[PATCH] D41651: AMDGPU: Add 32-bit constant address space

[Matt Arsenault via Phabricator via llvm-commits]

arsenm added a comment.

In https://reviews.llvm.org/D41651#988249, @mareko wrote:

> In https://reviews.llvm.org/D41651#986619, @nhaehnle wrote:
>
> > This needs documentation in AMDGPUUsage.rst.
> >
> > Relying on metadata for correctness is indeed not okay. We should either say that CONSTANT_ADDRESS_32BIT just assumes uniformness, and move the address to an SGPR (via v_readfirstlane) if required, *or* support this also with VMEM instructions.
>
>
> Here is why relying on metadata is OK.
>
> The behavior of 64-bit pointers:
>
> - If the address is in VGPRs and amdgpu.uniform is not dropped, you'll get readfirstlane and correct behavior.
> - If the address is in VGPRs and amdgpu.uniform is dropped by a random pass, you'll get SMEM opcodes reading descriptors from VGPRs, so you'll get an invalid binary without an error and a GPU hang.
>
>   The behavior for 32-bit pointers:
> - If the address is in VGPRs and amdgpu.uniform is not dropped, you'll get readfirstlane and correct behavior.
> - If the address is in VGPRs and amdgpu.uniform is dropped by a random pass, you'll get a compile error.


This is exactly why it's not OK? If it's dropped you get a compile error or miscompile


Repository:
  rL LLVM

https://reviews.llvm.org/D41651