[lldb] Remove ProcessStructReader from NSStringSummaryProvider (NFC)

[kastiglione]

Summary

Simplify getting the length of NSPathStore2 strings.

NSStringSummaryProvider uses a single field from NSPathStore2 instances, its first ivar: _lengthAndRefCount. This change uses GetSyntheticChildAtOffset to replace the use of ProcessStructReader, and removes the hard coded CompilerType definition of NSPathStore2.

[aprantl]

This doesn't look any less readable than the code it replaces and is definitely faster.

[kastiglione]

To add more context, ProcessStructReader is used in only a few places, and none seem like they should truly need it. This was a low hanging fruit.

Something I don't understand about this code is why it the objc metadata is incomplete. The class is known, but not its ivars, why? In theory this code should be able to request the _lengthAndRefCount child member directly by name.

[kastiglione]

give name to synthetic child

lldb/source/Plugins/Language/ObjC/NSString.cpp

 //===-- NSString.cpp ------------------------------------------------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 
 #include "NSString.h"
 
-#include "Plugins/TypeSystem/Clang/TypeSystemClang.h"
 #include "lldb/Core/ValueObject.h"
 #include "lldb/Core/ValueObjectConstResult.h"
 #include "lldb/DataFormatters/FormattersHelpers.h"
 #include "lldb/DataFormatters/StringPrinter.h"
 #include "lldb/Target/Language.h"
-#include "lldb/Target/ProcessStructReader.h"
 #include "lldb/Target/Target.h"
 #include "lldb/Utility/DataBufferHeap.h"
 #include "lldb/Utility/Endian.h"
 #include "lldb/Utility/Status.h"
 #include "lldb/Utility/Stream.h"
 
 using namespace lldb;
 using namespace lldb_private;
 using namespace lldb_private::formatters;
 
 std::map<ConstString, CXXFunctionSummaryFormat::Callback> &
 NSString_Additionals::GetAdditionalSummaries() {
   static std::map<ConstString, CXXFunctionSummaryFormat::Callback> g_map;
   return g_map;
 }
 
-static CompilerType GetNSPathStore2Type(Target &target) {
-  static ConstString g_type_name("__lldb_autogen_nspathstore2");
-
-  TypeSystemClang *ast_ctx = ScratchTypeSystemClang::GetForTarget(target);
-
-  if (!ast_ctx)
-    return CompilerType();
-
-  CompilerType voidstar =
-      ast_ctx->GetBasicType(lldb::eBasicTypeVoid).GetPointerType();
-  CompilerType uint32 =
-      ast_ctx->GetBuiltinTypeForEncodingAndBitSize(eEncodingUint, 32);
-
-  return ast_ctx->GetOrCreateStructForIdentifier(
-      g_type_name,
-      {{"isa", voidstar}, {"lengthAndRef", uint32}, {"buffer", voidstar}});
-}
-
 bool lldb_private::formatters::NSStringSummaryProvider(
     ValueObject &valobj, Stream &stream,
     const TypeSummaryOptions &summary_options) {
   static ConstString g_TypeHint("NSString");
 
   ProcessSP process_sp = valobj.GetProcessSP();
   if (!process_sp)
     return false;
if (is_mutable) {
     options.SetHasSourceSize(has_explicit_length);
     options.SetNeedsZeroTermination(!has_explicit_length);
     options.SetIgnoreMaxLength(summary_options.GetCapping() ==
                                TypeSummaryCapping::eTypeSummaryUncapped);
     options.SetBinaryZeroIsTerminator(!has_explicit_length);
     return StringPrinter::ReadStringAndDumpToStream<
         StringPrinter::StringElementType::UTF16>(options);
   } else if (is_path_store) {
-    ProcessStructReader reader(valobj.GetProcessSP().get(),
-                               valobj.GetValueAsUnsigned(0),
-                               GetNSPathStore2Type(*valobj.GetTargetSP()));
-    explicit_length =
-        reader.GetField<uint32_t>(ConstString("lengthAndRef")) >> 20;
+    // _lengthAndRefCount is the first ivar of NSPathStore2 (after the isa).
+    uint64_t length_ivar_offset = 1 * ptr_size;
+    CompilerType length_type = valobj.GetCompilerType().GetBasicTypeFromAST(
+        lldb::eBasicTypeUnsignedInt);
+    ValueObjectSP length_valobj_sp =
+        valobj.GetSyntheticChildAtOffset(length_ivar_offset, length_type, true);
+    if (!length_valobj_sp)
+      return false;
+    // Get the length out of _lengthAndRefCount.
+    explicit_length = length_valobj_sp->GetValueAsUnsigned(0) >> 20;
     lldb::addr_t location = valobj.GetValueAsUnsigned(0) + ptr_size + 4;
 
     options.SetLocation(location);
     options.SetTargetSP(valobj.GetTargetSP());
     options.SetStream(&stream);
     options.SetQuote('"');
     options.SetSourceSize(explicit_length);
     options.SetHasSourceSize(has_explicit_length);