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

4 files changed, 22 insertions, 21 deletions

diff --git a/lldb/include/lldb/Target/LanguageRuntime.h b/lldb/include/lldb/Target/LanguageRuntime.h
index d093dcc..954d454 100644
--- a/lldb/include/lldb/Target/LanguageRuntime.h
+++ b/lldb/include/lldb/Target/LanguageRuntime.h
@@ -170,9 +170,9 @@ public:
     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) {}
diff --git a/lldb/source/Plugins/LanguageRuntime/ObjC/ObjCLanguageRuntime.cpp b/lldb/source/Plugins/LanguageRuntime/ObjC/ObjCLanguageRuntime.cpp
index ba52444..a812ffb 100644
--- a/lldb/source/Plugins/LanguageRuntime/ObjC/ObjCLanguageRuntime.cpp
+++ b/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
diff --git a/lldb/source/Plugins/LanguageRuntime/ObjC/ObjCLanguageRuntime.h b/lldb/source/Plugins/LanguageRuntime/ObjC/ObjCLanguageRuntime.h
index 0a8b6e8d..ffe9725 100644
--- a/lldb/source/Plugins/LanguageRuntime/ObjC/ObjCLanguageRuntime.h
+++ b/lldb/source/Plugins/LanguageRuntime/ObjC/ObjCLanguageRuntime.h
@@ -107,7 +107,7 @@ public:
                                             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 @@ public:
     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".
diff --git a/lldb/source/Plugins/TypeSystem/Clang/TypeSystemClang.cpp b/lldb/source/Plugins/TypeSystem/Clang/TypeSystemClang.cpp
index 0270c67..eb7f623 100644
--- a/lldb/source/Plugins/TypeSystem/Clang/TypeSystemClang.cpp
+++ b/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;