[all-commits] [llvm/llvm-project] 7f8451: [RISCV] Use vsetvli instead of vlenb in Prologue/E...

[Kito Cheng via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: 7f8451c868cdc91481f9629517db3f53349514b7
      https://github.com/llvm/llvm-project/commit/7f8451c868cdc91481f9629517db3f53349514b7
  Author: Kito Cheng <kito.cheng at sifive.com>
  Date:   2025-03-21 (Fri, 21 Mar 2025)

  Changed paths:
    M llvm/lib/Target/RISCV/RISCVExpandPseudoInsts.cpp
    M llvm/lib/Target/RISCV/RISCVFeatures.td
    M llvm/lib/Target/RISCV/RISCVFrameLowering.cpp
    M llvm/lib/Target/RISCV/RISCVInstrInfoVPseudos.td
    M llvm/lib/Target/RISCV/RISCVRegisterInfo.cpp
    M llvm/test/CodeGen/RISCV/rvv/allocate-lmul-2-4-8.ll
    M llvm/test/CodeGen/RISCV/rvv/rv32-spill-vector-csr.ll
    M llvm/test/CodeGen/RISCV/rvv/rv32-spill-vector.ll
    M llvm/test/CodeGen/RISCV/rvv/rv32-spill-zvlsseg.ll
    M llvm/test/CodeGen/RISCV/rvv/rv64-spill-vector-csr.ll
    M llvm/test/CodeGen/RISCV/rvv/rv64-spill-vector.ll
    M llvm/test/CodeGen/RISCV/rvv/rv64-spill-zvlsseg.ll

  Log Message:
  -----------
  [RISCV] Use vsetvli instead of vlenb in Prologue/Epilogue (#113756)

Currently, we use `csrr` with `vlenb` to obtain the `VLEN`, but this is
not the only option. We can also use `vsetvli` with `e8`/`m1` to get
`VLENMAX`, which is equal to the VLEN. This method is preferable on some
microarchitectures and makes it easier to obtain values like `VLEN * 2`,
`VLEN * 4`, or `VLEN * 8`, reducing the number of instructions needed to
calculate VLEN multiples.

However, this approach is *NOT* always interchangeable, as it changes
the state of `VTYPE` and `VL`, which can alter the behavior of vector
instructions, potentially causing incorrect code generation if applied
after a vsetvli insertion. Therefore, we limit its use to the
prologue/epilogue for now, as there are no vector operations within the
prologue/epilogue sequence.

With further analysis, we may extend this approach beyond the
prologue/epilogue in the future, but starting here should be a good
first step.

This feature is gurded by the `+prefer-vsetvli-over-read-vlenb` feature,
which is disabled by default for now.



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