[LLDB][swift] Skip prologue when unwiding virtual frames

Variables are never available in the prologue of a function. By unwinding the pc
of the virtual frames as the first instruction of the continuation function,
LLDB is unable to display any variable information.

This patch addresses the issue by computing the continuation pointer from the
context of the function being unwound, and then querying the continuation's
SymbolContext to determine its prologue size.

Author: felipepiovezan

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

@@ -2745,9 +2745,31 @@ SwiftLanguageRuntime::GetRuntimeUnwindPlan(ProcessSP process_sp,
         regnums->dummy_regnum, g_dummy_dwarf_expression,
         sizeof(g_dummy_dwarf_expression), false);
   }
-  row->SetRegisterLocationToAtCFAPlusOffset(regnums->pc_regnum, ptr_size,
-                                            false);
 
+  std::optional<addr_t> pc_after_prologue = [&]() -> std::optional<addr_t> {
+    // In the prologue, use the async_reg as is, it has not been clobbered.
+    if (in_prologue)
+      return TrySkipVirtualParentProlog(GetAsyncContext(regctx), *process_sp,
+                                        indirect_context);
+
+    // Both ABIs (x86_64 and aarch64) guarantee the async reg is saved at:
+    // *(fp - 8).
+    Status error;
+    addr_t async_reg_entry_value = LLDB_INVALID_ADDRESS;
+    process_sp->ReadMemory(fp - ptr_size, &async_reg_entry_value, ptr_size,
+                           error);
+    if (error.Fail())
+      return {};
+    return TrySkipVirtualParentProlog(async_reg_entry_value, *process_sp,
+                                      indirect_context);
+  }();
+
+  if (pc_after_prologue)
+    row->SetRegisterLocationToIsConstant(regnums->pc_regnum, *pc_after_prologue,
+                                         false);
+  else
+    row->SetRegisterLocationToAtCFAPlusOffset(regnums->pc_regnum, ptr_size,
+                                              false);
   row->SetUnspecifiedRegistersAreUndefined(true);
 
   UnwindPlanSP plan = std::make_shared<UnwindPlan>(lldb::eRegisterKindDWARF);
@@ -2773,13 +2795,15 @@ UnwindPlanSP SwiftLanguageRuntime::GetFollowAsyncContextUnwindPlan(
   if (!regnums)
     return UnwindPlanSP();
 
+  const bool is_indirect =
+      regctx->ReadRegisterAsUnsigned(regnums->dummy_regnum, (uint64_t)-1ll) ==
+      (uint64_t)-1ll;
   // In the general case, the async register setup by the frame above us
   // should be dereferenced twice to get our context, except when the frame
   // above us is an async frame on the OS stack that takes its context directly
   // (see discussion in GetRuntimeUnwindPlan()). The availability of
   // dummy_regnum is used as a marker for this situation.
-  if (regctx->ReadRegisterAsUnsigned(regnums->dummy_regnum, (uint64_t)-1ll) !=
-      (uint64_t)-1ll) {
+  if (!is_indirect) {
     row->GetCFAValue().SetIsRegisterDereferenced(regnums->async_ctx_regnum);
     row->SetRegisterLocationToSame(regnums->async_ctx_regnum, false);
   } else {
@@ -2816,8 +2840,21 @@ UnwindPlanSP SwiftLanguageRuntime::GetFollowAsyncContextUnwindPlan(
         regnums->async_ctx_regnum, expression, expr_size - 1, false);
   }
 
-  row->SetRegisterLocationToAtCFAPlusOffset(regnums->pc_regnum, ptr_size,
-                                            false);
+  // Suppose this is unwinding frame #2 of a call stack. The value given for
+  // the async register has two possible values, depending on what frame #1
+  // expects:
+  // 1. The CFA of frame #1, direct ABI, dereferencing it once produces CFA of
+  // Frame #2.
+  // 2. The CFA of frame #0, indirect ABI, dereferencing it twice produces CFA
+  // of Frame #2.
+  const unsigned num_indirections = 1 + is_indirect;
+  if (std::optional<addr_t> pc_after_prologue = TrySkipVirtualParentProlog(
+          GetAsyncContext(regctx), *process_sp, num_indirections))
+    row->SetRegisterLocationToIsConstant(regnums->pc_regnum, *pc_after_prologue,
+                                         false);
+  else
+    row->SetRegisterLocationToAtCFAPlusOffset(regnums->pc_regnum, ptr_size,
+                                              false);
 
   row->SetUnspecifiedRegistersAreUndefined(true);
 
@@ -2831,4 +2868,51 @@ UnwindPlanSP SwiftLanguageRuntime::GetFollowAsyncContextUnwindPlan(
   return plan;
 }
 
+std::optional<lldb::addr_t> SwiftLanguageRuntime::TrySkipVirtualParentProlog(
+    lldb::addr_t async_reg_val, Process &process, unsigned num_indirections) {
+  assert(num_indirections <= 2 &&
+         "more than two dereferences should not be needed");
+  if (async_reg_val == LLDB_INVALID_ADDRESS || async_reg_val == 0)
+    return {};
+
+  const auto ptr_size = process.GetAddressByteSize();
+  Status error;
+
+  // Compute the CFA of this frame.
+  addr_t cfa = async_reg_val;
+  for (; num_indirections != 0; --num_indirections) {
+    process.ReadMemory(cfa, &cfa, ptr_size, error);
+    if (error.Fail())
+      return {};
+  }
+
+  // The last funclet will have a zero CFA, we don't want to read that.
+  if (cfa == 0)
+    return {};
+
+  // Get the PC of the parent frame, i.e. the continuation pointer, which is
+  // the second field of the CFA.
+  addr_t pc_location = cfa + ptr_size;
+  addr_t pc_value = LLDB_INVALID_ADDRESS;
+  process.ReadMemory(pc_location, &pc_value, ptr_size, error);
+  if (error.Fail())
+    return {};
+
+  Address pc;
+  Target &target = process.GetTarget();
+  pc.SetLoadAddress(pc_value, &target);
+  if (!pc.IsValid())
+    return {};
+
+  SymbolContext sc;
+  if (!pc.CalculateSymbolContext(&sc,
+                                 eSymbolContextFunction | eSymbolContextSymbol))
+    return {};
+  if (!sc.symbol && !sc.function)
+    return {};
+
+  auto prologue_size = sc.symbol ? sc.symbol->GetPrologueByteSize()
+                                 : sc.function->GetPrologueByteSize();
+  return pc_value + prologue_size;
+}
 } // namespace lldb_private

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

@@ -490,6 +490,13 @@ class SwiftLanguageRuntime : public LanguageRuntime {
   GetFollowAsyncContextUnwindPlan(lldb::ProcessSP process_sp,
                                   RegisterContext *regctx, ArchSpec &arch,
                                   bool &behaves_like_zeroth_frame);
+
+  /// Given the async register of a funclet, extract its continuation pointer,
+  /// compute the prologue size of the continuation function, and return the
+  /// address of the first non-prologue instruction.
+  std::optional<lldb::addr_t>
+  TrySkipVirtualParentProlog(lldb::addr_t async_reg_val, Process &process,
+                             unsigned num_indirections);
 };
 
 } // namespace lldb_private

lldb/test/API/lang/swift/async/frame/variables_multiple_frames/TestSwiftAsyncFrameVarMultipleFrames.py

@@ -44,7 +44,7 @@ def check_variables(self, async_frames, expected_values):
             lldbutil.check_variable(self, myvar, False, value=expected_value)
 
     @swiftTest
-    @skipIf(oslist=["windows", "linux", "macos"])
+    @skipIf(oslist=["windows", "linux"])
     def test(self):
         """Test `frame variable` in async functions"""
         self.build()

lldb/test/API/lang/swift/async/unwind/backtrace_locals/TestSwiftAsyncBacktraceLocals.py

@@ -76,8 +76,9 @@ def run_fibo_tests(self, target, process):
                     error = lldb.SBError()
                     ret_addr = process.ReadPointerFromMemory(
                         cfa[fibonacci_number-1] + target.addr_size, error)
+                    prologue_to_skip = frame.GetFunction().GetPrologueByteSize()
                     self.assertSuccess(error, "Managed to read context memory")
-                    self.assertEqual(ret_addr, frame.GetPC())
+                    self.assertEqual(ret_addr + prologue_to_skip, frame.GetPC())
 
             self.assertIn("Main.main", thread.GetFrameAtIndex(n+1).GetFunctionName())