[llvm] [AArch64][GlobalISel] Refactor BITCAST Legalization (PR #80505)

[Matt Arsenault via llvm-commits]

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@@ -4654,11 +4654,49 @@ LegalizerHelper::fewerElementsVector(MachineInstr &MI, unsigned TypeIdx,
     return fewerElementsVectorShuffle(MI, TypeIdx, NarrowTy);
   case G_FPOWI:
     return fewerElementsVectorMultiEltType(GMI, NumElts, {2 /*pow*/});
+  case G_BITCAST:
+    return fewerElementsBitcast(MI, TypeIdx, NarrowTy);
   default:
     return UnableToLegalize;
   }
 }
 
+LegalizerHelper::LegalizeResult
+LegalizerHelper::fewerElementsBitcast(MachineInstr &MI, unsigned int TypeIdx,
+                                      LLT NarrowTy) {
+  assert(MI.getOpcode() == TargetOpcode::G_BITCAST &&
+         "Not a bitcast operation");
+
+  if (TypeIdx != 0)
+    return UnableToLegalize;
+
+  auto [DstReg, DstTy, SrcReg, SrcTy] = MI.getFirst2RegLLTs();
+
+  unsigned SrcScalSize = SrcTy.getScalarSizeInBits();
+  LLT SrcNarrowTy =
+      LLT::fixed_vector(NarrowTy.getSizeInBits() / SrcScalSize, SrcScalSize);
+
+  // Split the Src and Dst Reg into smaller registers
+  LLT SrcLeftoverTy;
+  SmallVector<Register> SrcVRegs, SrcLeftoverVRegs, BitcastVRegs;
+  // MIRBuilder.setInstrAndDebugLoc(MI);
+  if (!extractParts(SrcReg, SrcTy, SrcNarrowTy, SrcLeftoverTy, SrcVRegs,
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arsenm wrote:

I was hoping to move away from using extractParts, as it falls back on using G_EXTRACT which we don't want 

https://github.com/llvm/llvm-project/pull/80505