Fixing arm subsystem version

[Petermarcu]

Being explicit about the windows subsystem for arm and getting rid of all the linker warnings about it.

LINK : warning LNK4010: invalid subsystem version number 6.00; default subsystem version assumed

[Petermarcu]

@gkhanna79 , PTAL

[gkhanna79]

Is there a specific reason we need to set this?

[Petermarcu]

Because we are passing an invalid version today, getting a bunch of warnings about it and then letting the toolset float us to whatever it decides to default to.

[jkotas]

Do we need similar fix for arm64 as well?

[gkhanna79]

Looking at https://ci.dot.net/job/dotnet_coreclr/job/master/job/arm64_cross_checked_windows_nt/81/consoleFull, we see the same warnings for Arm64 as well. Given this, I am wondering if we even need to specify the subsystem version given that the linker will pick the correct default version as described in the table at https://msdn.microsoft.com/en-us/library/fcc1zstk.aspx.

@jkotas What do you think?

[jkotas]

the linker will pick the correct default version

It will pick Vista (6.00) as the default for x86/x64 that is not quite right because of CoreCLR does not work on Vista. Even the Win8 (6.02) default for arm is not quite right because of I do not think CoreCLR will work on Win8 ARM.

It may be ok to use the default for simplicity though.

[gkhanna79]

The change (and the current implementation) actually implement the same rules that you call out below.

It may be ok to use the default for simplicity though.

Yes, that is my goal.

[Petermarcu]

I've seen problems in the past where we weren't explicit about things like this and pulling in a new toolset floats the default to something we didn't intend. Wouldn't it be most correct to get it set to the right value that is valid for each leg?

[gkhanna79]

What we are setting explicitly is what the linker will do (see the table in the link), independent of the toolset.

Wouldn't it be most correct to get it set to the right value that is valid for each leg?

Unless CMake has a bug that we wish to workaround, I would suggest we validate that linker does the right thing by default and work from there.

[Petermarcu]

BTW, I'm happy to make the arm64 fix as well. I can also update to 6.0.1 for the non-arm platforms to represent our Win7 minimum. Once we get this right, it won't show up again until we have a new platform where the existing non-arm value isn't correct or explicitly want to drop an older platform.

[Petermarcu]

My point is that those defaults are for that version of the linker. Those defaults move over time. New versions of the linker will have different defaults. For example, before 6.00 existed, the linker defaulted to something less.

[gkhanna79]

New versions of the linker will have different defaults.

If the toolset changes its defaults for the version of subsystem it supports, I believe that would imply that the previous versions of the OS may not be supported by that toolset (and thus, specifying an explicit version may not protect us against such updates).

I would expect such toolset updates to be effectively dropping support of platforms and thus, we should be cognizant about picking them up anyways. And if we do pick such updates, then we again want the toolset to specify its defaults as opposed to us specifying something that results in new warnings (about unsupported subsystem version).

[Petermarcu]

No, thats why there is a minimum column. You can compare these two pages... minimum support moves seperately from default.

https://msdn.microsoft.com/en-us/library/fcc1zstk(v=vs.140).aspx
https://msdn.microsoft.com/en-us/library/fcc1zstk(v=vs.100).aspx

Default is a policy decision not a correctness one and we want to control that policy. I was a part of an effort a few years ago where a bunch of people broke because the default jumped from 5 to 6 and that dropped XP support even though the toolset was totally able to target 5. The fix was to go through everyones build system's and make sure they were explicit about their minimum and pass it in.

[gkhanna79]

Interesting! Given this, I am fine making the subsystem version explicit. Also, please make the corresponding changes in CoreFX and Core-Setup since you are on the roll :)

[Petermarcu]

Happy to go do it in the other places :)