[llvm-branch-commits] [llvm] [AArch64][PAC] Support BLRA* instructions in SLS Hardening pass (PR #97605)

[Kristof Beyls via llvm-branch-commits]

================
@@ -274,40 +398,31 @@ void SLSHardeningInserter::convertBLRToBL(
 
   MachineInstr &BLR = *MBBI;
   assert(isBLR(BLR));
-  unsigned BLOpcode;
-  Register Reg;
-  bool RegIsKilled;
-  switch (BLR.getOpcode()) {
-  case AArch64::BLR:
-  case AArch64::BLRNoIP:
-    BLOpcode = AArch64::BL;
-    Reg = BLR.getOperand(0).getReg();
-    assert(Reg != AArch64::X16 && Reg != AArch64::X17 && Reg != AArch64::LR);
-    RegIsKilled = BLR.getOperand(0).isKill();
-    break;
-  case AArch64::BLRAA:
-  case AArch64::BLRAB:
-  case AArch64::BLRAAZ:
-  case AArch64::BLRABZ:
-    llvm_unreachable("BLRA instructions cannot yet be produced by LLVM, "
-                     "therefore there is no need to support them for now.");
-  default:
-    llvm_unreachable("unhandled BLR");
-  }
+  const ThunkKind &Kind = *getThunkKind(BLR.getOpcode());
----------------
kbeyls wrote:

In the 20-line-ish long comment above (which I cannot comment on directly in github),
I think it would also be useful to indicate which branch instructions might be the authenticating ones. I think that might help the reader a bit better with more quickly understanding what the intended transform is. E.g. something like:
```
// Before:
  //   |-----------------------------|
  //   |      ...                    |
  //   |  instI                      |
  //   |  BLR{A|B|}{Z} xN{, xM}      |
  //   |  instJ                      |
  //   |      ...                    |
  //   |-----------------------------|
  //
  // After:
  //   |-----------------------------|
  //   |      ...                    |
  //   |  instI                      |
  //   |  BL __llvm_slsblr_thunk_xN  |
  //   |  instJ                      |
  //   |      ...                    |
  //   |-----------------------------|
  //
  //   __llvm_slsblr_thunk_xN:
  //   |-----------------------------|
  //   |  BR{A|B|}{Z} xN{, xM}       |
  //   |  barrierInsts               |
  //   |-----------------------------|
//
```

https://github.com/llvm/llvm-project/pull/97605