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

[Matt Arsenault via llvm-commits]

================
@@ -1054,6 +1058,142 @@ bool RISCVLegalizerInfo::legalizeExtractSubvector(MachineInstr &MI,
   return true;
 }
 
+bool RISCVLegalizerInfo::legalizeInsertSubvector(MachineInstr &MI,
+                                                 LegalizerHelper &Helper,
+                                                 MachineIRBuilder &MIB) const {
+  GInsertSubvector &IS = cast<GInsertSubvector>(MI);
+
+  MachineRegisterInfo &MRI = *MIB.getMRI();
+
+  Register Dst = IS.getReg(0);
+  Register BigVec = IS.getBigVec();
+  Register LitVec = IS.getSubVec();
+  uint64_t Idx = IS.getIndexImm();
+
+  LLT BigTy = MRI.getType(BigVec);
+  LLT LitTy = MRI.getType(LitVec);
+
+  if (Idx == 0 ||
+      MRI.getVRegDef(BigVec)->getOpcode() == TargetOpcode::G_IMPLICIT_DEF)
+    return true;
+
+  // 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)) {
+    auto BigTyMinElts = BigTy.getElementCount().getKnownMinValue();
+    auto LitTyMinElts = LitTy.getElementCount().getKnownMinValue();
+    if (BigTyMinElts >= 8 && LitTyMinElts >= 8)
----------------
arsenm wrote:

Yes, if you require that you can't do it in the rule. But do you really need this to be in the rule? I would expect that folding out the undef part to only require an optimizing combine 

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