[llvm] [RISCV][GISEL] Legalize G_INSERT_SUBVECTOR (PR #108859)

[Michael Maitland via llvm-commits]

================
@@ -915,6 +921,152 @@ 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::legalizeInsertSubvector(MachineInstr &MI,
+                                                 MachineIRBuilder &MIB) const {
+  GInsertSubvector &IS = cast<GInsertSubvector>(MI);
+
+  MachineRegisterInfo &MRI = *MIB.getMRI();
+
+  Register Dst = IS.getOperand(0).getReg();
+  Register Src1 = IS.getBigVec();
+  Register Src2 = IS.getSubVec();
+  uint64_t Idx = IS.getIndexImm();
+
+  LLT BigTy = MRI.getType(Src1);
+  LLT LitTy = MRI.getType(Src2);
+  Register BigVec = Src1;
+  Register LitVec = Src2;
+
+  // We don't have the ability to slide mask vectors up indexed by their i1
+  // elements; the smallest we can do is i8. Often we are able to bitcast to
+  // equivalent i8 vectors. Otherwise, we can must zeroextend to equivalent i8
+  // vectors and truncate down after the insert.
+  if (LitTy.getElementType() == LLT::scalar(1) &&
+      (Idx != 0 ||
+       MRI.getVRegDef(BigVec)->getOpcode() != TargetOpcode::G_IMPLICIT_DEF)) {
+    auto BigTyMinElts = BigTy.getElementCount().getKnownMinValue();
+    auto LitTyMinElts = LitTy.getElementCount().getKnownMinValue();
+    if (BigTyMinElts >= 8 && LitTyMinElts >= 8) {
+      assert(Idx % 8 == 0 && "Invalid index");
+      assert(BigTyMinElts % 8 == 0 && LitTyMinElts % 8 == 0 &&
+             "Unexpected mask vector lowering");
+      Idx /= 8;
+      BigTy = LLT::vector(BigTy.getElementCount().divideCoefficientBy(8), 8);
+      LitTy = LLT::vector(LitTy.getElementCount().divideCoefficientBy(8), 8);
+      BigVec = MIB.buildBitcast(BigTy, BigVec).getReg(0);
+      LitVec = MIB.buildBitcast(LitTy, LitVec).getReg(0);
+    } else {
+      // We can't slide this mask vector up indexed by its i1 elements.
+      // This poses a problem when we wish to insert a scalable vector which
+      // can't be re-expressed as a larger type. Just choose the slow path and
+      // extend to a larger type, then truncate back down.
+      LLT ExtBigTy = BigTy.changeElementType(LLT::scalar(8));
+      LLT ExtLitTy = LitTy.changeElementType(LLT::scalar(8));
+      auto BigZExt = MIB.buildZExt(ExtBigTy, BigVec);
+      auto LitZExt = MIB.buildZExt(ExtLitTy, LitVec);
+      auto Insert = MIB.buildInsertSubvector(ExtBigTy, BigZExt, LitZExt, Idx);
+      auto SplatZero = MIB.buildSplatVector(
+          ExtBigTy, MIB.buildConstant(ExtBigTy.getElementType(), 0));
+      MIB.buildICmp(CmpInst::Predicate::ICMP_NE, Dst, Insert, SplatZero);
+      MI.eraseFromParent();
+      return true;
+    }
+  }
+
+  const RISCVRegisterInfo *TRI = STI.getRegisterInfo();
+  MVT LitTyMVT = getMVTForLLT(LitTy);
+  unsigned SubRegIdx, RemIdx;
+  std::tie(SubRegIdx, RemIdx) =
+      RISCVTargetLowering::decomposeSubvectorInsertExtractToSubRegs(
+          getMVTForLLT(BigTy), LitTyMVT, Idx, TRI);
+
+  RISCVII::VLMUL SubVecLMUL = RISCVTargetLowering::getLMUL(getMVTForLLT(LitTy));
+  bool IsSubVecPartReg = !RISCVVType::decodeVLMUL(SubVecLMUL).second;
----------------
michaelmaitland wrote:

@topperc 

This code was actually removed in f565b79f9fc27 for SelectionDAG. I wrote this patch before that code existed. I suppose we should take the new code instead?

https://github.com/llvm/llvm-project/pull/108859