[llvm] [X86][GlobalISel] Enable G_BRJT (PR #81811)

[Evgenii Kudriashov via llvm-commits]

================
@@ -549,6 +559,64 @@ X86LegalizerInfo::X86LegalizerInfo(const X86Subtarget &STI,
   verify(*STI.getInstrInfo());
 }
 
+bool X86LegalizerInfo::legalizeCustom(LegalizerHelper &Helper, MachineInstr &MI,
+                                      LostDebugLocObserver &LocObserver) const {
+  switch (MI.getOpcode()) {
+  case G_BRJT:
+    return legalizeBrJT(Helper, MI);
+  default:
+    llvm_unreachable("instruction is not in switch");
+  }
+}
+
+bool X86LegalizerInfo::legalizeBrJT(LegalizerHelper &Helper,
+                                    MachineInstr &MI) const {
+  MachineIRBuilder &MIB = Helper.MIRBuilder;
+  MachineFunction &MF = *MI.getMF();
+  MachineRegisterInfo &MRI = *MIB.getMRI();
+  const MachineJumpTableInfo *MJTI = MF.getJumpTableInfo();
+
+  unsigned EntrySize = MJTI->getEntrySize(MF.getDataLayout());
+
+  auto PtrReg = MI.getOperand(0).getReg();
+  auto PtrTy = MRI.getType(PtrReg);
+  auto IdxReg = MI.getOperand(2).getReg();
+  auto IdxTy = MRI.getType(IdxReg);
+
+  MachineMemOperand *MMO = MF.getMachineMemOperand(
+      MachinePointerInfo::getJumpTable(MF), MachineMemOperand::MOLoad,
+      PtrTy.getSizeInBytes(), Align(EntrySize));
+
+  auto ShiftAmt = MIB.buildConstant(IdxTy, Log2_32(PtrTy.getSizeInBytes()));
+  auto Shift = MIB.buildShl(IdxTy, IdxReg, ShiftAmt);
+  auto Target =
+      MIB.buildPtrAdd(PtrTy, PtrReg, Shift->getOperand(0).getReg()).getReg(0);
----------------
e-kud wrote:

You can simply pass `Shift` instead of `Shift->getOperand(0).getReg()`. Also you can drop `getReg(0)` when reassigning `Target` and invoke `getReg` once on the argument of `buildBRIndirect`.

https://github.com/llvm/llvm-project/pull/81811