[LLDB] Devirtualize coroutine promise types for std::coroutine_handle

This commit teaches the `std::coroutine_handle` pretty-printer to
devirtualize type-erased promise types. This is particularly useful to
resonstruct call stacks, either of asynchronous control flow or of
recursive invocations of `std::generator`. For the example recently
introduced by https://reviews.llvm.org/D132451, printing the `__promise`
variable now shows

```
(std::__coroutine_traits_sfinae<task, void>::promise_type) __promise = {
  continuation = coro frame = 0x555555562430 {
    resume = 0x0000555555556310 (a.out`task detail::chain_fn<1>() at llvm-nested-example.cpp:66)
    destroy = 0x0000555555556700 (a.out`task detail::chain_fn<1>() at llvm-nested-example.cpp:66)
    promise = {
      continuation = coro frame = 0x5555555623e0 {
        resume = 0x0000555555557070 (a.out`task detail::chain_fn<2>() at llvm-nested-example.cpp:66)
        destroy = 0x0000555555557460 (a.out`task detail::chain_fn<2>() at llvm-nested-example.cpp:66)
        promise = {
          ...
        }
      }
      result = 0
    }
  }
  result = 0
}
```

(shortened to keep the commit message readable) instead of

```
(std::__coroutine_traits_sfinae<task, void>::promise_type) __promise = {
  continuation = coro frame = 0x555555562430 {
    resume = 0x0000555555556310 (a.out`task detail::chain_fn<1>() at llvm-nested-example.cpp:66)
    destroy = 0x0000555555556700 (a.out`task detail::chain_fn<1>() at llvm-nested-example.cpp:66)
  }
  result = 0
}
```

Note how the new debug output reveals the complete asynchronous call
stack: our own function resumes `chain_fn<1>` which in turn will resume
`chain_fn<2>` and so on. Thereby this change allows users of lldb to
inspect the logical coroutine call stack without using any custom debug
scripts (although the display is still a bit clumsy. It would be nicer
to also integrate this into lldb's backtrace feature, but I don't know
how to do so)

The devirtualization currently works by introspecting the function
pointed to by the `destroy` pointer. (The `resume` pointer is not worth
much, given that for the final suspend point `resume` is set to a
nullptr. We have to use the `destroy` pointer instead.) We then look
for a `__promise` variable inside the `destroy` function. This
`__promise` variable is synthetically generated by LLVM, and looking at
its type reveals the type-erased promise_type.

This approach only works for clang-generated code, though. While gcc
also adds a `_Coro_promise` variable to the `resume` function, it does
not do so for the `destroy` function. However, we can't use the `resume`
function, as it will be reset to a nullptr at the final suspension
point. For the time being, I am happy with de-virtualization only working
for clang. A follow-up commit will further improve devirtualization and
also expose the variables spilled to the coroutine frame. As part of
this, I will also revisit gcc support.

Differential Revision: https://reviews.llvm.org/D132624

lldb/source/Plugins/Language/CPlusPlus/Coroutines.cpp

@@ -9,6 +9,8 @@
 #include "Coroutines.h"
 
 #include "Plugins/TypeSystem/Clang/TypeSystemClang.h"
+#include "lldb/Symbol/Function.h"
+#include "lldb/Symbol/VariableList.h"
 
 using namespace lldb;
 using namespace lldb_private;
@@ -32,6 +34,57 @@ static ValueObjectSP GetCoroFramePtrFromHandle(ValueObject &valobj) {
   return ptr_sp;
 }
 
+static Function *ExtractDestroyFunction(ValueObjectSP &frame_ptr_sp) {
+  lldb::TargetSP target_sp = frame_ptr_sp->GetTargetSP();
+  lldb::ProcessSP process_sp = frame_ptr_sp->GetProcessSP();
+  auto ptr_size = process_sp->GetAddressByteSize();
+
+  AddressType addr_type;
+  lldb::addr_t frame_ptr_addr = frame_ptr_sp->GetPointerValue(&addr_type);
+  if (!frame_ptr_addr || frame_ptr_addr == LLDB_INVALID_ADDRESS)
+    return nullptr;
+  lldbassert(addr_type == AddressType::eAddressTypeLoad);
+
+  Status error;
+  // The destroy pointer is the 2nd pointer inside the compiler-generated
+  // `pair<resumePtr,destroyPtr>`.
+  auto destroy_func_ptr_addr = frame_ptr_addr + ptr_size;
+  lldb::addr_t destroy_func_addr =
+      process_sp->ReadPointerFromMemory(destroy_func_ptr_addr, error);
+  if (error.Fail())
+    return nullptr;
+
+  Address destroy_func_address;
+  if (!target_sp->ResolveLoadAddress(destroy_func_addr, destroy_func_address))
+    return nullptr;
+
+  Function *destroy_func =
+      destroy_func_address.CalculateSymbolContextFunction();
+  if (!destroy_func)
+    return nullptr;
+
+  return destroy_func;
+}
+
+static CompilerType InferPromiseType(Function &destroy_func) {
+  SymbolContext sc;
+  Block &block = destroy_func.GetBlock(true);
+  auto variable_list = block.GetBlockVariableList(true);
+
+  // clang generates an artificial `__promise` variable inside the
+  // `destroy` function. Look for it.
+  auto promise_var = variable_list->FindVariable(ConstString("__promise"));
+  if (!promise_var)
+    return {};
+  if (!promise_var->IsArtificial())
+    return {};
+
+  Type *promise_type = promise_var->GetType();
+  if (!promise_type)
+    return {};
+  return promise_type->GetForwardCompilerType();
+}
+
 static CompilerType GetCoroutineFrameType(TypeSystemClang &ast_ctx,
                                           CompilerType promise_type) {
   CompilerType void_type = ast_ctx.GetBasicType(lldb::eBasicTypeVoid);
@@ -58,7 +111,11 @@ bool lldb_private::formatters::StdlibCoroutineHandleSummaryProvider(
   if (!ptr_sp)
     return false;
 
-  stream.Printf("coro frame = 0x%" PRIx64, ptr_sp->GetValueAsUnsigned(0));
+  if (!ptr_sp->GetValueAsUnsigned(0)) {
+    stream << "nullptr";
+  } else {
+    stream.Printf("coro frame = 0x%" PRIx64, ptr_sp->GetValueAsUnsigned(0));
+  }
   return true;
 }
 
@@ -100,15 +157,26 @@ bool lldb_private::formatters::StdlibCoroutineHandleSyntheticFrontEnd::
   if (!ptr_sp)
     return false;
 
-  TypeSystemClang *ast_ctx = llvm::dyn_cast_or_null<TypeSystemClang>(
-      valobj_sp->GetCompilerType().GetTypeSystem());
-  if (!ast_ctx)
-    return false;
-
+  // Get the `promise_type` from the template argument
   CompilerType promise_type(
       valobj_sp->GetCompilerType().GetTypeTemplateArgument(0));
   if (!promise_type)
     return false;
+
+  // Try to infer the promise_type if it was type-erased
+  if (promise_type.IsVoidType()) {
+    if (Function *destroy_func = ExtractDestroyFunction(ptr_sp)) {
+      if (CompilerType inferred_type = InferPromiseType(*destroy_func)) {
+        promise_type = inferred_type;
+      }
+    }
+  }
+
+  // Build the coroutine frame type
+  TypeSystemClang *ast_ctx = llvm::dyn_cast_or_null<TypeSystemClang>(
+      ptr_sp->GetCompilerType().GetTypeSystem());
+  if (!ast_ctx)
+    return {};
   CompilerType coro_frame_type = GetCoroutineFrameType(*ast_ctx, promise_type);
 
   m_frame_ptr_sp = ptr_sp->Cast(coro_frame_type.GetPointerType());

lldb/test/API/functionalities/data-formatter/data-formatter-stl/generic/coroutine_handle/TestCoroutineHandle.py

@@ -16,6 +16,7 @@ class TestCoroutineHandle(TestBase):
     def do_test(self, stdlib_type):
         """Test std::coroutine_handle is displayed correctly."""
         self.build(dictionary={stdlib_type: "1"})
+        is_clang = self.expectedCompiler(["clang"])
 
         test_generator_func_ptr_re = re.compile(
                 r"^\(a.out`my_generator_func\(\) at main.cpp:[0-9]*\)$")
@@ -37,14 +38,31 @@ def do_test(self, stdlib_type):
                     ValueCheck(name="current_value", value = "-1"),
                 ])
             ])
-        # For type-erased `coroutine_handle<>` we are missing the `promise`
-        # but still show `resume` and `destroy`.
-        self.expect_expr("type_erased_hdl",
-            result_summary=re.compile("^coro frame = 0x[0-9a-f]*$"),
-            result_children=[
-                ValueCheck(name="resume", summary = test_generator_func_ptr_re),
-                ValueCheck(name="destroy", summary = test_generator_func_ptr_re),
-            ])
+        if is_clang:
+            # For a type-erased `coroutine_handle<>`, we can still devirtualize
+            # the promise call and display the correctly typed promise.
+            self.expect_expr("type_erased_hdl",
+                result_summary=re.compile("^coro frame = 0x[0-9a-f]*$"),
+                result_children=[
+                    ValueCheck(name="resume", summary = test_generator_func_ptr_re),
+                    ValueCheck(name="destroy", summary = test_generator_func_ptr_re),
+                    ValueCheck(name="promise", children=[
+                        ValueCheck(name="current_value", value = "-1"),
+                    ])
+                ])
+            # For an incorrectly typed `coroutine_handle`, we use the user-supplied
+            # incorrect type instead of inferring the correct type. Strictly speaking,
+            # incorrectly typed coroutine handles are undefined behavior. However,
+            # it provides probably a better debugging experience if we display the
+            # promise as seen by the program instead of fixing this bug based on
+            # the available debug info.
+            self.expect_expr("incorrectly_typed_hdl",
+                result_summary=re.compile("^coro frame = 0x[0-9a-f]*$"),
+                result_children=[
+                    ValueCheck(name="resume", summary = test_generator_func_ptr_re),
+                    ValueCheck(name="destroy", summary = test_generator_func_ptr_re),
+                    ValueCheck(name="promise", value="-1")
+                ])
 
         # Run until after the `co_yield`
         process = self.process()
@@ -73,6 +91,18 @@ def do_test(self, stdlib_type):
                     ValueCheck(name="current_value", value = "42"),
                 ])
             ])
+        if is_clang:
+            # Devirtualization still works, also at the final suspension point, despite
+            # the `resume` pointer being reset to a nullptr
+            self.expect_expr("type_erased_hdl",
+                result_summary=re.compile("^coro frame = 0x[0-9a-f]*$"),
+                result_children=[
+                    ValueCheck(name="resume", value = "0x0000000000000000"),
+                    ValueCheck(name="destroy", summary = test_generator_func_ptr_re),
+                    ValueCheck(name="promise", children=[
+                        ValueCheck(name="current_value", value = "42"),
+                    ])
+                ])
 
     @add_test_categories(["libstdcxx"])
     def test_libstdcpp(self):

lldb/test/API/functionalities/data-formatter/data-formatter-stl/generic/coroutine_handle/main.cpp

@@ -43,6 +43,8 @@ int main() {
   bool is_supported = is_implementation_supported();
   int_generator gen = my_generator_func();
   std::coroutine_handle<> type_erased_hdl = gen.hdl;
+  std::coroutine_handle<int> incorrectly_typed_hdl =
+      std::coroutine_handle<int>::from_address(gen.hdl.address());
   gen.hdl.resume();                            // Break at initial_suspend
   gen.hdl.resume();                            // Break after co_yield
   empty_function_so_we_can_set_a_breakpoint(); // Break at final_suspend