[compiler-rt] [libcxxabi] [libunwind] [runtimes][PAC] Harden unwinding when possible (#138571) (PR #143230)

[Oliver Hunt via llvm-commits]

================
@@ -680,11 +682,19 @@ int CompactUnwinder_arm64<A>::stepWithCompactEncodingFrame(
     savedRegisterLoc -= 8;
   }
 
-  uint64_t fp = registers.getFP();
+  Registers_arm64::reg_t fp = registers.getFP();
   // fp points to old fp
   registers.setFP(addressSpace.get64(fp));
-  // old sp is fp less saved fp and lr
+
+  // old sp is fp less saved fp and lr. Set this before FP & LR because in
+  // arm64e it's the discriminator used for those registers.
   registers.setSP(fp + 16);
+
+  Registers_arm64::reg_t oldfp = addressSpace.get64(fp);
+
+  // fp points to old fp
+  registers.setFP(oldfp);
----------------
ojhunt wrote:

So this is an area I'm unsure of - I thought the addressSpace abstraction was for cross process. ... something? But I don't know if that was just a hypothetical future concept or is actually used somewhere?

It's also possible that this is just left over from a bunch of dead code I excised while preparing this

https://github.com/llvm/llvm-project/pull/143230