[llvm] [RISCV][GISEL] Generate VLMax using RISCV::VLMaxSentinel (PR #110778)

[Craig Topper via llvm-commits]

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@@ -859,19 +859,21 @@ static MachineInstrBuilder buildAllOnesMask(LLT VecTy, const SrcOp &VL,
 
 /// Gets the two common "VL" operands: an all-ones mask and the vector length.
 /// VecTy is a scalable vector type.
-static std::pair<MachineInstrBuilder, Register>
+static std::pair<MachineInstrBuilder, MachineInstrBuilder>
 buildDefaultVLOps(const DstOp &Dst, MachineIRBuilder &MIB,
                   MachineRegisterInfo &MRI) {
   LLT VecTy = Dst.getLLTTy(MRI);
   assert(VecTy.isScalableVector() && "Expecting scalable container type");
-  Register VL(RISCV::X0);
-  MachineInstrBuilder Mask = buildAllOnesMask(VecTy, VL, MIB, MRI);
+  const RISCVSubtarget &STI = MIB.getMF().getSubtarget<RISCVSubtarget>();
+  LLT XLenTy(STI.getXLenVT());
+  auto VL = MIB.buildConstant(XLenTy, RISCV::VLMaxSentinel);
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topperc wrote:

This should be `-1` not VLMaxSentinel. VLMaxSentinel encoding belongs to RISCV post-isel instructions. The specific value could be any value other than 0-31 since those are the only immediate values allowed post-isel.

The constant created here is a pre-isel value and we need it be a value larger than vlmax which can be as large as 65536. vmclr_vl/vmset_vl/etc. will eventually be provided VLs that come from users.

https://github.com/llvm/llvm-project/pull/110778