[all-commits] [llvm/llvm-project] c8136d: [RISCV] Correctly Decode Unsigned Immediates with ...

[Sam Elliott via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: c8136da26c56f44ab6a217853c58f79b88ceeb97
      https://github.com/llvm/llvm-project/commit/c8136da26c56f44ab6a217853c58f79b88ceeb97
  Author: Sam Elliott <quic_aelliott at quicinc.com>
  Date:   2025-02-25 (Tue, 25 Feb 2025)

  Changed paths:
    M llvm/lib/Target/RISCV/Disassembler/RISCVDisassembler.cpp
    M llvm/lib/Target/RISCV/RISCVInstrInfoXqci.td
    M llvm/lib/Target/RISCV/RISCVInstrInfoZc.td
    A llvm/test/MC/Disassembler/RISCV/xqci-invalid.txt

  Log Message:
  -----------
  [RISCV] Correctly Decode Unsigned Immediates with Ranges (#128584)

We currently have two operands upstream that are an unsigned immediate
with a range constraint - `uimm8ge32` (for `cm.jalt`) and `uimm5gt3`
(for `qc.shladd`).

Both of these were using `decodeUImmOperand<N>` for decoding. For `Zcmt`
this worked, because the generated decoder automatically checked for
`cm.jt` first because the 8 undefined bits in `cm.jalt` are `000?????`
in `cm.jt` (this is to do with the range lower-bound being a
power-of-two). For Zcmt, this patch is NFC.

We have less luck with `Xqciac` - `qc.shladd` is being decoded where the
`uimm5` field is 3 or lower. This patch fixes this by introducing a
`decodeUImmOperandGE<Width, LowerBound>` helper, which will corretly
return `MCDisassembler::Fail` when the immediate is below the lower
bound.

I have added a test to show the encoding where `uimm5` is equal to 3 is
no longer disassembled as `qc.shladd`.



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