[llvm] [RISCV][GISEL] Legalize G_EXTRACT_SUBVECTOR (PR #109426)

[Matt Arsenault via llvm-commits]

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@@ -931,6 +935,133 @@ bool RISCVLegalizerInfo::legalizeSplatVector(MachineInstr &MI,
   return true;
 }
 
+static LLT getLMUL1Ty(LLT VecTy) {
+  assert(VecTy.getElementType().getSizeInBits() <= 64 &&
+         "Unexpected vector LLT");
+  return LLT::scalable_vector(RISCV::RVVBitsPerBlock /
+                                  VecTy.getElementType().getSizeInBits(),
+                              VecTy.getElementType());
+}
+
+bool RISCVLegalizerInfo::legalizeExtractSubvector(MachineInstr &MI,
+                                                  MachineIRBuilder &MIB) const {
+  GExtractSubvector &ES = cast<GExtractSubvector>(MI);
+
+  MachineRegisterInfo &MRI = *MIB.getMRI();
+
+  Register Dst = ES.getReg(0);
+  Register Src = ES.getSrcVec();
+  uint64_t Idx = ES.getIndexImm();
+
+  // Only support vectors using custom legalization. We know the DstTy is a
+  // vector since we used that to decide whether to custom legalize or not.
+  LLT BigTy = MRI.getType(Src);
+  if (BigTy.isScalar())
+    return false;
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arsenm wrote:

This is malformed MIR 

https://github.com/llvm/llvm-project/pull/109426