[lldb] Skip prologue when stepping through swift_task_switch

Plans stepping through a swift_task_switch are often the last sub-plan
for a StepOver action, meaning the plan's destination is also the PC
where users will take control of the program. As such, it is crucial
that these plans skip over the prologue of their destination function,
otherwise most variables won't be in scope.

Because x86 and arm diverge on the codegen for `}` in a scope, I had to
add a `print` statement in the for loop of the affected test.

(cherry picked from commit 3ed5f1d)

Author: felipepiovezan

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

@@ -394,7 +394,18 @@ CreateRunThroughTaskSwitchThreadPlan(Thread &thread,
     return {};
 
   resume_fn_ptr = thread.GetProcess()->FixCodeAddress(resume_fn_ptr);
-  return std::make_shared<ThreadPlanRunToAddress>(thread, resume_fn_ptr,
+
+  llvm::Expected<uint64_t> maybe_prologue_size =
+      FindPrologueSize(*thread.GetProcess(), resume_fn_ptr);
+
+  uint64_t dest_address =
+      resume_fn_ptr + (maybe_prologue_size ? *maybe_prologue_size : 0);
+
+  if (!maybe_prologue_size)
+    LLDB_LOG_ERROR(GetLog(LLDBLog::Step), maybe_prologue_size.takeError(),
+                   "{1}::{0}", __FUNCTION__);
+
+  return std::make_shared<ThreadPlanRunToAddress>(thread, dest_address,
                                                   /*stop_others*/ false);
 }
 

lldb/test/API/lang/swift/async/stepping/step_over/TestSwiftAsyncStepOver.py

@@ -7,6 +7,11 @@
 @skipIfAsan  # rdar://138777205
 class TestCase(lldbtest.TestBase):
 
+    def check_x_is_available(self, frame):
+        x_var = frame.FindVariable("x")
+        self.assertTrue(x_var.IsValid(), f"Failed to find x in {frame}")
+        self.assertEqual(x_var.GetValueAsUnsigned(), 30)
+
     def check_is_in_line(self, thread, linenum):
         frame = thread.frames[0]
         line_entry = frame.GetLineEntry()
@@ -25,10 +30,11 @@ def test(self):
         bkpt.SetEnabled(False) # avoid hitting multiple locations in async breakpoints
 
         expected_line_nums = [4]  # print(x)
-        expected_line_nums += [5, 6, 7, 5, 6, 7, 5]  # two runs over the loop
-        expected_line_nums += [8, 9]  # if line + if block
+        expected_line_nums += [5, 6, 7, 8, 5, 6, 7, 8, 5]  # two runs over the loop
+        expected_line_nums += [9, 10]  # if line + if block
         for expected_line_num in expected_line_nums:
             thread.StepOver()
             stop_reason = thread.GetStopReason()
             self.assertStopReason(stop_reason, lldb.eStopReasonPlanComplete)
             self.check_is_in_line(thread, expected_line_num)
+            self.check_x_is_available(thread.frames[0])

lldb/test/API/lang/swift/async/stepping/step_over/main.swift

@@ -4,6 +4,7 @@
     print(x)
     for i in 1...2 {
       await f()
+      print("hello!")
     }
     if (await f() == 30) {
       print("here!")