[DWARF] Make the GC of child tags conservative by default

crates/cranelift/src/debug/gc.rs

@@ -109,23 +109,52 @@ fn build_unit_dependencies(
 }
 
 fn has_die_back_edge(die: &read::DebuggingInformationEntry<Reader<'_>>) -> read::Result<bool> {
+    // DIEs can be broadly divided into three categories:
+    // 1. Extensions of their parents; effectively attributes: DW_TAG_variable, DW_TAG_member, etc.
+    // 2. Standalone entities referred to by other DIEs via 'reference' class attributes: types.
+    // 3. Structural entities that organize how the above relate to each other: namespaces.
+    // Here, we must make sure to return 'true' for DIEs in the first category since stripping them,
+    // provided their parent is alive, is always wrong. To be conservatively correct in the face
+    // of new/vendor tags, we maintain a "(mostly) known good" list of tags of the latter categories.
     let result = match die.tag() {
-        constants::DW_TAG_variable
-        | constants::DW_TAG_constant
-        | constants::DW_TAG_inlined_subroutine
-        | constants::DW_TAG_lexical_block
-        | constants::DW_TAG_label
-        | constants::DW_TAG_with_stmt
-        | constants::DW_TAG_try_block
-        | constants::DW_TAG_catch_block
-        | constants::DW_TAG_template_type_parameter
-        | constants::DW_TAG_enumerator
-        | constants::DW_TAG_member
-        | constants::DW_TAG_variant_part
-        | constants::DW_TAG_variant
-        | constants::DW_TAG_formal_parameter => true,
+        constants::DW_TAG_array_type
+        | constants::DW_TAG_atomic_type
+        | constants::DW_TAG_base_type
+        | constants::DW_TAG_class_type
+        | constants::DW_TAG_const_type
+        | constants::DW_TAG_dwarf_procedure
+        | constants::DW_TAG_entry_point
+        | constants::DW_TAG_enumeration_type
+        | constants::DW_TAG_pointer_type
+        | constants::DW_TAG_ptr_to_member_type
+        | constants::DW_TAG_reference_type
+        | constants::DW_TAG_restrict_type
+        | constants::DW_TAG_rvalue_reference_type
+        | constants::DW_TAG_string_type
+        | constants::DW_TAG_structure_type
+        | constants::DW_TAG_typedef
+        | constants::DW_TAG_union_type
+        | constants::DW_TAG_unspecified_type
+        | constants::DW_TAG_volatile_type
+        | constants::DW_TAG_coarray_type
+        | constants::DW_TAG_common_block
+        | constants::DW_TAG_dynamic_type
+        | constants::DW_TAG_file_type
+        | constants::DW_TAG_immutable_type
+        | constants::DW_TAG_interface_type
+        | constants::DW_TAG_set_type
+        | constants::DW_TAG_shared_type
+        | constants::DW_TAG_subroutine_type
+        | constants::DW_TAG_packed_type
+        | constants::DW_TAG_template_alias
+        | constants::DW_TAG_namelist
+        | constants::DW_TAG_namespace
+        | constants::DW_TAG_imported_unit
+        | constants::DW_TAG_imported_declaration
+        | constants::DW_TAG_imported_module
+        | constants::DW_TAG_module => false,
         constants::DW_TAG_subprogram => die.attr(constants::DW_AT_declaration)?.is_some(),
-        _ => false,
+        _ => true,
     };
     Ok(result)
 }

tests/all/debug/lldb.rs

@@ -181,15 +181,14 @@ pub fn test_debug_dwarf_generic_lldb() -> Result<()> {
             "-Ddebug-info",
             "tests/all/debug/testsuite/generic.wasm",
         ],
-        r#"b MainDefinedFunction
+        r#"br set -n debug_break -C up
 r
 p __vmctx->set()
-n
 p (x + x)
-b SatelliteFunction
 c
-n
 p (x + x)
+c
+p inst.BaseValue + inst.DerivedValue
 c"#,
     )?;
 
@@ -198,8 +197,10 @@ c"#,
         r#"
 check: stop reason = breakpoint 1.1
 check: 2
-check: stop reason = breakpoint 2.1
+check: stop reason = breakpoint 1.1
 check: 4
+check: stop reason = breakpoint 1.1
+check: 3
 check: exited with status = 0
 "#,
     )?;

tests/all/debug/testsuite/generic-satellite.cpp

@@ -2,5 +2,6 @@
 
 int SomeClass::SatelliteFunction(int x) {
   x *= 2;
+  debug_break();
   return x;
 }

tests/all/debug/testsuite/generic.cpp

@@ -1,10 +1,12 @@
-// clang generic.cpp generic-satellite.cpp -o generic.wasm -g -target
-// wasm32-unknown-wasip1
+// clang-format off
+// clang generic.cpp generic-satellite.cpp -o generic.wasm -g -target wasm32-unknown-wasip1
+// clang-format on
 //
 #include "generic.h"
 
 int SomeClass::MainDefinedFunction() {
   int x = HIDE_FROM_CHECKER(1);
+  debug_break();
   int y = SatelliteFunction(x);
   return x + y;
 }
@@ -14,8 +16,22 @@ int TestClassDefinitionSpreadAcrossCompileUnits() {
   return result != 3 ? 1 : 0;
 }
 
+struct BaseType {
+  int BaseValue = 1;
+};
+struct DerivedType : BaseType {
+  long long DerivedValue = 2;
+};
+
+int TestInheritance() {
+  DerivedType inst;
+  debug_break();
+  return inst.BaseValue + inst.DerivedValue != 3 ? 1 : 0;
+}
+
 int main() {
   int exitCode = 0;
   exitCode += TestClassDefinitionSpreadAcrossCompileUnits();
+  exitCode += TestInheritance();
   return exitCode;
 }

tests/all/debug/testsuite/generic.h

@@ -5,3 +5,5 @@ class SomeClass {
   static int MainDefinedFunction();
   static int SatelliteFunction(int x);
 };
+
+inline void debug_break() {}