[PATCH] D78570: [AMDGPU] Use RegClass helper functions in getRegForInlineAsmConstraint.

Tue Apr 21 10:14:50 PDT 2020

arsenm added inline comments.

================
Comment at: llvm/lib/Target/AMDGPU/SIISelLowering.cpp:10597-10599
       case 64:
         RC = &AMDGPU::SGPR_64RegClass;
         break;
----------------
foad wrote:
> I don't like having exceptions like this, where we override the class that getSGPRClassForBitWidth would return. There are similar things in e.g. SIRegisterInfo::getEquivalentSGPRClass.
> 
> Perhaps getSGPRClassForBitWidth et al should be split into two different helpers, "get allocatable class" and "get *something else* class". But I really don't understand the purpose and the naming of the SReg/VReg/AReg vs SGPR/VGPR/AGPR classes, so I'm not sure how to do that.
For the VGPR and AGPR cases, I think it's a naming mistake based on how these are defined in tablegen. We ended up with the register list as VGPR_* and then the class as VReg_*s for > 32.

For SGPRs, there is a distinction. SReg_* is more inclusive. SGPR_* only include number SGPRs. The various SReg_*s include different combinations of special registers like m0 and vcc or ttmp

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  https://reviews.llvm.org/D78570/new/

https://reviews.llvm.org/D78570