AArch64: Don't use RETA[AB] when ShadowCallStack is enabled.

[pcc]

Summary

When returning from a function with both SCS and PAC-RET enabled, we need to
authenticate the return address from the stack and then load from the SCS,
but this was happening in the reverse order when RETA[AB] were being used.
Fix it by disabling the use of RETA[AB] when SCS is enabled.

Fixes pr58072.

[danielkiss]

LGTM, Thanks!

llvm/lib/Target/AArch64/AArch64FrameLowering.cpp

static void InsertReturnAddressAuth(MachineFunction &MF,
   if (MBBI != MBB.end())
     DL = MBBI->getDebugLoc();
 
   // The AUTIASP instruction assembles to a hint instruction before v8.3a so
   // this instruction can safely used for any v8a architecture.
   // From v8.3a onwards there are optimised authenticate LR and return
   // instructions, namely RETA{A,B}, that can be used instead. In this case the
   // DW_CFA_AARCH64_negate_ra_state can't be emitted.
-  if (Subtarget.hasPAuth() && MBBI != MBB.end() &&
-      MBBI->getOpcode() == AArch64::RET_ReallyLR) {
+  if (Subtarget.hasPAuth() &&
+      !MF.getFunction().hasFnAttribute(Attribute::ShadowCallStack) &&
+      MBBI != MBB.end() && MBBI->getOpcode() == AArch64::RET_ReallyLR) {
     BuildMI(MBB, MBBI, DL,
             TII->get(MFI.shouldSignWithBKey() ? AArch64::RETAB : AArch64::RETAA))
         .copyImplicitOps(*MBBI);
     MBB.erase(MBBI);
   } else {
     BuildMI(
         MBB, MBBI, DL,
         TII->get(MFI.shouldSignWithBKey() ? AArch64::AUTIBSP : AArch64::AUTIASP))

llvm/test/CodeGen/AArch64/sign-return-address.ll

 ; CHECK-NEXT:        .cfi_negate_ra_state
 ; CHECK:             ret
 ; CHECK-V83A:        retaa
 define i32 @non_leaf_sign_non_leaf(i32 %x) "sign-return-address"="non-leaf"  {
   %call = call i32 @foo(i32 %x)
   ret i32 %call
 }
 
+; CHECK-LABEL: @non_leaf_scs
+; CHECK-NOT:   retaa
+define i32 @non_leaf_scs(i32 %x) "sign-return-address"="non-leaf" shadowcallstack "target-features"="+v8.3a,+reserve-x18"  {
+  %call = call i32 @foo(i32 %x)
+  ret i32 %call
+}
+
 ; CHECK-LABEL: @leaf_sign_all_v83
 ; CHECK:        pacia x30, sp
 ; CHECK-NEXT:  .cfi_negate_ra_state
 ; CHECK-NOT:   ret
 ; CHECK:       retaa
 ; CHECK-NOT:   ret
 define i32 @leaf_sign_all_v83(i32 %x) "sign-return-address"="all" "target-features"="+v8.3a" {
   ret i32 %x