[PATCH] D98783: [CUDA][HIP] Remove unused addr space casts

[Yaxun Liu via Phabricator via cfe-commits]

yaxunl added a comment.

In D98783#2633261 <https://reviews.llvm.org/D98783#2633261>, @tra wrote:

> In D98783#2632244 <https://reviews.llvm.org/D98783#2632244>, @yaxunl wrote:
>
>> In D98783#2632143 <https://reviews.llvm.org/D98783#2632143>, @tra wrote:
>>
>>> Shouldn't the unused ASCs be eliminated by DCE? We seem to be cleaning up the consequences of a problem that happened somewhere else.
>>
>> DCE does not eliminate these unused ASCs because normally they should not exist. That's why we do not want them to be kept in the IR emitted by clang.
>
> TBH, these 'invisible' ASCs do bother me. I wonder what else is present in the IR that we can't easily examine with existing tools?
> In principle I'm OK with eliminating such dangling ASCs here as a short-term workaround, but I think it's potentially a more general issue which needs a more general solution.

The invisible LLVM constants only happen when they are created but not used later, which is rare since usually a constant is created and used immediately, making them visible.

> One way to consistently deal with something like that is to codegen something using those ASCs, but which would be considered unused and later DCE'd along with unused ASCs. 
> We could use some sort of counterpart for `@llvm.used` only for potentially unused IR we create.
> Tying them to such `@llvm_maybe_unused` would avoid the problem.

That's one solution. Another solution is to add a member function to LLVM module to clean up unused LLVM constants and add check to LLVM IR verifier to make sure there is no unused constants.


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