[llvm] [BPF] Support Jump Table (PR #149715)

[via llvm-commits]

================
@@ -259,3 +264,40 @@ unsigned BPFInstrInfo::removeBranch(MachineBasicBlock &MBB,
 
   return Count;
 }
+
+int BPFInstrInfo::getJumpTableIndex(const MachineInstr &MI) const {
+  // The pattern looks like:
+  // %0 = LD_imm64 %jump-table.0   ; load jump-table address
+  // %1 = ADD_rr %0, $another_reg  ; address + offset
+  // %2 = LDD %1, 0                ; load the actual label
+  // JX %2
+  const MachineFunction &MF = *MI.getParent()->getParent();
+  const MachineRegisterInfo &MRI = MF.getRegInfo();
+
+  Register Reg = MI.getOperand(0).getReg();
+  if (!Reg.isVirtual())
+    return -1;
+  MachineInstr *Ldd = MRI.getUniqueVRegDef(Reg);
+  if (Ldd == nullptr || Ldd->getOpcode() != BPF::LDD)
----------------
eddyz87 wrote:

I inserted a print here, and `getJumpTableIndex` always returns `-1` for [this](https://gist.github.com/eddyz87/c2f7ff145260d52d5ff7e968e8b06dea) test case.
The reason is that `Ldd` is not `BPF::LDD`: `Ldd=%16:gpr = LDWSX killed %15:gpr, 0 :: (load (s32) from jump-table)` .
Given that this method is optional maybe add a test case that is sensitive to matches here or just drop it?

https://github.com/llvm/llvm-project/pull/149715