Initial back-edge CFI implementation (#3606)

Give the user the option to sign and to authenticate function
return addresses with the operations introduced by the Pointer
Authentication extension to the Arm instruction set architecture.

Copyright (c) 2021, Arm Limited.

cranelift/codegen/src/isa/aarch64/mod.rs

@@ -14,7 +14,7 @@ use crate::settings as shared_settings;
 use alloc::{boxed::Box, vec::Vec};
 use core::fmt;
 use regalloc2::MachineEnv;
-use target_lexicon::{Aarch64Architecture, Architecture, Triple};
+use target_lexicon::{Aarch64Architecture, Architecture, OperatingSystem, Triple};
 
 // New backend:
 mod abi;
@@ -59,7 +59,7 @@ impl AArch64Backend {
         flags: shared_settings::Flags,
     ) -> CodegenResult<(VCode<inst::Inst>, regalloc2::Output)> {
         let emit_info = EmitInfo::new(flags.clone());
-        let abi = Box::new(abi::AArch64ABICallee::new(func, self)?);
+        let abi = Box::new(abi::AArch64ABICallee::new(func, self, &self.isa_flags)?);
         compile::compile::<AArch64Backend>(func, self, abi, &self.machine_env, emit_info)
     }
 }
@@ -147,6 +147,21 @@ impl TargetIsa for AArch64Backend {
 
     #[cfg(feature = "unwind")]
     fn create_systemv_cie(&self) -> Option<gimli::write::CommonInformationEntry> {
+        let is_apple_os = match self.triple.operating_system {
+            OperatingSystem::Darwin
+            | OperatingSystem::Ios
+            | OperatingSystem::MacOSX { .. }
+            | OperatingSystem::Tvos => true,
+            _ => false,
+        };
+
+        if self.isa_flags.sign_return_address()
+            && self.isa_flags.sign_return_address_with_bkey()
+            && !is_apple_os
+        {
+            unimplemented!("Specifying that the B key is used with pointer authentication instructions in the CIE is not implemented.");
+        }
+
         Some(inst::unwind::systemv::create_cie())
     }