[NVPTX] Use .common linkage for common globals

[AlexMaclean]

Switch from .weak to .common linkage for common global variables where possible. The .common linkage is described in PTX ISA 11.6.4. Linking Directives: .common

Declares identifier to be globally visible but “common”.

Common symbols are similar to globally visible symbols. However multiple object files may declare the same common symbol and they may have different types and sizes and references to a symbol get resolved against a common symbol with the largest size.

Only one object file can initialize a common symbol and that must have the largest size among all other definitions of that common symbol from different object files.

.common linking directive can be used only on variables with .global storage. It cannot be used on function symbols or on symbols with opaque type.

[Artem-B]

@MaskRay can you double check that PTX's idea of common matches what LLVM promises for GVar->hasCommonLinkage()?

The patch LGTM otherwise.

[MaskRay]

I am not familiar with NVPTX. Is the common symbol behavior similar to things described on https://maskray.me/blog/2022-02-06-all-about-common-symbols ?

[Artem-B]

My understanding is that we should follow the "standard" linking rules for C++ and ELF objects.

The quote in the PR description is all NVIDIA docs say about the .common directive. On the surface it does seem to match the standard behavior of common data, but I'm not very familiar with it. I mostly dealt with the problems it caused when some things were made common when they should not have been.

[Artem-B]

Looks like with this patch we're generating .common even when we're compiling for older PTX version.

The common-linkage.ll test actually fails if you happen to run it with ptxas enabled.

[Sterling-Augustine]

I went ahead and reverted this in d4a8e97 due to the breakage under ptxas.