Modernize macros to use `do { } while (0)` instead of `{ }` about zstd

[terrelln]

Based on this discussion VS2012 does emit this warning.

[terrelln]

And in the same discussion, looks like it was fixed in VS2015.

[terrelln]

I suspect V2013 does have the bug, since it was mentioned it was fixed in 2015.

[terrelln]

It turns out we already have a few macros that use do { } while (0) in our error_private.h header. And we've made a release and had no complaints. So we should be able to add it no problem.

[Cyan4973]

Yes, Visual Studio has been the main reason to not employ while (0).

Which minimal version of Visual Studio is required to avoid triggering a "constant branch" warning ?

[terrelln]

Which minimal version of Visual Studio is required to avoid triggering a "constant branch" warning ?

I'm not really sure. I tried to trigger it in godbolt, but I don't think they have a old enough version.

[Cyan4973]

While CI seems safe, by virtue of supporting only fairly recent versions of Visual Studio,
this decision may have consequences on https://github.com/facebook/zstd/tree/dev/build,
since we are supporting Visual Studio since 2008 here.

Depending on how we want to handle that, we might have to drop a few versions.
Your investigation suggests that versions <= 2012 would trigger a warning.
So I think that leaves only one version, VS2013, as a question mark.

[Cyan4973]

I wonder if there would be a way to disable this warning from within the project file,
or possibly even better, from within the cmake script that will generate the project file.

[terrelln]

I don't really know what our policy should be here. We will still compile with older Visual Studios, it will just emit warnings. So our users would have to turn off warnings-as-errors for older Visual Studios.

We should be able to disable the warning using __pragma(warning(suppress:4127)). So we could probably push that pragma in our portability header, and then use do { } while (0) without generating those warnings.