<algorithm>: reverse_copy optimization is not activated under C++20/C++23 mode

[statementreply]

Describe the bug

STL/stl/inc/algorithm

Lines 4366 to 4387 in 2f9f567

	#pragma warning(suppress : 6326) // Potential comparison of a constant with another constant
	if constexpr (_Allow_vectorization && _Nx <= 8 && (_Nx & (_Nx - 1)) == 0) {
	#if _HAS_CXX20
	if (!_STD is_constant_evaluated())
	#endif // _HAS_CXX20
	{
	if constexpr (_Nx == 1) {
	__std_reverse_copy_trivially_copyable_1(_To_address(_UFirst), _To_address(_ULast), _To_address(_UDest));
	} else if constexpr (_Nx == 2) {
	__std_reverse_copy_trivially_copyable_2(_To_address(_UFirst), _To_address(_ULast), _To_address(_UDest));
	} else if constexpr (_Nx == 4) {
	__std_reverse_copy_trivially_copyable_4(_To_address(_UFirst), _To_address(_ULast), _To_address(_UDest));
	} else {
	__std_reverse_copy_trivially_copyable_8(_To_address(_UFirst), _To_address(_ULast), _To_address(_UDest));
	}
	_UDest += _ULast - _UFirst;
	_Seek_wrapped(_Dest, _UDest);
	return _Dest;
	}
	}
	#endif // _USE_STD_VECTOR_ALGORITHMS

There is an optimized code path in reverse_copy for contiguous ranges of small trivial types. However, for some reasons it is only activated under /std:c++14 and /std:c++17, not under /std:c++20 or /std:c++latest.

Command-line test case
https://godbolt.org/z/9bc17xT8e

C:\Users\He\source\temp>type reverse_copy.cpp

#include <algorithm>
#include <chrono>
#include <iostream>
#include <iterator>
#include <random>

constexpr int len = 100'000;
constexpr int iters = 100'000;

unsigned char src[len];
unsigned char dst[len];

int main() {
    std::mt19937 rng;
    std::uniform_int_distribution<> dist{0, 255};
    for (auto& x : src) {
        x = static_cast<unsigned char>(dist(rng));
    }

    const auto t0 = std::chrono::steady_clock::now();
    for (int i = 0; i < iters; ++i) {
        std::reverse_copy(std::cbegin(src), std::cend(src), std::begin(dst));
    }
    const auto t1 = std::chrono::steady_clock::now();

    std::cout << std::chrono::duration<double>{t1 - t0}.count() << " s\n";
    return 0;
}

C:\Users\He\source\temp>cl /EHsc /W4 /O2 /std:c++17 reverse_copy.cpp
用于 x64 的 Microsoft (R) C/C++ 优化编译器 19.31.30818 版
版权所有(C) Microsoft Corporation。保留所有权利。

reverse_copy.cpp
Microsoft (R) Incremental Linker Version 14.31.30818.0
Copyright (C) Microsoft Corporation.  All rights reserved.

/out:reverse_copy.exe
reverse_copy.obj

C:\Users\He\source\temp>.\reverse_copy.exe
0.194523 s

C:\Users\He\source\temp>cl /EHsc /W4 /O2 /std:c++20 reverse_copy.cpp
用于 x64 的 Microsoft (R) C/C++ 优化编译器 19.31.30818 版
版权所有(C) Microsoft Corporation。保留所有权利。

reverse_copy.cpp
Microsoft (R) Incremental Linker Version 14.31.30818.0
Copyright (C) Microsoft Corporation.  All rights reserved.

/out:reverse_copy.exe
reverse_copy.obj

C:\Users\He\source\temp>.\reverse_copy.exe
3.23843 s

STL version
2f9f567

[AlexGuteniev]

Check for contiguous_iterator fails for a pointer with top level const

template<std::contiguous_iterator I>
struct S {};

int main()
{
    S<char*> s1;
    S<const char*> s2;
    S<char* const> s3; // fails
}

[miscco]

As discussed in discord, this is actually a copy & paste bug:

STL/stl/inc/algorithm

Line 4362 in 2f9f567

bool_constant<_Iterators_are_contiguous<decltype(_UFirst), decltype(_UDest)>>, is_trivially_copyable<_Elem>,

We are checking the type of _UFirst, which is declared as a const variable as we are actually iterating over _ULast, because this is a reverse algorithm.

So the solution would be to change that line to use _ULast rather than _UFirst