Bug in zgetrs() under Windows 10 2004

[vssalbert]

When running under Windows 10 2004, if the CRT function fmod(x, y) is called with 0 for the x argument prior to calling zgetrs(), zgetrs() returns -nan(ind) for many of the values of the solved for x. The call to fmod() can occur far away and long before.

This is with Visual Studio Professional 2019 versions 16.7.2 and 16.2.3. Attached is code illustrating the problem, when run under Release the output for the vector pB contains several -nan(ind) values. 

zgetrs() returns the expected results if any of the following is true:

- the x argument to fmod() is not 0.

- Debug build is run.

- It is run under Windows 10 1909 or 1903.

- The environment variable MKL_CBWR=COMPATIBLE or MKL_CBWR=SSE2 is set.

This was tested using:

- MKL versions 2019 build 4, 2019 build 5, and 2020 build 2.

- Intel(R) Core(TM) i7-4770 CPU @ 3.40GHz 3.40 GHz, 16 GB RAM

- Intel(R) Core(TM) i7-7820HQ CPU @ 2.90GHz 2.90GHz, 32 GB RAM

- Windows 10 Pro 2004

Our production product is experiencing this problem under Windows 10 Pro 2004 with MKL version 2019 build 5. The call to fmod() is occurring much earlier than the call to zgetrs(), and is known to occur in the same thread (it may also happen in separate threads, I have not checked that scenario).

Thanks,

Albert Santos.

[RyanLivingston]

Likely related:

https://developercommunity.visualstudio.com/content/problem/1207405/fmod-after-an-update-to-windows-2004-is-causing-a.html?childToView=1243113

[Gennady_F_Intel]

yes, that's exactly the cause of this issue