.h>
+#include <__config>
+#include <__iterator/iterator_traits.h>
+#include <__iterator/move_iterator.h>
+#include <__utility/move.h>
+#include <__utility/pair.h>
 #include <__utility/swap.h>
 #include <memory>
+#include <type_traits>
 
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #pragma GCC system_header
 #endif
 
 _LIBCPP_BEGIN_NAMESPACE_STD
 
-// stable, 2-3 compares, 0-2 swaps
+namespace __sorting_network {
 
-template <class _Compare, class _ForwardIterator>
-_LIBCPP_CONSTEXPR_AFTER_CXX11 unsigned
-__sort3(_ForwardIterator __x, _ForwardIterator __y, _ForwardIterator __z, _Compare __c)
-{
-    unsigned __r = 0;
-    if (!__c(*__y, *__x))          // if x <= y
-    {
-        if (!__c(*__z, *__y))      // if y <= z
-            return __r;            // x <= y && y <= z
-                                   // x <= y && y > z
-        swap(*__y, *__z);          // x <= z && y < z
-        __r = 1;
-        if (__c(*__y, *__x))       // if x > y
-        {
-            swap(*__x, *__y);      // x < y && y <= z
-            __r = 2;
-        }
-        return __r;                // x <= y && y < z
-    }
-    if (__c(*__z, *__y))           // x > y, if y > z
-    {
-        swap(*__x, *__z);          // x < y && y < z
-        __r = 1;
-        return __r;
-    }
-    swap(*__x, *__y);              // x > y && y <= z
-    __r = 1;                       // x < y && x <= z
-    if (__c(*__z, *__y))           // if y > z
-    {
-        swap(*__y, *__z);          // x <= y && y < z
-        __r = 2;
-    }
-    return __r;
-}                                  // x <= y && y <= z
-
-// stable, 3-6 compares, 0-5 swaps
-
-template <class _Compare, class _ForwardIterator>
-unsigned
-__sort4(_ForwardIterator __x1, _ForwardIterator __x2, _ForwardIterator __x3,
-            _ForwardIterator __x4, _Compare __c)
-{
-    unsigned __r = _VSTD::__sort3<_Compare>(__x1, __x2, __x3, __c);
-    if (__c(*__x4, *__x3))
-    {
-        swap(*__x3, *__x4);
-        ++__r;
-        if (__c(*__x3, *__x2))
-        {
-            swap(*__x2, *__x3);
-            ++__r;
-            if (__c(*__x2, *__x1))
-            {
-                swap(*__x1, *__x2);
-                ++__r;
-            }
-        }
-    }
-    return __r;
-}
-
-// stable, 4-10 compares, 0-9 swaps
-
-template <class _Compare, class _ForwardIterator>
-_LIBCPP_HIDDEN
-unsigned
-__sort5(_ForwardIterator __x1, _ForwardIterator __x2, _ForwardIterator __x3,
-            _ForwardIterator __x4, _ForwardIterator __x5, _Compare __c)
-{
-    unsigned __r = _VSTD::__sort4<_Compare>(__x1, __x2, __x3, __x4, __c);
-    if (__c(*__x5, *__x4))
-    {
-        swap(*__x4, *__x5);
-        ++__r;
-        if (__c(*__x4, *__x3))
-        {
-            swap(*__x3, *__x4);
-            ++__r;
-            if (__c(*__x3, *__x2))
-            {
-                swap(*__x2, *__x3);
-                ++__r;
-                if (__c(*__x2, *__x1))
-                {
-                    swap(*__x1, *__x2);
-                    ++__r;
-                }
-            }
-        }
-    }
-    return __r;
-}
-
-// Assumes size > 0
-template <class _Compare, class _BidirectionalIterator>
-_LIBCPP_CONSTEXPR_AFTER_CXX11 void
-__selection_sort(_BidirectionalIterator __first, _BidirectionalIterator __last, _Compare __comp)
-{
-    _BidirectionalIterator __lm1 = __last;
-    for (--__lm1; __first != __lm1; ++__first)
-    {
-        _BidirectionalIterator __i = _VSTD::min_element<_BidirectionalIterator, _Compare&>(__first, __last, __comp);
-        if (__i != __first)
-            swap(*__first, *__i);
-    }
-}
-
-template <class _Compare, class _BidirectionalIterator>
-void
-__insertion_sort(_BidirectionalIterator __first, _BidirectionalIterator __last, _Compare __comp)
-{
-    typedef typename iterator_traits<_BidirectionalIterator>::value_type value_type;
-    if (__first != __last)
-    {
-        _BidirectionalIterator __i = __first;
-        for (++__i; __i != __last; ++__i)
-        {
-            _BidirectionalIterator __j = __i;
-            value_type __t(_VSTD::move(*__j));
-            for (_BidirectionalIterator __k = __i; __k != __first && __comp(__t,  *--__k); --__j)
-                *__j = _VSTD::move(*__k);
-            *__j = _VSTD::move(__t);
-        }
+template <class _RandomAccessIterator, class _Compare>
+class __conditional_swap {
+  _Compare comp_;
+
+ public:
+  _Compare get() const { return comp_; }
+  __conditional_swap(_Compare __comp) : comp_(__comp) {}
+  inline void operator()(_RandomAccessIterator __x,
+                         _RandomAccessIterator __y) {
+    typedef typename _VSTD::iterator_traits<_RandomAccessIterator>::value_type
+        value_type;
+    bool __result = comp_(*__x, *__y);
+    // Expect a compiler would short-circuit the following if-block.
+    // 4 * sizeof(size_t) is a magic number. Expect a compiler to use SIMD
+    // instruction on them.
+    if (_VSTD::is_trivially_copy_constructible<value_type>::value &&
+        _VSTD::is_trivially_copy_assignable<value_type>::value &&
+        sizeof(value_type) <= 4 * sizeof(size_t)) {
+      value_type __min = __result ? _VSTD::move(*__x) : _VSTD::move(*__y);
+      *__y = __result ? _VSTD::move(*__y) : _VSTD::move(*__x);
+      *__x = _VSTD::move(__min);
+    } else {
+      if (!__result) {
+        _VSTD::iter_swap(__x, __y);
     }
   }
+};
 
-template <class _Compare, class _RandomAccessIterator>
-void
-__insertion_sort_3(_RandomAccessIterator __first, _RandomAccessIterator __last, _Compare __comp)
-{
-    typedef typename iterator_traits<_RandomAccessIterator>::value_type value_type;
-    _RandomAccessIterator __j = __first+2;
-    _VSTD::__sort3<_Compare>(__first, __first+1, __j, __comp);
-    for (_RandomAccessIterator __i = __j+1; __i != __last; ++__i)
-    {
-        if (__comp(*__i, *__j))
-        {
-            value_type __t(_VSTD::move(*__i));
-            _RandomAccessIterator __k = __j;
-            __j = __i;
-            do
-            {
-                *__j = _VSTD::move(*__k);
-                __j = __k;
-            } while (__j != __first && __comp(__t, *--__k));
-            *__j = _VSTD::move(__t);
-        }
-        __j = __i;
+template <class _RandomAccessIterator, class _Compare>
+class __reverse_conditional_swap {
+  _Compare comp_;
+
+ public:
+  _Compare get() const { return comp_; }
+  __reverse_conditional_swap(_Compare __comp) : comp_(__comp) {}
+  inline void operator()(_RandomAccessIterator __x,
+                         _RandomAccessIterator __y) {
+    typedef typename _VSTD::iterator_traits<_RandomAccessIterator>::value_type
+        value_type;
+    bool __result = !comp_(*__x, *__y);
+    // Expect a compiler would short-circuit the following if-block.
+    if (_VSTD::is_trivially_copy_constructible<value_type>::value &&
+        _VSTD::is_trivially_copy_assignable<value_type>::value &&
+        sizeof(value_type) <= 4 * sizeof(size_t)) {
+      value_type __min = __result ? _VSTD::move(*__x) : _VSTD::move(*__y);
+      *__y = __result ? _VSTD::move(*__y) : _VSTD::move(*__x);
+      *__x = _VSTD::move(__min);
+    } else {
+      if (__result) {
+        _VSTD::iter_swap(__x, __y);
     }
   }
+};
 
-template <class _Compare, class _RandomAccessIterator>
-bool
-__insertion_sort_incomplete(_RandomAccessIterator __first, _RandomAccessIterator __last, _Compare __comp)
-{
-    switch (__last - __first)
-    {
+template <class _RandomAccessIterator, class _ConditionalSwap>
+void __sort2(_RandomAccessIterator __a, _ConditionalSwap __cond_swap) {
+  __cond_swap(__a + 0, __a + 1);
+}
+
+template <class _RandomAccessIterator, class _ConditionalSwap>
+void __sort3(_RandomAccessIterator __a, _ConditionalSwap __cond_swap) {
+  __cond_swap(__a + 1, __a + 2);
+  __cond_swap(__a + 0, __a + 2);
+  __cond_swap(__a + 0, __a + 1);
+}
+
+template <class _RandomAccessIterator, class _ConditionalSwap>
+void __sort4(_RandomAccessIterator __a, _ConditionalSwap __cond_swap) {
+  __cond_swap(__a + 0, __a + 1);
+  __cond_swap(__a + 2, __a + 3);
+  __cond_swap(__a + 0, __a + 2);
+  __cond_swap(__a + 1, __a + 3);
+  __cond_swap(__a + 1, __a + 2);
+}
+
+template <class _RandomAccessIterator, class _ConditionalSwap>
+void __sort5(_RandomAccessIterator __a, _ConditionalSwap __cond_swap) {
+  __cond_swap(__a + 0, __a + 1);
+  __cond_swap(__a + 3, __a + 4);
+  __cond_swap(__a + 2, __a + 4);
+  __cond_swap(__a + 2, __a + 3);
+  __cond_swap(__a + 0, __a + 3);
+  __cond_swap(__a + 1, __a + 4);
+  __cond_swap(__a + 0, __a + 2);
+  __cond_swap(__a + 1, __a + 3);
+  __cond_swap(__a + 1, __a + 2);
+}
+
+template <class _RandomAccessIterator, class _ConditionalSwap>
+void __sort6(_RandomAccessIterator __a, _ConditionalSwap __cond_swap) {
+  __cond_swap(__a + 1, __a + 2);
+  __cond_swap(__a + 4, __a + 5);
+  __cond_swap(__a + 0, __a + 2);
+  __cond_swap(__a + 3, __a + 5);
+  __cond_swap(__a + 0, __a + 1);
+  __cond_swap(__a + 3, __a + 4);
+  __cond_swap(__a + 0, __a + 3);
+  __cond_swap(__a + 1, __a + 4);
+  __cond_swap(__a + 2, __a + 5);
+  __cond_swap(__a + 2, __a + 4);
+  __cond_swap(__a + 1, __a + 3);
+  __cond_swap(__a + 2, __a + 3);
+}
+template <class _RandomAccessIterator, class _ConditionalSwap>
+void __sort7(_RandomAccessIterator __a, _ConditionalSwap __cond_swap) {
+  __cond_swap(__a + 1, __a + 2);
+  __cond_swap(__a + 3, __a + 4);
+  __cond_swap(__a + 5, __a + 6);
+  __cond_swap(__a + 0, __a + 2);
+  __cond_swap(__a + 3, __a + 5);
+  __cond_swap(__a + 4, __a + 6);
+  __cond_swap(__a + 0, __a + 1);
+  __cond_swap(__a + 4, __a + 5);
+  __cond_swap(__a + 0, __a + 4);
+  __cond_swap(__a + 1, __a + 5);
+  __cond_swap(__a + 2, __a + 6);
+  __cond_swap(__a + 0, __a + 3);
+  __cond_swap(__a + 2, __a + 5);
+  __cond_swap(__a + 1, __a + 3);
+  __cond_swap(__a + 2, __a + 4);
+  __cond_swap(__a + 2, __a + 3);
+}
+
+template <class _RandomAccessIterator, class _ConditionalSwap>
+void __sort8(_RandomAccessIterator __a, _ConditionalSwap __cond_swap) {
+  __cond_swap(__a + 0, __a + 1);
+  __cond_swap(__a + 2, __a + 3);
+  __cond_swap(__a + 4, __a + 5);
+  __cond_swap(__a + 6, __a + 7);
+  __cond_swap(__a + 0, __a + 2);
+  __cond_swap(__a + 1, __a + 3);
+  __cond_swap(__a + 4, __a + 6);
+  __cond_swap(__a + 5, __a + 7);
+  __cond_swap(__a + 1, __a + 2);
+  __cond_swap(__a + 5, __a + 6);
+  __cond_swap(__a + 0, __a + 4);
+  __cond_swap(__a + 1, __a + 5);
+  __cond_swap(__a + 2, __a + 6);
+  __cond_swap(__a + 3, __a + 7);
+  __cond_swap(__a + 1, __a + 4);
+  __cond_swap(__a + 3, __a + 6);
+  __cond_swap(__a + 2, __a + 4);
+  __cond_swap(__a + 3, __a + 5);
+  __cond_swap(__a + 3, __a + 4);
+}
+
+template <class _RandomAccessIterator, class _ConditionalSwap>
+void __sort1to8(
+    _RandomAccessIterator __a,
+    typename _VSTD::iterator_traits<_RandomAccessIterator>::difference_type __len,
+    _ConditionalSwap __cond_swap) {
+  switch (__len) {
     case 0:
     case 1:
-        return true;
+      return;
     case 2:
-        if (__comp(*--__last, *__first))
-            swap(*__first, *__last);
-        return true;
+      __sort2(__a, __cond_swap);
+      return;
     case 3:
-        _VSTD::__sort3<_Compare>(__first, __first+1, --__last, __comp);
-        return true;
+      __sort3(__a, __cond_swap);
+      return;
     case 4:
-        _VSTD::__sort4<_Compare>(__first, __first+1, __first+2, --__last, __comp);
-        return true;
+      __sort4(__a, __cond_swap);
+      return;
     case 5:
-        _VSTD::__sort5<_Compare>(__first, __first+1, __first+2, __first+3, --__last, __comp);
-        return true;
-    }
-    typedef typename iterator_traits<_RandomAccessIterator>::value_type value_type;
-    _RandomAccessIterator __j = __first+2;
-    _VSTD::__sort3<_Compare>(__first, __first+1, __j, __comp);
-    const unsigned __limit = 8;
-    unsigned __count = 0;
-    for (_RandomAccessIterator __i = __j+1; __i != __last; ++__i)
-    {
-        if (__comp(*__i, *__j))
-        {
-            value_type __t(_VSTD::move(*__i));
-            _RandomAccessIterator __k = __j;
-            __j = __i;
-            do
-            {
-                *__j = _VSTD::move(*__k);
-                __j = __k;
-            } while (__j != __first && __comp(__t, *--__k));
-            *__j = _VSTD::move(__t);
-            if (++__count == __limit)
-                return ++__i == __last;
-        }
-        __j = __i;
-    }
-    return true;
+      __sort5(__a, __cond_swap);
+      return;
+    case 6:
+      __sort6(__a, __cond_swap);
+      return;
+    case 7:
+      __sort7(__a, __cond_swap);
+      return;
+    case 8:
+      __sort8(__a, __cond_swap);
+      return;
+  }
+  // ignore
+}
+template <class _RandomAccessIterator, class _ConditionalSwap>
+void __sort3(_RandomAccessIterator __a0, _RandomAccessIterator __a1, _RandomAccessIterator __a2, _ConditionalSwap __cond_swap) {
+  __cond_swap(__a1, __a2);
+  __cond_swap(__a0, __a2);
+  __cond_swap(__a0, __a1);
+}
+}  // namespace __sorting_network
+
+namespace __bitonic {
+class __detail {
+ public:
+  enum {
+    __batch = 8,
+    __bitonic_batch = __batch * 2,
+    __small_sort_max = __bitonic_batch * 2,
+  };
+};
+
+template <class _RandomAccessIterator, class _ConditionalSwap,
+          class _ReverseConditionalSwap>
+void __enforce_order(_RandomAccessIterator __first,
+                     _RandomAccessIterator __last, _ConditionalSwap __cond_swap,
+                     _ReverseConditionalSwap __reverse_cond_swap) {
+  _RandomAccessIterator __i = __first;
+  while (__i + __detail::__bitonic_batch <= __last) {
+    __sorting_network::__sort8(__i, __cond_swap);
+    __sorting_network::__sort8(__i + __detail::__batch, __reverse_cond_swap);
+    __i += __detail::__bitonic_batch;
+  }
+  if (__i + __detail::__batch <= __last) {
+    __sorting_network::__sort8(__i, __cond_swap);
+    __i += __detail::__batch;
+    __sorting_network::__sort1to8(__i, __last - __i, __reverse_cond_swap);
+  } else {
+    __sorting_network::__sort1to8(__i, __last - __i, __cond_swap);
+  }
+}
+
+class __construct {
+ public:
+  template <class _Type>
+  static inline void __op(_Type* __result, _Type&& __val) {
+    new (__result) _Type(_VSTD::move(__val));
+  }
+};
+
+class __move_assign {
+ public:
+  template <class _Type>
+  static inline void __op(_Type* __result, _Type&& __val) {
+    *__result = _VSTD::move(__val);
+  }
+};
+
+template <class _Copy, class _InputIterator, class _OutputIterator,
+          class _Compare>
+void __forward_merge(_InputIterator __first, _InputIterator __last,
+                     _OutputIterator __result, _Compare __comp) {
+  --__last;
+  typename _VSTD::iterator_traits<_InputIterator>::difference_type __len =
+      __last - __first;
+  for (; __len > 0; __len--) {
+    if (__comp(*__first, *__last)) {
+      _Copy::__op(&*__result, _VSTD::move(*__first++));
+    } else {
+      _Copy::__op(&*__result, _VSTD::move(*__last--));
+    }
+    __result++;
+  }
+  _Copy::__op(&*__result, _VSTD::move(*__first));
+}
+
+template <class _Copy, class _InputIterator, class _OutputIterator,
+          class _Compare>
+void __backward_merge(_InputIterator __first, _InputIterator __last,
+                      _OutputIterator __result, _Compare __comp) {
+  --__last;
+  __result += __last - __first;
+  typename _VSTD::iterator_traits<_InputIterator>::difference_type __len =
+      __last - __first;
+  for (; __len > 0; __len--) {
+    if (__comp(*__first, *__last)) {
+      _Copy::__op(&*__result, _VSTD::move(*__first++));
+    } else {
+      _Copy::__op(&*__result, _VSTD::move(*__last--));
+    }
+    __result--;
+  }
+  _Copy::__op(&*__result, _VSTD::move(*__first));
+}
+
+template <class _Copy, class _InputIterator, class _OutputIterator,
+          class _Compare>
+void __forward_and_backward_merge(_InputIterator __first, _InputIterator __last,
+                                  _InputIterator __rlast,
+                                  _OutputIterator __result, _Compare __comp) {
+  _InputIterator __rfirst = __last;
+  __last--;
+  __rlast--;
+  typename _VSTD::iterator_traits<_InputIterator>::difference_type len =
+      __last - __first;
+  _OutputIterator __rout = __result + (__rlast - __first);
+
+  for (; len > 0; len--) {
+    if (__comp(*__first, *__last)) {
+      _Copy::__op(&*__result, _VSTD::move(*__first++));
+    } else {
+      _Copy::__op(&*__result, _VSTD::move(*__last--));
+    }
+    __result++;
+    if (__comp(*__rfirst, *__rlast)) {
+      _Copy::__op(&*__rout, _VSTD::move(*__rfirst++));
+    } else {
+      _Copy::__op(&*__rout, _VSTD::move(*__rlast--));
+    }
+    __rout--;
+  }
+  _Copy::__op(&*__result, _VSTD::move(*__first));
+  _Copy::__op(&*__rout, _VSTD::move(*__rfirst));
+}
+
+template <class _RandomAccessIterator, class _ConditionalSwap,
+          class _ReverseConditionalSwap>
+inline bool __small_sort(
+    _RandomAccessIterator __first,
+    typename _VSTD::iterator_traits<_RandomAccessIterator>::difference_type __len,
+    typename _VSTD::iterator_traits<_RandomAccessIterator>::value_type* __buff,
+    _ConditionalSwap __cond_swap,
+    _ReverseConditionalSwap __reverse_cond_swap) {
+  typedef
+      typename _VSTD::iterator_traits<_RandomAccessIterator>::value_type value_type;
+  if (__len > __detail::__small_sort_max) {
+    return false;
+  }
+  _RandomAccessIterator __last = __first + __len;
+  __enforce_order(__first, __last, __cond_swap, __reverse_cond_swap);
+  if (__len <= __detail::__batch) {
+    // sorted.
+    return true;
+  }
+  auto __comp = __cond_swap.get();
+  if (__len <= __detail::__bitonic_batch) {
+    // single bitonic order merge.
+    __forward_merge<__bitonic::__construct>(__first, __last, __buff, __comp);
+    copy(_VSTD::make_move_iterator(__buff), _VSTD::make_move_iterator(__buff + __len),
+         __first);
+    for (auto __iter = __buff; __iter < __buff + __len; __iter++) {
+      (*__iter).~value_type();
+    }
+  // double bitonic order merge.
+  __forward_merge<__construct>(__first, __first + __detail::__bitonic_batch,
+                               __buff, __comp);
+  __backward_merge<__construct>(__first + __detail::__bitonic_batch, __last,
+                                __buff + __detail::__bitonic_batch, __comp);
+  __forward_merge<__move_assign>(__buff, __buff + __len, __first, __comp);
+  for (auto __iter = __buff; __iter < __buff + __len; __iter++) {
+    (*__iter).~value_type();
+  }
+  return true;
+}
+}  // namespace __bitonic
+
+namespace __bitsetsort {
+struct __64bit_set {
+  typedef uint64_t __storage_t;
+  enum { __block_size = 64 };
+  static __storage_t __blsr(__storage_t x) {
+    // _blsr_u64 can be used here but it did not make any performance
+    // difference in practice.
+    return x ^ (x & -x);
+  }
+  static int __clz(__storage_t x) { return __builtin_clzll(x); }
+  static int __ctz(__storage_t x) { return __builtin_ctzll(x); }
+};
+
+struct __32bit_set {
+  typedef uint32_t __storage_t;
+  enum { __block_size = 32 };
+  static __storage_t __blsr(__storage_t x) {
+    // _blsr_u32 can be used here but it did not make any performance
+    // difference in practice.
+    return x ^ (x & -x);
+  }
+  static int __clz(__storage_t x) { return __builtin_clzl(x); }
+  static int __ctz(__storage_t x) { return __builtin_ctzl(x); }
+};
+
+template <int _Width>
+struct __set_selector {
+    typedef __64bit_set __set;
+};
+
+template<>
+struct __set_selector<4> {
+    typedef __32bit_set __set;
+};
+
+template <class _Bitset, class _RandomAccessIterator>
+inline void __swap_bitmap_pos(_RandomAccessIterator __first,
+                              _RandomAccessIterator __last,
+                              typename _Bitset::__storage_t& __left_bitset,
+                              typename _Bitset::__storage_t& __right_bitset) {
+  while (__left_bitset != 0 & __right_bitset != 0) {
+    int tz_left = _Bitset::__ctz(__left_bitset);
+    __left_bitset = _Bitset::__blsr(__left_bitset);
+    int tz_right = _Bitset::__ctz(__right_bitset);
+    __right_bitset = _Bitset::__blsr(__right_bitset);
+    _VSTD::iter_swap(__first + tz_left, __last - tz_right);
+  }
+}
+
+template <class _Bitset, class _RandomAccessIterator>
+inline void __swap_bitmap(_RandomAccessIterator __first,
+                          _RandomAccessIterator __last,
+                          typename _Bitset::__storage_t& __left_bitset,
+                          typename _Bitset::__storage_t& __right_bitset) {
+  if (__left_bitset == 0 || __right_bitset == 0) {
+    return;
+  }
+  int tz_left;
+  int tz_right;
+
+  tz_left = _Bitset::__ctz(__left_bitset);
+  __left_bitset = _Bitset::__blsr(__left_bitset);
+
+  tz_right = _Bitset::__ctz(__right_bitset);
+  __right_bitset = _Bitset::__blsr(__right_bitset);
+
+  _RandomAccessIterator l = __first + tz_left;
+  _RandomAccessIterator r = __last - tz_right;
+  typename _VSTD::iterator_traits<_RandomAccessIterator>::value_type tmp(
+      _VSTD::move(*l));
+  *l = _VSTD::move(*r);
+  while (__left_bitset != 0 & __right_bitset != 0) {
+    tz_left = _Bitset::__ctz(__left_bitset);
+    __left_bitset = _Bitset::__blsr(__left_bitset);
+    tz_right = _Bitset::__ctz(__right_bitset);
+    __right_bitset = _Bitset::__blsr(__right_bitset);
+
+    l = __first + tz_left;
+    *r = _VSTD::move(*l);
+    r = __last - tz_right;
+    *l = _VSTD::move(*r);
+  }
+  *r = _VSTD::move(tmp);
+}
+
+template <class _Bitset, class _RandomAccessIterator, class _Compare>
+_VSTD::pair<_RandomAccessIterator, bool> __bitset_partition(
+    _RandomAccessIterator __first, _RandomAccessIterator __last,
+    _Compare __comp) {
+  typedef typename _VSTD::iterator_traits<_RandomAccessIterator>::value_type
+      value_type;
+  typedef typename _VSTD::iterator_traits<_RandomAccessIterator>::difference_type
+      difference_type;
+  typedef typename _Bitset::__storage_t __storage_t;
+  _RandomAccessIterator __begin = __first;
+  value_type __pivot = _VSTD::move(*__first);
+
+  if (__comp(__pivot, *(__last - 1))) {
+    // Guarded.
+    while (!__comp(__pivot, *++__first)) {}
+  } else {
+    while (++__first < __last && !__comp(__pivot, *__first)) {}
+  }
+
+  if (__first < __last) {
+    // It will be always guarded because __bitset_sort will do the median-of-three before calling this.
+    while (__comp(__pivot, *--__last)) {}
+  }
+  bool __already_partitioned = __first >= __last;
+  if (!__already_partitioned) {
+      _VSTD::iter_swap(__first, __last);
+      ++__first;
+  }
+
+  // [__first, __last) - __last is not inclusive. From now one, it uses last minus one to be inclusive on both sides.
+  _RandomAccessIterator __lm1 = __last - 1;
+  __storage_t __left_bitset = 0;
+  __storage_t __right_bitset = 0;
+
+  // Reminder: length = __lm1 - __first + 1.
+  while (__lm1 - __first >= 2 * _Bitset::__block_size - 1) {
+    if (__left_bitset == 0) {
+      // Possible vectorization. With a proper "-march" flag, the following loop
+      // will be compiled into a set of SIMD instructions.
+      _RandomAccessIterator __iter = __first;
+      for (int __j = 0; __j < _Bitset::__block_size;) {
+        __left_bitset |= (static_cast<__storage_t>(__comp(__pivot, *__iter)) << __j);
+        __j++;
+        __iter++;
+      }
+    }
+
+    if (__right_bitset == 0) {
+      // Possible vectorization. With a proper "-march" flag, the following loop
+      // will be compiled into a set of SIMD instructions.
+      _RandomAccessIterator __iter = __lm1;
+      for (int __j = 0; __j < _Bitset::__block_size;) {
+        __right_bitset |=
+            (static_cast<__storage_t>(!__comp(__pivot, *__iter)) << __j);
+        __j++;
+        __iter--;
+      }
+    }
+
+    __swap_bitmap_pos<_Bitset>(__first, __lm1, __left_bitset, __right_bitset);
+    __first += (__left_bitset == 0) ? _Bitset::__block_size : 0;
+    __lm1 -= (__right_bitset == 0) ? _Bitset::__block_size : 0;
+  }
+  // Now, we have a less-than a block on each side.
+  difference_type __remaining_len = __lm1 - __first + 1;
+  difference_type __l_size;
+  difference_type __r_size;
+  if (__left_bitset == 0 && __right_bitset == 0) {
+    __l_size = __remaining_len / 2;
+    __r_size = __remaining_len - __l_size;
+  } else if (__left_bitset == 0) {
+    // We know at least one side is a full block.
+    __l_size = __remaining_len - _Bitset::__block_size;
+    __r_size = _Bitset::__block_size;
+  } else {  // if (right == 0)
+    __l_size = _Bitset::__block_size;
+    __r_size = __remaining_len - _Bitset::__block_size;
+  }
+  if (__left_bitset == 0) {
+    _RandomAccessIterator __iter = __first;
+    for (int j = 0; j < __l_size; j++) {
+      __left_bitset |=
+          (static_cast<__storage_t>(__comp(__pivot, *(__iter))) << j);
+      __iter++;
+    }
+  }
+  if (__right_bitset == 0) {
+    _RandomAccessIterator __iter = __lm1;
+    for (int j = 0; j < __r_size; j++) {
+      __right_bitset |=
+          (static_cast<__storage_t>(!__comp(__pivot, *(__iter))) << j);
+      --__iter;
+    }
+  }
+  __swap_bitmap_pos<_Bitset>(__first, __lm1, __left_bitset, __right_bitset);
+  __first += (__left_bitset == 0) ? __l_size : 0;
+  __lm1 -= (__right_bitset == 0) ? __r_size : 0;
+
+  if (__left_bitset) {
+    // Swap within the right side.
+    int __tz_left;
+
+    // Need to find set positions in the reverse order.
+    while (__left_bitset != 0) {
+      __tz_left = _Bitset::__block_size - 1 - _Bitset::__clz(__left_bitset);
+      __left_bitset &= (static_cast<__storage_t>(1) << __tz_left) - 1;
+      _VSTD::iter_swap(__first + __tz_left, __lm1--);
+    }
+    __first = __lm1 + 1;
+  } else if (__right_bitset) {
+    // Swap within the left side.
+    int __tz_right;
+    // Need to find set positions in the reverse order.
+    while (__right_bitset != 0) {
+      __tz_right = _Bitset::__block_size - 1 - _Bitset::__clz(__right_bitset);
+      __right_bitset &= (static_cast<__storage_t>(1) << __tz_right) - 1;
+      _VSTD::iter_swap(__lm1 - __tz_right, __first++);
+    }
+  }
+
+  _RandomAccessIterator __pivot_pos = __first - 1;
+  *__begin = _VSTD::move(*__pivot_pos);
+  *__pivot_pos = _VSTD::move(__pivot);
+  return _VSTD::make_pair(__pivot_pos, __already_partitioned);
+}
+
+template <class _RandomAccessIterator, class _Compare>
+inline bool __partial_insertion_sort(_RandomAccessIterator __first,
+                                     _RandomAccessIterator __last,
+                                     _Compare __comp) {
+  typedef typename _VSTD::iterator_traits<_RandomAccessIterator>::value_type
+      value_type;
+  if (__first == __last) return true;
+
+  _RandomAccessIterator __j = __first;
+  for (_RandomAccessIterator __i = __j + 1; __i != __last; ++__i) {
+    if (__comp(*__i, *__j)) {
+      do {
+      if (++__count == __limit) return ++__i == __last;
 }
 
-template <class _Compare, class _BidirectionalIterator>
-void
-__insertion_sort_move(_BidirectionalIterator __first1, _BidirectionalIterator __last1,
-                      typename iterator_traits<_BidirectionalIterator>::value_type* __first2, _Compare __comp)
-{
-    typedef typename iterator_traits<_BidirectionalIterator>::value_type value_type;
-    if (__first1 != __last1)
-    {
-        __destruct_n __d(0);
-        unique_ptr<value_type, __destruct_n&> __h(__first2, __d);
-        value_type* __last2 = __first2;
-        ::new ((void*)__last2) value_type(_VSTD::move(*__first1));
-        __d.template __incr<value_type>();
-        for (++__last2; ++__first1 != __last1; ++__last2)
-        {
-            value_type* __j2 = __last2;
-            value_type* __i2 = __j2;
-            if (__comp(*__first1, *--__i2))
-            {
-                ::new ((void*)__j2) value_type(_VSTD::move(*__i2));
-                __d.template __incr<value_type>();
-                for (--__j2; __i2 != __first2 && __comp(*__first1,  *--__i2); --__j2)
-                    *__j2 = _VSTD::move(*__i2);
-                *__j2 = _VSTD::move(*__first1);
-            }
-            else
-            {
-                ::new ((void*)__j2) value_type(_VSTD::move(*__first1));
-                __d.template __incr<value_type>();
-            }
-        }
-        __h.release();
-    }
-}
-
 template <class _Compare, class _RandomAccessIterator>
-void
-__sort(_RandomAccessIterator __first, _RandomAccessIterator __last, _Compare __comp)
-{
-    typedef typename iterator_traits<_RandomAccessIterator>::difference_type difference_type;
-    typedef typename iterator_traits<_RandomAccessIterator>::value_type value_type;
-    const difference_type __limit = is_trivially_copy_constructible<value_type>::value &&
-                                    is_trivially_copy_assignable<value_type>::value ? 30 : 6;
-    while (true)
-    {
-    __restart:
-        difference_type __len = __last - __first;
-        switch (__len)
-        {
-        case 0:
-        case 1:
-            return;
-        case 2:
-            if (__comp(*--__last, *__first))
-                swap(*__first, *__last);
-            return;
-        case 3:
-            _VSTD::__sort3<_Compare>(__first, __first+1, --__last, __comp);
-            return;
-        case 4:
-            _VSTD::__sort4<_Compare>(__first, __first+1, __first+2, --__last, __comp);
-            return;
-        case 5:
-            _VSTD::__sort5<_Compare>(__first, __first+1, __first+2, __first+3, --__last, __comp);
-            return;
-        }
-        if (__len <= __limit)
-        {
-            _VSTD::__insertion_sort_3<_Compare>(__first, __last, __comp);
-            return;
-        }
-        // __len > 5
-        _RandomAccessIterator __m = __first;
-        _RandomAccessIterator __lm1 = __last;
-        --__lm1;
-        unsigned __n_swaps;
-        {
-        difference_type __delta;
-        if (__len >= 1000)
-        {
-            __delta = __len/2;
-            __m += __delta;
-            __delta /= 2;
-            __n_swaps = _VSTD::__sort5<_Compare>(__first, __first + __delta, __m, __m+__delta, __lm1, __comp);
-        }
-        else
-        {
-            __delta = __len/2;
-            __m += __delta;
-            __n_swaps = _VSTD::__sort3<_Compare>(__first, __m, __lm1, __comp);
-        }
-        }
-        // *__m is median
-        // partition [__first, __m) < *__m and *__m <= [__m, __last)
-        // (this inhibits tossing elements equivalent to __m around unnecessarily)
-        _RandomAccessIterator __i = __first;
-        _RandomAccessIterator __j = __lm1;
-        // j points beyond range to be tested, *__m is known to be <= *__lm1
-        // The search going up is known to be guarded but the search coming down isn't.
-        // Prime the downward search with a guard.
-        if (!__comp(*__i, *__m))  // if *__first == *__m
-        {
-            // *__first == *__m, *__first doesn't go in first part
-            // manually guard downward moving __j against __i
-            while (true)
-            {
-                if (__i == --__j)
-                {
-                    // *__first == *__m, *__m <= all other elements
-                    // Parition instead into [__first, __i) == *__first and *__first < [__i, __last)
-                    ++__i;  // __first + 1
-                    __j = __last;
-                    if (!__comp(*__first, *--__j))  // we need a guard if *__first == *(__last-1)
-                    {
-                        while (true)
-                        {
-                            if (__i == __j)
-                                return;  // [__first, __last) all equivalent elements
-                            if (__comp(*__first, *__i))
-                            {
-                                swap(*__i, *__j);
-                                ++__n_swaps;
-                                ++__i;
-                                break;
-                            }
-                            ++__i;
-                        }
-                    }
-                    // [__first, __i) == *__first and *__first < [__j, __last) and __j == __last - 1
-                    if (__i == __j)
-                        return;
-                    while (true)
-                    {
-                        while (!__comp(*__first, *__i))
-                            ++__i;
-                        while (__comp(*__first, *--__j))
-                            ;
-                        if (__i >= __j)
-                            break;
-                        swap(*__i, *__j);
-                        ++__n_swaps;
-                        ++__i;
-                    }
-                    // [__first, __i) == *__first and *__first < [__i, __last)
-                    // The first part is sorted, sort the second part
-                    // _VSTD::__sort<_Compare>(__i, __last, __comp);
-                    __first = __i;
-                    goto __restart;
-                }
-                if (__comp(*__j, *__m))
-                {
-                    swap(*__i, *__j);
-                    ++__n_swaps;
-                    break;  // found guard for downward moving __j, now use unguarded partition
-                }
-            }
-        }
-        // It is known that *__i < *__m
-        ++__i;
-        // j points beyond range to be tested, *__m is known to be <= *__lm1
-        // if not yet partitioned...
-        if (__i < __j)
-        {
-            // known that *(__i - 1) < *__m
-            // known that __i <= __m
-            while (true)
-            {
-                // __m still guards upward moving __i
-                while (__comp(*__i, *__m))
-                    ++__i;
-                // It is now known that a guard exists for downward moving __j
-                while (!__comp(*--__j, *__m))
-                    ;
-                if (__i > __j)
-                    break;
-                swap(*__i, *__j);
-                ++__n_swaps;
-                // It is known that __m != __j
-                // If __m just moved, follow it
-                if (__m == __i)
-                    __m = __j;
-                ++__i;
-            }
-        }
-        // [__first, __i) < *__m and *__m <= [__i, __last)
-        if (__i != __m && __comp(*__m, *__i))
-        {
-            swap(*__i, *__m);
-            ++__n_swaps;
-        }
-        // [__first, __i) < *__i and *__i <= [__i+1, __last)
-        // If we were given a perfect partition, see if insertion sort is quick...
-        if (__n_swaps == 0)
-        {
-            bool __fs = _VSTD::__insertion_sort_incomplete<_Compare>(__first, __i, __comp);
-            if (_VSTD::__insertion_sort_incomplete<_Compare>(__i+1, __last, __comp))
-            {
-                if (__fs)
-                    return;
-                __last = __i;
-                continue;
-            }
-            else
-            {
-                if (__fs)
-                {
-                    __first = ++__i;
-                    continue;
-                }
-            }
-        }
-        // sort smaller range with recursive call and larger with tail recursion elimination
-        if (__i - __first < __last - __i)
-        {
-            _VSTD::__sort<_Compare>(__first, __i, __comp);
-            // _VSTD::__sort<_Compare>(__i+1, __last, __comp);
-            __first = ++__i;
-        }
-        else
-        {
-            _VSTD::__sort<_Compare>(__i+1, __last, __comp);
-            // _VSTD::__sort<_Compare>(__first, __i, __comp);
-            __last = __i;
-        }
-    }
+void __bitsetsort_loop(
+    _RandomAccessIterator __first, _RandomAccessIterator __last,
+    _Compare __comp,
+    typename _VSTD::iterator_traits<_RandomAccessIterator>::value_type* __buff,
+    typename _VSTD::iterator_traits<_RandomAccessIterator>::difference_type __limit) {
+  _LIBCPP_CONSTEXPR_AFTER_CXX11 int __ninther_threshold = 128;
+  typedef typename _VSTD::iterator_traits<_RandomAccessIterator>::difference_type
+      difference_type;
+  __sorting_network::__conditional_swap<_RandomAccessIterator, _Compare>
+      __cond_swap(__comp);
+  __sorting_network::__reverse_conditional_swap<_RandomAccessIterator, _Compare>
+      __reverse_cond_swap(__comp);
+  while (true) {
+    if (__limit == 0) {
+      // Fallback to heap sort as Introsort suggests.
+      _VSTD::make_heap(__first, __last, __comp);
+      _VSTD::sort_heap(__first, __last, __comp);
+    }
+    __limit--;
+    difference_type __len = __last - __first;
+    if (__len <= __bitonic::__detail::__batch) {
+      __sorting_network::__sort1to8(__first, __len, __cond_swap);
+    } else if (__len <= 32) {
+      __bitonic::__small_sort(__first, __len, __buff, __cond_swap,
+                               __reverse_cond_swap);
+      // __bitonic::__sort9to32(__first, __len, __buff, __cond_swap,
+      //                           __reverse_cond_swap);
+    difference_type __half_len = __len / 2;
+    if (__len > __ninther_threshold) {
+      __sorting_network::__sort3(__first, __first + __half_len, __last - 1, __cond_swap);
+      __sorting_network::__sort3(__first + 1, __first + (__half_len - 1), __last - 2, __cond_swap);
+      __sorting_network::__sort3(__first + 2, __first + (__half_len + 1), __last - 3, __cond_swap);
+      __sorting_network::__sort3(__first + (__half_len - 1), __first + __half_len,
+              __first + (__half_len + 1), __cond_swap);
+      _VSTD::iter_swap(__first, __first + __half_len);
+    } else {
+      __sorting_network::__sort3(__first + __half_len, __first, __last - 1, __cond_swap);
+    auto __ret = __bitset_partition<__64bit_set>(__first, __last, __comp);
+    if (__ret.second) {
+      bool __left = __partial_insertion_sort(__first, __ret.first, __comp);
+      if (__partial_insertion_sort(__ret.first + 1, __last, __comp)) {
+        if (__left) return;
+        __last = __ret.first;
+        continue;
+      } else {
+        if (__left) {
+          __first = ++__ret.first;
+
+    // Sort smaller range with recursive call and larger with tail recursion
+    // elimination.
+    if (__ret.first - __first < __last - __ret.first) {
+      __bitsetsort_loop<_Compare>(__first, __ret.first, __comp, __buff, __limit);
+      __first = ++__ret.first;
+    } else {
+      __bitsetsort_loop<_Compare>(__ret.first + 1, __last, __comp, __buff, __limit);
+      __last = __ret.first;
 }
 
-template <class _Compare, class _Tp>
-inline _LIBCPP_INLINE_VISIBILITY
-void
-__sort(_Tp** __first, _Tp** __last, __less<_Tp*>&)
-{
-    __less<uintptr_t> __comp;
-    _VSTD::__sort<__less<uintptr_t>&, uintptr_t*>((uintptr_t*)__first, (uintptr_t*)__last, __comp);
-}
-
-_LIBCPP_EXTERN_TEMPLATE(_LIBCPP_FUNC_VIS void __sort<__less<char>&, char*>(char*, char*, __less<char>&))
-_LIBCPP_EXTERN_TEMPLATE(_LIBCPP_FUNC_VIS void __sort<__less<wchar_t>&, wchar_t*>(wchar_t*, wchar_t*, __less<wchar_t>&))
-_LIBCPP_EXTERN_TEMPLATE(_LIBCPP_FUNC_VIS void __sort<__less<signed char>&, signed char*>(signed char*, signed char*, __less<signed char>&))
-_LIBCPP_EXTERN_TEMPLATE(_LIBCPP_FUNC_VIS void __sort<__less<unsigned char>&, unsigned char*>(unsigned char*, unsigned char*, __less<unsigned char>&))
-_LIBCPP_EXTERN_TEMPLATE(_LIBCPP_FUNC_VIS void __sort<__less<short>&, short*>(short*, short*, __less<short>&))
-_LIBCPP_EXTERN_TEMPLATE(_LIBCPP_FUNC_VIS void __sort<__less<unsigned short>&, unsigned short*>(unsigned short*, unsigned short*, __less<unsigned short>&))
-_LIBCPP_EXTERN_TEMPLATE(_LIBCPP_FUNC_VIS void __sort<__less<int>&, int*>(int*, int*, __less<int>&))
-_LIBCPP_EXTERN_TEMPLATE(_LIBCPP_FUNC_VIS void __sort<__less<unsigned>&, unsigned*>(unsigned*, unsigned*, __less<unsigned>&))
-_LIBCPP_EXTERN_TEMPLATE(_LIBCPP_FUNC_VIS void __sort<__less<long>&, long*>(long*, long*, __less<long>&))
-_LIBCPP_EXTERN_TEMPLATE(_LIBCPP_FUNC_VIS void __sort<__less<unsigned long>&, unsigned long*>(unsigned long*, unsigned long*, __less<unsigned long>&))
-_LIBCPP_EXTERN_TEMPLATE(_LIBCPP_FUNC_VIS void __sort<__less<long long>&, long long*>(long long*, long long*, __less<long long>&))
-_LIBCPP_EXTERN_TEMPLATE(_LIBCPP_FUNC_VIS void __sort<__less<unsigned long long>&, unsigned long long*>(unsigned long long*, unsigned long long*, __less<unsigned long long>&))
-_LIBCPP_EXTERN_TEMPLATE(_LIBCPP_FUNC_VIS void __sort<__less<float>&, float*>(float*, float*, __less<float>&))
-_LIBCPP_EXTERN_TEMPLATE(_LIBCPP_FUNC_VIS void __sort<__less<double>&, double*>(double*, double*, __less<double>&))
-_LIBCPP_EXTERN_TEMPLATE(_LIBCPP_FUNC_VIS void __sort<__less<long double>&, long double*>(long double*, long double*, __less<long double>&))
-
-_LIBCPP_EXTERN_TEMPLATE(_LIBCPP_FUNC_VIS bool __insertion_sort_incomplete<__less<char>&, char*>(char*, char*, __less<char>&))
-_LIBCPP_EXTERN_TEMPLATE(_LIBCPP_FUNC_VIS bool __insertion_sort_incomplete<__less<wchar_t>&, wchar_t*>(wchar_t*, wchar_t*, __less<wchar_t>&))
-_LIBCPP_EXTERN_TEMPLATE(_LIBCPP_FUNC_VIS bool __insertion_sort_incomplete<__less<signed char>&, signed char*>(signed char*, signed char*, __less<signed char>&))
-_LIBCPP_EXTERN_TEMPLATE(_LIBCPP_FUNC_VIS bool __insertion_sort_incomplete<__less<unsigned char>&, unsigned char*>(unsigned char*, unsigned char*, __less<unsigned char>&))
-_LIBCPP_EXTERN_TEMPLATE(_LIBCPP_FUNC_VIS bool __insertion_sort_incomplete<__less<short>&, short*>(short*, short*, __less<short>&))
-_LIBCPP_EXTERN_TEMPLATE(_LIBCPP_FUNC_VIS bool __insertion_sort_incomplete<__less<unsigned short>&, unsigned short*>(unsigned short*, unsigned short*, __less<unsigned short>&))
-_LIBCPP_EXTERN_TEMPLATE(_LIBCPP_FUNC_VIS bool __insertion_sort_incomplete<__less<int>&, int*>(int*, int*, __less<int>&))
-_LIBCPP_EXTERN_TEMPLATE(_LIBCPP_FUNC_VIS bool __insertion_sort_incomplete<__less<unsigned>&, unsigned*>(unsigned*, unsigned*, __less<unsigned>&))
-_LIBCPP_EXTERN_TEMPLATE(_LIBCPP_FUNC_VIS bool __insertion_sort_incomplete<__less<long>&, long*>(long*, long*, __less<long>&))
-_LIBCPP_EXTERN_TEMPLATE(_LIBCPP_FUNC_VIS bool __insertion_sort_incomplete<__less<unsigned long>&, unsigned long*>(unsigned long*, unsigned long*, __less<unsigned long>&))
-_LIBCPP_EXTERN_TEMPLATE(_LIBCPP_FUNC_VIS bool __insertion_sort_incomplete<__less<long long>&, long long*>(long long*, long long*, __less<long long>&))
-_LIBCPP_EXTERN_TEMPLATE(_LIBCPP_FUNC_VIS bool __insertion_sort_incomplete<__less<unsigned long long>&, unsigned long long*>(unsigned long long*, unsigned long long*, __less<unsigned long long>&))
-_LIBCPP_EXTERN_TEMPLATE(_LIBCPP_FUNC_VIS bool __insertion_sort_incomplete<__less<float>&, float*>(float*, float*, __less<float>&))
-_LIBCPP_EXTERN_TEMPLATE(_LIBCPP_FUNC_VIS bool __insertion_sort_incomplete<__less<double>&, double*>(double*, double*, __less<double>&))
-_LIBCPP_EXTERN_TEMPLATE(_LIBCPP_FUNC_VIS bool __insertion_sort_incomplete<__less<long double>&, long double*>(long double*, long double*, __less<long double>&))
+template<typename _Number>
+inline _LIBCPP_INLINE_VISIBILITY _Number __log2i(_Number __n) {
+    _Number __log2 = 0;
+    while (__n > 1) {
+      __log2++;
+      __n >>= 1;
+    }
+    return __log2;
+}
 
-_LIBCPP_EXTERN_TEMPLATE(_LIBCPP_FUNC_VIS unsigned __sort5<__less<long double>&, long double*>(long double*, long double*, long double*, long double*, long double*, __less<long double>&))
+template <class _Compare, class _RandomAccessIterator>
+inline _LIBCPP_INLINE_VISIBILITY void __bitsetsort_internal(
+    _RandomAccessIterator __first, _RandomAccessIterator __last,
+    _Compare __comp) {
+  typedef typename _VSTD::iterator_traits<_RandomAccessIterator>::value_type
+      value_type;
+  typedef typename _VSTD::iterator_traits<_RandomAccessIterator>::difference_type
+      difference_type;
+  typename _VSTD::aligned_storage<sizeof(value_type)>::type
+      __buff[__bitonic::__detail::__small_sort_max];
+
+  // 2*log2 comes from Introsort https://reviews.llvm.org/D36423.
+  difference_type __depth_limit = 2 * __log2i(__last - __first);
+  __bitsetsort_loop(__first, __last, __comp,
+                    reinterpret_cast<value_type*>(&__buff[0]),
+                    __depth_limit);
+}
+}  // namespace __bitsetsort
 
 template <class _RandomAccessIterator, class _Compare>
 inline _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
-void
-sort(_RandomAccessIterator __first, _RandomAccessIterator __last, _Compare __comp)
-{
-    typedef typename __comp_ref_type<_Compare>::type _Comp_ref;
-    if (__libcpp_is_constant_evaluated()) {
-        _VSTD::__partial_sort<_Comp_ref>(__first, __last, __last, _Comp_ref(__comp));
-    } else {
-        _VSTD::__sort<_Comp_ref>(_VSTD::__unwrap_iter(__first), _VSTD::__unwrap_iter(__last), _Comp_ref(__comp));
-    }
+void sort(_RandomAccessIterator __first, _RandomAccessIterator __last, _Compare __comp) {
+    __bitsetsort::__bitsetsort_internal<_Comp_ref>(_VSTD::__unwrap_iter(__first), _VSTD::__unwrap_iter(__last), _Comp_ref(__comp));
 }
 
 template <class _RandomAccessIterator>
 inline _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
-void
-sort(_RandomAccessIterator __first, _RandomAccessIterator __last)
-{
-    _VSTD::sort(__first, __last, __less<typename iterator_traits<_RandomAccessIterator>::value_type>());
+void sort(_RandomAccessIterator __first, _RandomAccessIterator __last) {
 }
 
 _LIBCPP_END_NAMESPACE_STD
 
 #endif // _LIBCPP___ALGORITHM_SORT_H

libcxx/include/__algorithm/stable_sort.h

 #include <type_traits> // swap
 
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #pragma GCC system_header
 #endif
 
 _LIBCPP_BEGIN_NAMESPACE_STD
 
+template <class _Compare, class _BidirectionalIterator>
+void
+__insertion_sort(_BidirectionalIterator __first, _BidirectionalIterator __last, _Compare __comp)
+{
+    typedef typename iterator_traits<_BidirectionalIterator>::value_type value_type;
+    if (__first != __last)
+    {
+        _BidirectionalIterator __i = __first;
+        for (++__i; __i != __last; ++__i)
+        {
+            _BidirectionalIterator __j = __i;
+            value_type __t(_VSTD::move(*__j));
+            for (_BidirectionalIterator __k = __i; __k != __first && __comp(__t,  *--__k); --__j)
+                *__j = _VSTD::move(*__k);
+            *__j = _VSTD::move(__t);
+        }
+    }
+}
+
+template <class _Compare, class _BidirectionalIterator>
+void
+__insertion_sort_move(_BidirectionalIterator __first1, _BidirectionalIterator __last1,
+                      typename iterator_traits<_BidirectionalIterator>::value_type* __first2, _Compare __comp)
+{
+    typedef typename iterator_traits<_BidirectionalIterator>::value_type value_type;
+    if (__first1 != __last1)
+    {
+        __destruct_n __d(0);
+        unique_ptr<value_type, __destruct_n&> __h(__first2, __d);
+        value_type* __last2 = __first2;
+        ::new ((void*)__last2) value_type(_VSTD::move(*__first1));
+        __d.template __incr<value_type>();
+        for (++__last2; ++__first1 != __last1; ++__last2)
+        {
+            value_type* __j2 = __last2;
+            value_type* __i2 = __j2;
+            if (__comp(*__first1, *--__i2))
+            {
+                ::new ((void*)__j2) value_type(_VSTD::move(*__i2));
+                __d.template __incr<value_type>();
+                for (--__j2; __i2 != __first2 && __comp(*__first1,  *--__i2); --__j2)
+                    *__j2 = _VSTD::move(*__i2);
+                *__j2 = _VSTD::move(*__first1);
+            }
+            else
+            {
+                ::new ((void*)__j2) value_type(_VSTD::move(*__first1));
+                __d.template __incr<value_type>();
+            }
+        }
+        __h.release();
+    }
+}
+
 template <class _Compare, class _InputIterator1, class _InputIterator2>
 void
 __merge_move_construct(_InputIterator1 __first1, _InputIterator1 __last1,
         _InputIterator2 __first2, _InputIterator2 __last2,
         typename iterator_traits<_InputIterator1>::value_type* __result, _Compare __comp)
 {
     typedef typename iterator_traits<_InputIterator1>::value_type value_type;
     __destruct_n __d(0);

libcxx/src/CMakeLists.txt

 set(LIBCXX_LIB_CMAKEFILES_DIR "${CMAKE_CURRENT_BINARY_DIR}${CMAKE_FILES_DIRECTORY}"  PARENT_SCOPE)
 
 # Get sources
 set(LIBCXX_SOURCES
-  algorithm.cpp
   any.cpp
   atomic.cpp
   barrier.cpp
   bind.cpp
   charconv.cpp
   chrono.cpp
   condition_variable.cpp
   condition_variable_destructor.cpp
   exception.cpp
   functional.cpp
   future.cpp
   hash.cpp
   include/apple_availability.h
   include/atomic_support.h
   include/config_elast.h
   include/refstring.h
+  legacy-sort.cpp
   memory.cpp
   mutex.cpp
   mutex_destructor.cpp
   new.cpp
   optional.cpp
   random_shuffle.cpp
   shared_mutex.cpp
   stdexcept.cpp

libcxx/src/algorithm.cpp

-//===----------------------- algorithm.cpp --------------------------------===//
-//
-// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
-// See https://llvm.org/LICENSE.txt for license information.
-// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
-//
-//===----------------------------------------------------------------------===//
-
-#include "algorithm"
-
-_LIBCPP_BEGIN_NAMESPACE_STD
-
-template void __sort<__less<char>&, char*>(char*, char*, __less<char>&);
-template void __sort<__less<wchar_t>&, wchar_t*>(wchar_t*, wchar_t*, __less<wchar_t>&);
-template void __sort<__less<signed char>&, signed char*>(signed char*, signed char*, __less<signed char>&);
-template void __sort<__less<unsigned char>&, unsigned char*>(unsigned char*, unsigned char*, __less<unsigned char>&);
-template void __sort<__less<short>&, short*>(short*, short*, __less<short>&);
-template void __sort<__less<unsigned short>&, unsigned short*>(unsigned short*, unsigned short*, __less<unsigned short>&);
-template void __sort<__less<int>&, int*>(int*, int*, __less<int>&);
-template void __sort<__less<unsigned>&, unsigned*>(unsigned*, unsigned*, __less<unsigned>&);
-template void __sort<__less<long>&, long*>(long*, long*, __less<long>&);
-template void __sort<__less<unsigned long>&, unsigned long*>(unsigned long*, unsigned long*, __less<unsigned long>&);
-template void __sort<__less<long long>&, long long*>(long long*, long long*, __less<long long>&);
-template void __sort<__less<unsigned long long>&, unsigned long long*>(unsigned long long*, unsigned long long*, __less<unsigned long long>&);
-template void __sort<__less<float>&, float*>(float*, float*, __less<float>&);
-template void __sort<__less<double>&, double*>(double*, double*, __less<double>&);
-template void __sort<__less<long double>&, long double*>(long double*, long double*, __less<long double>&);
-
-template bool __insertion_sort_incomplete<__less<char>&, char*>(char*, char*, __less<char>&);
-template bool __insertion_sort_incomplete<__less<wchar_t>&, wchar_t*>(wchar_t*, wchar_t*, __less<wchar_t>&);
-template bool __insertion_sort_incomplete<__less<signed char>&, signed char*>(signed char*, signed char*, __less<signed char>&);
-template bool __insertion_sort_incomplete<__less<unsigned char>&, unsigned char*>(unsigned char*, unsigned char*, __less<unsigned char>&);
-template bool __insertion_sort_incomplete<__less<short>&, short*>(short*, short*, __less<short>&);
-template bool __insertion_sort_incomplete<__less<unsigned short>&, unsigned short*>(unsigned short*, unsigned short*, __less<unsigned short>&);
-template bool __insertion_sort_incomplete<__less<int>&, int*>(int*, int*, __less<int>&);
-template bool __insertion_sort_incomplete<__less<unsigned>&, unsigned*>(unsigned*, unsigned*, __less<unsigned>&);
-template bool __insertion_sort_incomplete<__less<long>&, long*>(long*, long*, __less<long>&);
-template bool __insertion_sort_incomplete<__less<unsigned long>&, unsigned long*>(unsigned long*, unsigned long*, __less<unsigned long>&);
-template bool __insertion_sort_incomplete<__less<long long>&, long long*>(long long*, long long*, __less<long long>&);
-template bool __insertion_sort_incomplete<__less<unsigned long long>&, unsigned long long*>(unsigned long long*, unsigned long long*, __less<unsigned long long>&);
-template bool __insertion_sort_incomplete<__less<float>&, float*>(float*, float*, __less<float>&);
-template bool __insertion_sort_incomplete<__less<double>&, double*>(double*, double*, __less<double>&);
-template bool __insertion_sort_incomplete<__less<long double>&, long double*>(long double*, long double*, __less<long double>&);
-
-template unsigned __sort5<__less<long double>&, long double*>(long double*, long double*, long double*, long double*, long double*, __less<long double>&);
-
-_LIBCPP_END_NAMESPACE_STD

libcxx/src/legacy-sort.cpp

 //===----------------------------------------------------------------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 
-#ifndef _LIBCPP___ALGORITHM_SORT_H
-#define _LIBCPP___ALGORITHM_SORT_H
+//
+// This file contains the legacy implementation of std::sort that used to ship
+// with libc++. Since we used to explicitly instantiate some specializations of
+// the sorting functions in the built library (to improve code size), we can't
+// just remove those symbols without breaking ABI.
+//
+// Hence, the old std::sort implementation remains in this file and we keep
+// exporting the necessary symbols to keep our ABI stable, however programs
+// compiled against newer versions of libc++ will never actually rely on those
+// symbols.
+//
 
-#include <__config>
-#include <__algorithm/comp.h>
-#include <__algorithm/comp_ref_type.h>
-#include <__algorithm/min_element.h>
-#include <__algorithm/partial_sort.h>
-#include <__algorithm/unwrap_iter.h>
-#include <__utility/swap.h>
-#include <memory>
-
-#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
-#pragma GCC system_header
-#endif
+#include "algorithm"
+#include "memory"
+#include "utility"
 
 _LIBCPP_BEGIN_NAMESPACE_STD
 
 // stable, 2-3 compares, 0-2 swaps
 
 template <class _Compare, class _ForwardIterator>
 _LIBCPP_CONSTEXPR_AFTER_CXX11 unsigned
 __sort3(_ForwardIterator __x, _ForwardIterator __y, _ForwardIterator __z, _Compare __c)
if (__c(*__x5, *__x4))
                     ++__r;
                 }
             }
         }
     }
     return __r;
 }
 
-// Assumes size > 0
-template <class _Compare, class _BidirectionalIterator>
-_LIBCPP_CONSTEXPR_AFTER_CXX11 void
-__selection_sort(_BidirectionalIterator __first, _BidirectionalIterator __last, _Compare __comp)
-{
-    _BidirectionalIterator __lm1 = __last;
-    for (--__lm1; __first != __lm1; ++__first)
-    {
-        _BidirectionalIterator __i = _VSTD::min_element<_BidirectionalIterator, _Compare&>(__first, __last, __comp);
-        if (__i != __first)
-            swap(*__first, *__i);
-    }
-}
-
-template <class _Compare, class _BidirectionalIterator>
-void
-__insertion_sort(_BidirectionalIterator __first, _BidirectionalIterator __last, _Compare __comp)
-{
-    typedef typename iterator_traits<_BidirectionalIterator>::value_type value_type;
-    if (__first != __last)
-    {
-        _BidirectionalIterator __i = __first;
-        for (++__i; __i != __last; ++__i)
-        {
-            _BidirectionalIterator __j = __i;
-            value_type __t(_VSTD::move(*__j));
-            for (_BidirectionalIterator __k = __i; __k != __first && __comp(__t,  *--__k); --__j)
-                *__j = _VSTD::move(*__k);
-            *__j = _VSTD::move(__t);
-        }
-    }
-}
-
 template <class _Compare, class _RandomAccessIterator>
 void
 __insertion_sort_3(_RandomAccessIterator __first, _RandomAccessIterator __last, _Compare __comp)
 {
     typedef typename iterator_traits<_RandomAccessIterator>::value_type value_type;
     _RandomAccessIterator __j = __first+2;
     _VSTD::__sort3<_Compare>(__first, __first+1, __j, __comp);
     for (_RandomAccessIterator __i = __j+1; __i != __last; ++__i)
for (_RandomAccessIterator __i = __j+1; __i != __last; ++__i)
             if (++__count == __limit)
                 return ++__i == __last;
         }
         __j = __i;
     }
     return true;
 }
 
-template <class _Compare, class _BidirectionalIterator>
-void
-__insertion_sort_move(_BidirectionalIterator __first1, _BidirectionalIterator __last1,
-                      typename iterator_traits<_BidirectionalIterator>::value_type* __first2, _Compare __comp)
-{
-    typedef typename iterator_traits<_BidirectionalIterator>::value_type value_type;
-    if (__first1 != __last1)
-    {
-        __destruct_n __d(0);
-        unique_ptr<value_type, __destruct_n&> __h(__first2, __d);
-        value_type* __last2 = __first2;
-        ::new ((void*)__last2) value_type(_VSTD::move(*__first1));
-        __d.template __incr<value_type>();
-        for (++__last2; ++__first1 != __last1; ++__last2)
-        {
-            value_type* __j2 = __last2;
-            value_type* __i2 = __j2;
-            if (__comp(*__first1, *--__i2))
-            {
-                ::new ((void*)__j2) value_type(_VSTD::move(*__i2));
-                __d.template __incr<value_type>();
-                for (--__j2; __i2 != __first2 && __comp(*__first1,  *--__i2); --__j2)
-                    *__j2 = _VSTD::move(*__i2);
-                *__j2 = _VSTD::move(*__first1);
-            }
-            else
-            {
-                ::new ((void*)__j2) value_type(_VSTD::move(*__first1));
-                __d.template __incr<value_type>();
-            }
-        }
-        __h.release();
-    }
-}
-
 template <class _Compare, class _RandomAccessIterator>
 void
 __sort(_RandomAccessIterator __first, _RandomAccessIterator __last, _Compare __comp)
 {
     typedef typename iterator_traits<_RandomAccessIterator>::difference_type difference_type;
     typedef typename iterator_traits<_RandomAccessIterator>::value_type value_type;
     const difference_type __limit = is_trivially_copy_constructible<value_type>::value &&
                                     is_trivially_copy_assignable<value_type>::value ? 30 : 6;
__restart:
                 // It is now known that a guard exists for downward moving __j
                 while (!__comp(*--__j, *__m))
                     ;
                 if (__i > __j)
                     break;
                 swap(*__i, *__j);
                 ++__n_swaps;
                 // It is known that __m != __j
                 // If __m just moved, follow it
                 if (__m == __i)
                     __m = __j;
                 ++__i;
             }
         }
         // [__first, __i) < *__m and *__m <= [__i, __last)
         if (__i != __m && __comp(*__m, *__i))
         {
             swap(*__i, *__m);
             ++__n_swaps;
         }
         // [__first, __i) < *__i and *__i <= [__i+1, __last)
         // If we were given a perfect partition, see if insertion sort is quick...
         if (__n_swaps == 0)
         {
             bool __fs = _VSTD::__insertion_sort_incomplete<_Compare>(__first, __i, __comp);
             if (_VSTD::__insertion_sort_incomplete<_Compare>(__i+1, __last, __comp))
             {
                 if (__fs)
                     return;
                 __last = __i;
                 continue;
             }
             else
             {
                 if (__fs)
                 {
                     __first = ++__i;
                     continue;
                 }
             }
         }
         // sort smaller range with recursive call and larger with tail recursion elimination
         if (__i - __first < __last - __i)
         {
             _VSTD::__sort<_Compare>(__first, __i, __comp);
             // _VSTD::__sort<_Compare>(__i+1, __last, __comp);
             __first = ++__i;
         }
         else
         {
             _VSTD::__sort<_Compare>(__i+1, __last, __comp);
             // _VSTD::__sort<_Compare>(__first, __i, __comp);
             __last = __i;
         }
     }
 }
 
-template <class _Compare, class _Tp>
-inline _LIBCPP_INLINE_VISIBILITY
-void
-__sort(_Tp** __first, _Tp** __last, __less<_Tp*>&)
-{
-    __less<uintptr_t> __comp;
-    _VSTD::__sort<__less<uintptr_t>&, uintptr_t*>((uintptr_t*)__first, (uintptr_t*)__last, __comp);
-}
-
-_LIBCPP_EXTERN_TEMPLATE(_LIBCPP_FUNC_VIS void __sort<__less<char>&, char*>(char*, char*, __less<char>&))
-_LIBCPP_EXTERN_TEMPLATE(_LIBCPP_FUNC_VIS void __sort<__less<wchar_t>&, wchar_t*>(wchar_t*, wchar_t*, __less<wchar_t>&))
-_LIBCPP_EXTERN_TEMPLATE(_LIBCPP_FUNC_VIS void __sort<__less<signed char>&, signed char*>(signed char*, signed char*, __less<signed char>&))
-_LIBCPP_EXTERN_TEMPLATE(_LIBCPP_FUNC_VIS void __sort<__less<unsigned char>&, unsigned char*>(unsigned char*, unsigned char*, __less<unsigned char>&))
-_LIBCPP_EXTERN_TEMPLATE(_LIBCPP_FUNC_VIS void __sort<__less<short>&, short*>(short*, short*, __less<short>&))
-_LIBCPP_EXTERN_TEMPLATE(_LIBCPP_FUNC_VIS void __sort<__less<unsigned short>&, unsigned short*>(unsigned short*, unsigned short*, __less<unsigned short>&))
-_LIBCPP_EXTERN_TEMPLATE(_LIBCPP_FUNC_VIS void __sort<__less<int>&, int*>(int*, int*, __less<int>&))
-_LIBCPP_EXTERN_TEMPLATE(_LIBCPP_FUNC_VIS void __sort<__less<unsigned>&, unsigned*>(unsigned*, unsigned*, __less<unsigned>&))
-_LIBCPP_EXTERN_TEMPLATE(_LIBCPP_FUNC_VIS void __sort<__less<long>&, long*>(long*, long*, __less<long>&))
-_LIBCPP_EXTERN_TEMPLATE(_LIBCPP_FUNC_VIS void __sort<__less<unsigned long>&, unsigned long*>(unsigned long*, unsigned long*, __less<unsigned long>&))
-_LIBCPP_EXTERN_TEMPLATE(_LIBCPP_FUNC_VIS void __sort<__less<long long>&, long long*>(long long*, long long*, __less<long long>&))
-_LIBCPP_EXTERN_TEMPLATE(_LIBCPP_FUNC_VIS void __sort<__less<unsigned long long>&, unsigned long long*>(unsigned long long*, unsigned long long*, __less<unsigned long long>&))
-_LIBCPP_EXTERN_TEMPLATE(_LIBCPP_FUNC_VIS void __sort<__less<float>&, float*>(float*, float*, __less<float>&))
-_LIBCPP_EXTERN_TEMPLATE(_LIBCPP_FUNC_VIS void __sort<__less<double>&, double*>(double*, double*, __less<double>&))
-_LIBCPP_EXTERN_TEMPLATE(_LIBCPP_FUNC_VIS void __sort<__less<long double>&, long double*>(long double*, long double*, __less<long double>&))
-
-_LIBCPP_EXTERN_TEMPLATE(_LIBCPP_FUNC_VIS bool __insertion_sort_incomplete<__less<char>&, char*>(char*, char*, __less<char>&))
-_LIBCPP_EXTERN_TEMPLATE(_LIBCPP_FUNC_VIS bool __insertion_sort_incomplete<__less<wchar_t>&, wchar_t*>(wchar_t*, wchar_t*, __less<wchar_t>&))
-_LIBCPP_EXTERN_TEMPLATE(_LIBCPP_FUNC_VIS bool __insertion_sort_incomplete<__less<signed char>&, signed char*>(signed char*, signed char*, __less<signed char>&))
-_LIBCPP_EXTERN_TEMPLATE(_LIBCPP_FUNC_VIS bool __insertion_sort_incomplete<__less<unsigned char>&, unsigned char*>(unsigned char*, unsigned char*, __less<unsigned char>&))
-_LIBCPP_EXTERN_TEMPLATE(_LIBCPP_FUNC_VIS bool __insertion_sort_incomplete<__less<short>&, short*>(short*, short*, __less<short>&))
-_LIBCPP_EXTERN_TEMPLATE(_LIBCPP_FUNC_VIS bool __insertion_sort_incomplete<__less<unsigned short>&, unsigned short*>(unsigned short*, unsigned short*, __less<unsigned short>&))
-_LIBCPP_EXTERN_TEMPLATE(_LIBCPP_FUNC_VIS bool __insertion_sort_incomplete<__less<int>&, int*>(int*, int*, __less<int>&))
-_LIBCPP_EXTERN_TEMPLATE(_LIBCPP_FUNC_VIS bool __insertion_sort_incomplete<__less<unsigned>&, unsigned*>(unsigned*, unsigned*, __less<unsigned>&))
-_LIBCPP_EXTERN_TEMPLATE(_LIBCPP_FUNC_VIS bool __insertion_sort_incomplete<__less<long>&, long*>(long*, long*, __less<long>&))
-_LIBCPP_EXTERN_TEMPLATE(_LIBCPP_FUNC_VIS bool __insertion_sort_incomplete<__less<unsigned long>&, unsigned long*>(unsigned long*, unsigned long*, __less<unsigned long>&))
-_LIBCPP_EXTERN_TEMPLATE(_LIBCPP_FUNC_VIS bool __insertion_sort_incomplete<__less<long long>&, long long*>(long long*, long long*, __less<long long>&))
-_LIBCPP_EXTERN_TEMPLATE(_LIBCPP_FUNC_VIS bool __insertion_sort_incomplete<__less<unsigned long long>&, unsigned long long*>(unsigned long long*, unsigned long long*, __less<unsigned long long>&))
-_LIBCPP_EXTERN_TEMPLATE(_LIBCPP_FUNC_VIS bool __insertion_sort_incomplete<__less<float>&, float*>(float*, float*, __less<float>&))
-_LIBCPP_EXTERN_TEMPLATE(_LIBCPP_FUNC_VIS bool __insertion_sort_incomplete<__less<double>&, double*>(double*, double*, __less<double>&))
-_LIBCPP_EXTERN_TEMPLATE(_LIBCPP_FUNC_VIS bool __insertion_sort_incomplete<__less<long double>&, long double*>(long double*, long double*, __less<long double>&))
+template _LIBCPP_FUNC_VIS void __sort<__less<char>&, char*>(char*, char*, __less<char>&);
+template _LIBCPP_FUNC_VIS void __sort<__less<wchar_t>&, wchar_t*>(wchar_t*, wchar_t*, __less<wchar_t>&);
+template _LIBCPP_FUNC_VIS void __sort<__less<signed char>&, signed char*>(signed char*, signed char*, __less<signed char>&);
+template _LIBCPP_FUNC_VIS void __sort<__less<unsigned char>&, unsigned char*>(unsigned char*, unsigned char*, __less<unsigned char>&);
+template _LIBCPP_FUNC_VIS void __sort<__less<short>&, short*>(short*, short*, __less<short>&);
+template _LIBCPP_FUNC_VIS void __sort<__less<unsigned short>&, unsigned short*>(unsigned short*, unsigned short*, __less<unsigned short>&);
+template _LIBCPP_FUNC_VIS void __sort<__less<int>&, int*>(int*, int*, __less<int>&);
+template _LIBCPP_FUNC_VIS void __sort<__less<unsigned>&, unsigned*>(unsigned*, unsigned*, __less<unsigned>&);
+template _LIBCPP_FUNC_VIS void __sort<__less<long>&, long*>(long*, long*, __less<long>&);
+template _LIBCPP_FUNC_VIS void __sort<__less<unsigned long>&, unsigned long*>(unsigned long*, unsigned long*, __less<unsigned long>&);
+template _LIBCPP_FUNC_VIS void __sort<__less<long long>&, long long*>(long long*, long long*, __less<long long>&);
+template _LIBCPP_FUNC_VIS void __sort<__less<unsigned long long>&, unsigned long long*>(unsigned long long*, unsigned long long*, __less<unsigned long long>&);
+template _LIBCPP_FUNC_VIS void __sort<__less<float>&, float*>(float*, float*, __less<float>&);
+template _LIBCPP_FUNC_VIS void __sort<__less<double>&, double*>(double*, double*, __less<double>&);
+template _LIBCPP_FUNC_VIS void __sort<__less<long double>&, long double*>(long double*, long double*, __less<long double>&);
+
+template _LIBCPP_FUNC_VIS bool __insertion_sort_incomplete<__less<char>&, char*>(char*, char*, __less<char>&);
+template _LIBCPP_FUNC_VIS bool __insertion_sort_incomplete<__less<wchar_t>&, wchar_t*>(wchar_t*, wchar_t*, __less<wchar_t>&);
+template _LIBCPP_FUNC_VIS bool __insertion_sort_incomplete<__less<signed char>&, signed char*>(signed char*, signed char*, __less<signed char>&);
+template _LIBCPP_FUNC_VIS bool __insertion_sort_incomplete<__less<unsigned char>&, unsigned char*>(unsigned char*, unsigned char*, __less<unsigned char>&);
+template _LIBCPP_FUNC_VIS bool __insertion_sort_incomplete<__less<short>&, short*>(short*, short*, __less<short>&);
+template _LIBCPP_FUNC_VIS bool __insertion_sort_incomplete<__less<unsigned short>&, unsigned short*>(unsigned short*, unsigned short*, __less<unsigned short>&);
+template _LIBCPP_FUNC_VIS bool __insertion_sort_incomplete<__less<int>&, int*>(int*, int*, __less<int>&);
+template _LIBCPP_FUNC_VIS bool __insertion_sort_incomplete<__less<unsigned>&, unsigned*>(unsigned*, unsigned*, __less<unsigned>&);
+template _LIBCPP_FUNC_VIS bool __insertion_sort_incomplete<__less<long>&, long*>(long*, long*, __less<long>&);
+template _LIBCPP_FUNC_VIS bool __insertion_sort_incomplete<__less<unsigned long>&, unsigned long*>(unsigned long*, unsigned long*, __less<unsigned long>&);
+template _LIBCPP_FUNC_VIS bool __insertion_sort_incomplete<__less<long long>&, long long*>(long long*, long long*, __less<long long>&);
+template _LIBCPP_FUNC_VIS bool __insertion_sort_incomplete<__less<unsigned long long>&, unsigned long long*>(unsigned long long*, unsigned long long*, __less<unsigned long long>&);
+template _LIBCPP_FUNC_VIS bool __insertion_sort_incomplete<__less<float>&, float*>(float*, float*, __less<float>&);
+template _LIBCPP_FUNC_VIS bool __insertion_sort_incomplete<__less<double>&, double*>(double*, double*, __less<double>&);
+template _LIBCPP_FUNC_VIS bool __insertion_sort_incomplete<__less<long double>&, long double*>(long double*, long double*, __less<long double>&);
 
-_LIBCPP_EXTERN_TEMPLATE(_LIBCPP_FUNC_VIS unsigned __sort5<__less<long double>&, long double*>(long double*, long double*, long double*, long double*, long double*, __less<long double>&))
-
-template <class _RandomAccessIterator, class _Compare>
-inline _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
-void
-sort(_RandomAccessIterator __first, _RandomAccessIterator __last, _Compare __comp)
-{
-    typedef typename __comp_ref_type<_Compare>::type _Comp_ref;
-    if (__libcpp_is_constant_evaluated()) {
-        _VSTD::__partial_sort<_Comp_ref>(__first, __last, __last, _Comp_ref(__comp));
-    } else {
-        _VSTD::__sort<_Comp_ref>(_VSTD::__unwrap_iter(__first), _VSTD::__unwrap_iter(__last), _Comp_ref(__comp));
-    }
-}
-
-template <class _RandomAccessIterator>
-inline _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
-void
-sort(_RandomAccessIterator __first, _RandomAccessIterator __last)
-{
-    _VSTD::sort(__first, __last, __less<typename iterator_traits<_RandomAccessIterator>::value_type>());
-}
+template _LIBCPP_FUNC_VIS unsigned __sort5<__less<long double>&, long double*>(long double*, long double*, long double*, long double*, long double*, __less<long double>&);
 
 _LIBCPP_END_NAMESPACE_STD
-
-#endif // _LIBCPP___ALGORITHM_SORT_H