[AArch64] Emit CFI instruction for updating x18 when using ShadowCallStack with exception unwinding

PR45875 notes an instance where exception handling crashes on aarch64-fuchsia
where SCS is enabled by default. The underlying issue seems to be that within libunwind,
various _Unwind_* functions, the x18 register is not updated if a function is marked
with nounwind. This removes the check for nounwind and emits the CFI instruction that updates x18.

Differential Revision: https://reviews.llvm.org/D79822

Author: PiJoules

llvm/lib/Target/AArch64/AArch64FrameLowering.cpp

@@ -2494,22 +2494,20 @@ bool AArch64FrameLowering::spillCalleeSavedRegisters(
       BuildMI(MBB, MI, DL, TII.get(AArch64::SEH_Nop))
           .setMIFlag(MachineInstr::FrameSetup);
 
-    if (!MF.getFunction().hasFnAttribute(Attribute::NoUnwind)) {
-      // Emit a CFI instruction that causes 8 to be subtracted from the value of
-      // x18 when unwinding past this frame.
-      static const char CFIInst[] = {
-          dwarf::DW_CFA_val_expression,
-          18, // register
-          2,  // length
-          static_cast<char>(unsigned(dwarf::DW_OP_breg18)),
-          static_cast<char>(-8) & 0x7f, // addend (sleb128)
-      };
-      unsigned CFIIndex = MF.addFrameInst(MCCFIInstruction::createEscape(
-          nullptr, StringRef(CFIInst, sizeof(CFIInst))));
-      BuildMI(MBB, MI, DL, TII.get(AArch64::CFI_INSTRUCTION))
-          .addCFIIndex(CFIIndex)
-          .setMIFlag(MachineInstr::FrameSetup);
-    }
+    // Emit a CFI instruction that causes 8 to be subtracted from the value of
+    // x18 when unwinding past this frame.
+    static const char CFIInst[] = {
+        dwarf::DW_CFA_val_expression,
+        18, // register
+        2,  // length
+        static_cast<char>(unsigned(dwarf::DW_OP_breg18)),
+        static_cast<char>(-8) & 0x7f, // addend (sleb128)
+    };
+    unsigned CFIIndex = MF.addFrameInst(MCCFIInstruction::createEscape(
+        nullptr, StringRef(CFIInst, sizeof(CFIInst))));
+    BuildMI(MBB, MI, DL, TII.get(AArch64::CFI_INSTRUCTION))
+        .addCFIIndex(CFIIndex)
+        .setMIFlag(MachineInstr::FrameSetup);
 
     // This instruction also makes x18 live-in to the entry block.
     MBB.addLiveIn(AArch64::X18);

llvm/test/CodeGen/AArch64/shadow-call-stack.ll

@@ -58,3 +58,10 @@ define i32 @f5() shadowcallstack nounwind {
   ret i32 %res
 }
 
+define i32 @f6() shadowcallstack nounwind uwtable {
+  ; CHECK: f6:
+  ; CHECK: .cfi_escape 0x16, 0x12, 0x02, 0x82, 0x78
+  %res = call i32 @bar()
+  %res1 = add i32 %res, 1
+  ret i32 %res
+}