[llvm-branch-commits] [llvm] [AArch64][PAC] Emit tail calls more efficiently (PR #220193)

[Victor Campos via llvm-branch-commits]

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@@ -11800,6 +11796,16 @@ void AArch64InstrInfo::createPauthEpilogueInstr(MachineBasicBlock &MBB,
   RS.enterBasicBlockEnd(MBB);
   RS.backward(InsertPt);
 
+  bool MayCheckLR = Subtarget.getAuthenticatedLRCheckMethod(MF) !=
+                    AArch64PAuth::AuthCheckMethod::None;
+  if (ImplicitDefs.empty() && MayCheckLR) {
+    // If we may have to check LR at this point, we need a scratch register -
+    // try to pick a free one among X16 and X17.
+    Register ScratchRegForCheckVA =
+        LiveRegs.available(MRI, AArch64::X16) ? AArch64::X16 : AArch64::X17;
+    ImplicitDefs.push_back(ScratchRegForCheckVA);
+  }
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vhscampos wrote:

This is really brittle because both registers might be unavailable at this point. An available register should be chosen using `RegScavenger`.

Besides, I am not sure if this is the right place to find and commit to a scratch register. In my PACMAN PR, a scratch register is chosen in `AArch64PointerAuth.cpp`. I really don't know which one is better than the other, but we must carefully think about this.

I am generally in favour of keeping `CreatePauthEpilogueInstr` as simple as possible, and perform register spilling and scratch register picking in `AArch64PointerAuth.cpp`.

https://github.com/llvm/llvm-project/pull/220193