AArch64: don't sign the return address in noreturn functions

[t.p.northover]

Summary

A simple little change that could save a few bytes space on occasion. If a function doesn't return then signing its return address is a bit pointless.

[t.p.northover]

Add context to diff.

[danielkiss]

The produced code for the non return function might save an unsigned LR to the stack and that might be usable to craft a gadget IMHO. The probability is quite low but still.

llvm/lib/Target/AArch64/AArch64FrameLowering.cpp

   // - sign-return-address=non-leaf and the functions spills the LR
 
   const Function &F = MF.getFunction();
+  if (F.hasFnAttribute(Attribute::NoReturn))
+    return false;
+
   if (!F.hasFnAttribute("sign-return-address"))
     return false;
 

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

 ; CHECK: pacibsp
 ; CHECK-NOT: ret
 ; CHECK: retab
-; CHECK-NOT: ret
+; CHECK-NOT: ret{{$}}
 define i32 @leaf_sign_all_v83_b_key(i32 %x) "sign-return-address"="all" "target-features"="+v8.3a" "sign-return-address-key"="b_key" {
   ret i32 %x
 }
+
+; CHECK-LABEL: @noreturn_nosign
+; CHECK-NOT: pacibsp
+define i32 @noreturn_nosign(i32 %x) "sign-return-address"="all" noreturn {
+  ret i32 %x
+}