[lldb][swift] Change unwind heuristic for Q funclets in arm64e

With arm64e, many more branches are generated, which cause the debugger
to stop more often while stepping. Among those stops are code regions
where the debugger gets confused about what is stored in the async
register stack slot.

This commit revives a heuristic that was previously used by default for
all architectures and that works in all PC addresses, but that
sacrifices unwinding o recursive functions. It is only used if the
target is arm64e.

Author: felipepiovezan

lldb/source/Plugins/LanguageRuntime/Swift/SwiftLanguageRuntime.cpp

@@ -2663,20 +2663,48 @@ static llvm::Expected<addr_t> ReadAsyncContextRegisterFromUnwind(
   return async_reg;
 }
 
+static llvm::Expected<bool>
+DoesContinuationPointToSameFunction(addr_t async_reg, SymbolContext &sc,
+                                    Process &process) {
+  llvm::Expected<addr_t> continuation_ptr = ReadPtrFromAddr(
+      process, async_reg, /*offset*/ process.GetAddressByteSize());
+  if (!continuation_ptr)
+    return continuation_ptr.takeError();
+
+  Address continuation_addr;
+  continuation_addr.SetLoadAddress(process.FixCodeAddress(*continuation_ptr),
+                                   &process.GetTarget());
+  if (sc.function)
+    return sc.function->GetAddressRange().ContainsLoadAddress(
+        continuation_addr, &process.GetTarget());
+  assert(sc.symbol);
+  return sc.symbol->ContainsFileAddress(continuation_addr.GetFileAddress());
+}
+
 /// Returns true if the async register should be dereferenced once to obtain the
 /// CFA of the currently executing function. This is the case at the start of
 /// "Q" funclets, before the low level code changes the meaning of the async
 /// register to not require the indirection.
-/// The end of the prologue approximates the transition point.
+/// The end of the prologue approximates the transition point well in non-arm64e
+/// targets.
 /// FIXME: In the few instructions between the end of the prologue and the
 /// transition point, this approximation fails. rdar://139676623
 static llvm::Expected<bool> IsIndirectContext(Process &process,
                                               StringRef mangled_name,
-                                              Address pc, SymbolContext &sc) {
+                                              Address pc, SymbolContext &sc,
+                                              addr_t async_reg) {
   if (!SwiftLanguageRuntime::IsSwiftAsyncAwaitResumePartialFunctionSymbol(
           mangled_name))
     return false;
 
+  // For arm64e, pointer authentication generates branches that cause stepping
+  // algorithms to stop & unwind in more places. The "end of the prologue"
+  // approximation fails in those; instead, check whether the continuation
+  // pointer still points to the currently executing function. This works for
+  // all instructions, but fails when direct recursion is involved.
+  if (process.GetTarget().GetArchitecture().GetTriple().isArm64e())
+    return DoesContinuationPointToSameFunction(async_reg, sc, process);
+
   // This is checked prior to calling this function.
   assert(sc.function || sc.symbol);
   uint32_t prologue_size = sc.function ? sc.function->GetPrologueByteSize()
@@ -2759,7 +2787,7 @@ SwiftLanguageRuntime::GetRuntimeUnwindPlan(ProcessSP process_sp,
     return log_expected(async_reg.takeError());
 
   llvm::Expected<bool> maybe_indirect_context =
-      IsIndirectContext(*process_sp, mangled_name, pc, sc);
+      IsIndirectContext(*process_sp, mangled_name, pc, sc, *async_reg);
   if (!maybe_indirect_context)
     return log_expected(maybe_indirect_context.takeError());