[PATCH] D115661: [clang][amdgpu] - Choose when to promote VarDecl to address space 4.

Tue Dec 14 12:39:37 PST 2021

arsenm added a comment.

In D115661#3193152 <https://reviews.llvm.org/D115661#3193152>, @yaxunl wrote:

> In D115661#3192983 <https://reviews.llvm.org/D115661#3192983>, @estewart08 wrote:
>
>> In D115661#3190477 <https://reviews.llvm.org/D115661#3190477>, @yaxunl wrote:
>>
>>> This may cause perf regressions for HIP.
>>
>> Do you have a test that would show such a regression? Emitting a store to address space (4) in a constructor seems the wrong thing to do.
>
> The two lit tests which changed from addr space 4 to 1 demonstrated that. In alias analysis, if a variable is in addr space 4, the backend knows that it is constant and can do optimizations on it. After changing to addr space 1, those optimizations are gone.

The backend also knows because the constant flag is set on the global variable. Addrspace(4) is a kludge which is largely redundant with other mechanisms for indicating constants

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