AArch64: Don't emit CFI for SCS register in nounwind functions.

All that you can legitimately do with the CFI for a nounwind function
is get a backtrace, and adjusting the SCS register is not (currently)
required for this purpose.

Differential Revision: https://reviews.llvm.org/D54988

llvm-svn: 348035

llvm/lib/Target/AArch64/AArch64FrameLowering.cpp

@@ -1733,20 +1733,22 @@ bool AArch64FrameLowering::spillCalleeSavedRegisters(
       BuildMI(MBB, MI, DL, TII.get(AArch64::SEH_Nop))
           .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, CFIInst));
-    BuildMI(MBB, MI, DL, TII.get(AArch64::CFI_INSTRUCTION))
-        .addCFIIndex(CFIIndex)
-        .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, 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

@@ -46,3 +46,11 @@ define i32 @f4() shadowcallstack {
   ; CHECK: ret
   ret i32 %res1234
 }
+
+define i32 @f5() shadowcallstack nounwind {
+  ; CHECK: f5:
+  ; CHECK-NOT: .cfi_escape
+  %res = call i32 @bar()
+  %res1 = add i32 %res, 1
+  ret i32 %res
+}