Increase preallocated buffer for scanning the process objc classes

[jasonmolenda]

Summary

lldb allocates a buffer in the inferior, and runs an expression to fill it with information about all of the classes defined in the objective-c runtime. We don't want to run two expressions for perf reasons, one to find the number of classes, then a second expression to fill in the buffer. (I think we had that code in the past tbh, and went with "this is big enough")

Our class list in some processes is approaching this limit. This patch doubles it. It increases the memory buffer from ~150kb to ~300kb, and it is freed shortly after the expression completes. I also changed the name of the variable to make it easier to understand the nature of the failure if the assert is hit.

[JDevlieghere]

LGTM with the nits about the comments.

lldb/source/Plugins/LanguageRuntime/ObjC/AppleObjCRuntime/AppleObjCRuntimeV2.cpp

2011

Nit: LLVM coding style says comments need to be proper sentences starting with a capital letter and end with a period.

2100–2101

Same nit

[jasonmolenda]

Update to address Jonas' feedback.

lldb/source/Plugins/LanguageRuntime/ObjC/AppleObjCRuntime/AppleObjCRuntimeV2.cpp

AppleObjCRuntimeV2::SharedCacheClassInfoExtractor::UpdateISAToDescriptorMap() {
   const lldb::addr_t objc_opt_ptr = m_runtime.GetSharedCacheReadOnlyAddress();
   const lldb::addr_t shared_cache_base_addr =
       m_runtime.GetSharedCacheBaseAddress();
 
   if (objc_opt_ptr == LLDB_INVALID_ADDRESS ||
       shared_cache_base_addr == LLDB_INVALID_ADDRESS)
     return DescriptorMapUpdateResult::Fail();
 
-  const uint32_t num_classes = 128 * 1024;
+  // The number of entries to pre-allocate room for.
+  const uint32_t max_num_classes = 256 * 1024;
 
   UtilityFunction *get_class_info_code = GetClassInfoUtilityFunction(exe_ctx);
   if (!get_class_info_code) {
     // The callee will have already logged a useful error message.
     return DescriptorMapUpdateResult::Fail();
   }
 
   FunctionCaller *get_shared_cache_class_info_function =
       get_class_info_code->GetFunctionCaller();
 
   if (!get_shared_cache_class_info_function) {
     LLDB_LOGF(log, "Failed to get implementation lookup function caller.");
     return DescriptorMapUpdateResult::Fail();
   }
 
   ValueList arguments =
       get_shared_cache_class_info_function->GetArgumentValues();
 
   DiagnosticManager diagnostics;
 
   const uint32_t class_info_byte_size = addr_size + 4;
-  const uint32_t class_infos_byte_size = num_classes * class_info_byte_size;
+  const uint32_t class_infos_byte_size = max_num_classes * class_info_byte_size;
   lldb::addr_t class_infos_addr = process->AllocateMemory(
       class_infos_byte_size, ePermissionsReadable | ePermissionsWritable, err);
   const uint32_t relative_selector_offset_addr_size = 64;
   lldb::addr_t relative_selector_offset_addr =
       process->AllocateMemory(relative_selector_offset_addr_size,
                               ePermissionsReadable | ePermissionsWritable, err);
 
   if (class_infos_addr == LLDB_INVALID_ADDRESS) {
ExpressionResults results =
         get_shared_cache_class_info_function->ExecuteFunction(
             exe_ctx, &m_args, options, diagnostics, return_value);
 
     if (results == eExpressionCompleted) {
       // The result is the number of ClassInfo structures that were filled in
       num_class_infos = return_value.GetScalar().ULong();
       LLDB_LOG(log, "Discovered {0} Objective-C classes in the shared cache",
                num_class_infos);
-      assert(num_class_infos <= num_classes);
+      // Assert if there were more classes than we pre-allocated
+      // room for.
+      assert(num_class_infos <= max_num_classes);
       if (num_class_infos > 0) {
-        if (num_class_infos > num_classes) {
-          num_class_infos = num_classes;
+        if (num_class_infos > max_num_classes) {
+          num_class_infos = max_num_classes;
 
           success = false;
         } else {
           success = true;
         }
 
         // Read the relative selector offset.
         DataBufferHeap relative_selector_offset_buffer(64, 0);