[lldb] Make LanguageRuntime::GetTypeBitSize return an optional (NFC) (llvm#96013)

Make LanguageRuntime::GetTypeBitSize return an optional. This should be
NFC, though the ObjCLanguageRuntime implementation is (possibly) more
defensive against returning 0.

I'm not sure if it's possible for both `m_ivar.size` and `m_ivar.offset`
to be zero. Previously, we'd return 0 and cache it, only to discard it
the next time when finding it in the cache, and recomputing it again.
The new code will avoid putting it in the cache in the first place.

Author: JDevlieghere

lldb/include/lldb/Target/LanguageRuntime.h

@@ -170,9 +170,9 @@ class LanguageRuntime : public Runtime, public PluginInterface {
     return m_process->GetTarget().GetSearchFilterForModule(nullptr);
   }
 
-  virtual bool GetTypeBitSize(const CompilerType &compiler_type,
-                              uint64_t &size) {
-    return false;
+  virtual std::optional<uint64_t>
+  GetTypeBitSize(const CompilerType &compiler_type) {
+    return {};
   }
 
   virtual void SymbolsDidLoad(const ModuleList &module_list) {}

lldb/source/Plugins/LanguageRuntime/ObjC/ObjCLanguageRuntime.cpp

@@ -350,18 +350,17 @@ ObjCLanguageRuntime::EncodingToTypeSP ObjCLanguageRuntime::GetEncodingToType() {
   return nullptr;
 }
 
-bool ObjCLanguageRuntime::GetTypeBitSize(const CompilerType &compiler_type,
-                                         uint64_t &size) {
+std::optional<uint64_t>
+ObjCLanguageRuntime::GetTypeBitSize(const CompilerType &compiler_type) {
   void *opaque_ptr = compiler_type.GetOpaqueQualType();
-  size = m_type_size_cache.Lookup(opaque_ptr);
-  // an ObjC object will at least have an ISA, so 0 is definitely not OK
-  if (size > 0)
-    return true;
+  uint64_t cached_size = m_type_size_cache.Lookup(opaque_ptr);
+  if (cached_size > 0)
+    return cached_size;
 
   ClassDescriptorSP class_descriptor_sp =
       GetClassDescriptorFromClassName(compiler_type.GetTypeName());
   if (!class_descriptor_sp)
-    return false;
+    return {};
 
   int32_t max_offset = INT32_MIN;
   uint64_t sizeof_max = 0;
@@ -377,11 +376,13 @@ bool ObjCLanguageRuntime::GetTypeBitSize(const CompilerType &compiler_type,
     }
   }
 
-  size = 8 * (max_offset + sizeof_max);
-  if (found)
+  uint64_t size = 8 * (max_offset + sizeof_max);
+  if (found && size > 0) {
     m_type_size_cache.Insert(opaque_ptr, size);
+    return size;
+  }
 
-  return found;
+  return {};
 }
 
 lldb::BreakpointPreconditionSP

lldb/source/Plugins/LanguageRuntime/ObjC/ObjCLanguageRuntime.h

@@ -107,7 +107,7 @@ class ObjCLanguageRuntime : public LanguageRuntime {
                                             int64_t *value_bits = nullptr,
                                             uint64_t *payload = nullptr) = 0;
     /// @}
- 
+
     virtual uint64_t GetInstanceSize() = 0;
 
     // use to implement version-specific additional constraints on pointers
@@ -321,8 +321,8 @@ class ObjCLanguageRuntime : public LanguageRuntime {
     m_negative_complete_class_cache.clear();
   }
 
-  bool GetTypeBitSize(const CompilerType &compiler_type,
-                      uint64_t &size) override;
+  std::optional<uint64_t>
+  GetTypeBitSize(const CompilerType &compiler_type) override;
 
   /// Check whether the name is "self" or "_cmd" and should show up in
   /// "frame variable".

lldb/source/Plugins/TypeSystem/Clang/TypeSystemClang.cpp

@@ -4744,11 +4744,11 @@ TypeSystemClang::GetBitSize(lldb::opaque_compiler_type_t type,
       ExecutionContext exe_ctx(exe_scope);
       Process *process = exe_ctx.GetProcessPtr();
       if (process) {
-        ObjCLanguageRuntime *objc_runtime = ObjCLanguageRuntime::Get(*process);
-        if (objc_runtime) {
-          uint64_t bit_size = 0;
-          if (objc_runtime->GetTypeBitSize(GetType(qual_type), bit_size))
-            return bit_size;
+        if (ObjCLanguageRuntime *objc_runtime =
+                ObjCLanguageRuntime::Get(*process)) {
+          if (std::optional<uint64_t> bit_size =
+                  objc_runtime->GetTypeBitSize(GetType(qual_type)))
+            return *bit_size;
         }
       } else {
         static bool g_printed = false;