fixes #2568. MSVC optimizations causing segmentation faults in optimized builds.

[sg-james]

Works around an msvc optimizer bug which occurs when shape.size() == 0.

Surrounding the problematic region to prevent illegal access that occurs on windows MSVC optimized builds (/O1, /O2).

• allows construction of 0-sized xarray and xtensors (e.g. xt::xtensor<size_t, 0>)
• also fixes xt::argmax() crashing

Checklist

• The title and commit message(s) are descriptive.
• Small commits made to fix your PR have been squashed to avoid history pollution.
• Tests have been added for new features or bug fixes.
• API of new functions and classes are documented.

Description

Fixes #2551
Fixes #2568
Fixes #2596

[tdegeus]

Thanks a lot!

It could be nice to do the workaround only on MSVC (on potentially only on the affected version) using some pre-processor statement?

[sg-james]

pushed with msvc guards. I also added a test which I think covers all 3 issues.

[tdegeus]

I would have probably only added

        #if defined(_MSC_VER) && (1931 <= _MSC_VER)
             // Workaround MSVC compiler optimization bug, xtensor#2568
             if (shape.size() == 0) {
                 return static_cast<std::size_t>(data_size);
             }
        #endif 

to keep the implementation cleaner. That should work too no?

[sg-james]

oh. yes, that's much better. I'll fix that up

[SylvainCorlay]

This is amazing!

It may be worth adding a comment and a reference to this compiler issue in https://xtensor.readthedocs.io/en/latest/compilers.html.

[sg-james]

Would something like the following do? (not so familiar with .rst)

Visual Studio 2022 (19.31+) workaround inline compiler optimization bug
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

In `xstrides.hpp`, added an early return inside `compute_strides` when ``shape.size() == 0`` to 
prevent a run time crash from occuring. Without this guard statement, instructions from inside the 
for loop were somehow being reached, despite being logically unreachable. 
Original issue  `here. <https://github.com/xtensor-stack/xtensor/issues/2568>`_ 
Upstream issue `here. <https://developercommunity.visualstudio.com/t/Runtime-Crash-msvc--1931-with-optimiz/10134617>`_

[tdegeus]

Great! Good to go. Thanks a lot for getting to the bottom of this