[llvm] [RISCV] Refactor subreg indices. (PR #77173)

[Craig Topper via llvm-commits]

topperc wrote:

> > > I don't think any of these names are an improvement. Why are you of the view Zacas needs this renaming?
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> > Zacas needs a pair of GPRs. sub_32 is clearly not the right name for the lower half a RV64 GPR pair.
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> But the flip side is SubRegIndex<32> isn't half of a GPR pair for RV64. What you've changed the name to only applies to RV32, whilst it was previously true for RV64 too (even if not so useful).
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> > Should we make SubReg size and offsets HwMode aware?
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> Ideally yes; for CHERI-RISC-V downstream we make the X-in-C subreg be size -1 as a result of not having that.
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> > Or have different regclasses for pairs for RV32 and RV64
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> Would be sad to go down that road...
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> > If we make subregs hwmode aware then we definitely need different indices for GPR and FPR.
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> This is true, but I don't see the point of renaming them until that's possible.

Would setting the size of sub_gpr_even/odd to -1 work?

https://github.com/llvm/llvm-project/pull/77173