[GISel] Support narrowing G_ICMP with more than 2 parts. (llvm#119335)

This allows us to support i128 G_ICMP on RV32. I'm not sure how to test
the "left over" part of this as RISC-V always widens to a power of 2
before narrowing.

Author: topperc

llvm/lib/CodeGen/GlobalISel/LegalizerHelper.cpp

@@ -1716,14 +1716,9 @@ LegalizerHelper::LegalizeResult LegalizerHelper::narrowScalar(MachineInstr &MI,
   case TargetOpcode::G_ICMP: {
     Register LHS = MI.getOperand(2).getReg();
     LLT SrcTy = MRI.getType(LHS);
-    uint64_t SrcSize = SrcTy.getSizeInBits();
     CmpInst::Predicate Pred =
         static_cast<CmpInst::Predicate>(MI.getOperand(1).getPredicate());
 
-    // TODO: Handle the non-equality case for weird sizes.
-    if (NarrowSize * 2 != SrcSize && !ICmpInst::isEquality(Pred))
-      return UnableToLegalize;
-
     LLT LeftoverTy; // Example: s88 -> s64 (NarrowTy) + s24 (leftover)
     SmallVector<Register, 4> LHSPartRegs, LHSLeftoverRegs;
     if (!extractParts(LHS, SrcTy, NarrowTy, LeftoverTy, LHSPartRegs,
@@ -1775,19 +1770,59 @@ LegalizerHelper::LegalizeResult LegalizerHelper::narrowScalar(MachineInstr &MI,
         Or = MIRBuilder.buildOr(NarrowTy, Or, Xors[I]);
       MIRBuilder.buildICmp(Pred, Dst, Or, Zero);
     } else {
-      // TODO: Handle non-power-of-two types.
-      assert(LHSPartRegs.size() == 2 && "Expected exactly 2 LHS part regs?");
-      assert(RHSPartRegs.size() == 2 && "Expected exactly 2 RHS part regs?");
-      Register LHSL = LHSPartRegs[0];
-      Register LHSH = LHSPartRegs[1];
-      Register RHSL = RHSPartRegs[0];
-      Register RHSH = RHSPartRegs[1];
-      MachineInstrBuilder CmpH = MIRBuilder.buildICmp(Pred, ResTy, LHSH, RHSH);
-      MachineInstrBuilder CmpHEQ =
-          MIRBuilder.buildICmp(CmpInst::Predicate::ICMP_EQ, ResTy, LHSH, RHSH);
-      MachineInstrBuilder CmpLU = MIRBuilder.buildICmp(
-          ICmpInst::getUnsignedPredicate(Pred), ResTy, LHSL, RHSL);
-      MIRBuilder.buildSelect(Dst, CmpHEQ, CmpLU, CmpH);
+      Register CmpIn;
+      for (unsigned I = 0, E = LHSPartRegs.size(); I != E; ++I) {
+        Register CmpOut;
+        CmpInst::Predicate PartPred;
+
+        if (I == E - 1 && LHSLeftoverRegs.empty()) {
+          PartPred = Pred;
+          CmpOut = Dst;
+        } else {
+          PartPred = ICmpInst::getUnsignedPredicate(Pred);
+          CmpOut = MRI.createGenericVirtualRegister(ResTy);
+        }
+
+        if (!CmpIn) {
+          MIRBuilder.buildICmp(PartPred, CmpOut, LHSPartRegs[I],
+                               RHSPartRegs[I]);
+        } else {
+          auto Cmp = MIRBuilder.buildICmp(PartPred, ResTy, LHSPartRegs[I],
+                                          RHSPartRegs[I]);
+          auto CmpEq = MIRBuilder.buildICmp(CmpInst::Predicate::ICMP_EQ, ResTy,
+                                            LHSPartRegs[I], RHSPartRegs[I]);
+          MIRBuilder.buildSelect(CmpOut, CmpEq, CmpIn, Cmp);
+        }
+
+        CmpIn = CmpOut;
+      }
+
+      for (unsigned I = 0, E = LHSLeftoverRegs.size(); I != E; ++I) {
+        Register CmpOut;
+        CmpInst::Predicate PartPred;
+
+        if (I == E - 1 && LHSLeftoverRegs.empty()) {
+          PartPred = Pred;
+          CmpOut = Dst;
+        } else {
+          PartPred = ICmpInst::getUnsignedPredicate(Pred);
+          CmpOut = MRI.createGenericVirtualRegister(ResTy);
+        }
+
+        if (!CmpIn) {
+          MIRBuilder.buildICmp(PartPred, CmpOut, LHSLeftoverRegs[I],
+                               RHSLeftoverRegs[I]);
+        } else {
+          auto Cmp = MIRBuilder.buildICmp(PartPred, ResTy, LHSLeftoverRegs[I],
+                                          RHSLeftoverRegs[I]);
+          auto CmpEq =
+              MIRBuilder.buildICmp(CmpInst::Predicate::ICMP_EQ, ResTy,
+                                   LHSLeftoverRegs[I], RHSLeftoverRegs[I]);
+          MIRBuilder.buildSelect(CmpOut, CmpEq, CmpIn, Cmp);
+        }
+
+        CmpIn = CmpOut;
+      }
     }
     MI.eraseFromParent();
     return Legalized;