[lldb] Adapt shared cache ObjC class metadata extraction for long term growth (#167579)

bulbazord · web-flow · commit ec4207baed7b · 2025-11-12T10:52:26.000-08:00
On Apple's platforms, the size of the shared cache grows steadily. As it
grows, so does its list of ObjC classes. LLDB currently accepts an upper
limit to the number of classes when it extracts the class information.
Every few years we will hit the limit and increase the upper limit of
classes.

This approach is fundamentally unsustainable. On top of needing to
manually adjust the number every few years, our current method requires
us to allocate memory in the inferior process. On macOS this is usually
not a problem, but on embedded devices there is usually a limit to how
much memory a process can allocate before they are killed by the OS.

My solution involves running the metadata extraction logic multiple
times. I've added a new parameter to our utility function `start_idx`
that keeps track of where it stopped during the previous run so that it
may pick up again where it stopped.

rdar://91398396
diff --git a/lldb/source/Plugins/LanguageRuntime/ObjC/AppleObjCRuntime/AppleObjCRuntimeV2.cpp b/lldb/source/Plugins/LanguageRuntime/ObjC/AppleObjCRuntime/AppleObjCRuntimeV2.cpp
@@ -418,6 +418,7 @@ __lldb_apple_objc_v2_get_shared_cache_class_info (void *objc_opt_ro_ptr,
                                                   void *class_infos_ptr,
                                                   uint64_t *relative_selector_offset,
                                                   uint32_t class_infos_byte_size,
+                                                  uint32_t *start_idx,
                                                   uint32_t should_log)
 {
     *relative_selector_offset = 0;
@@ -426,6 +427,7 @@ __lldb_apple_objc_v2_get_shared_cache_class_info (void *objc_opt_ro_ptr,
     DEBUG_PRINTF ("shared_cache_base_ptr = %p\n", shared_cache_base_ptr);
     DEBUG_PRINTF ("class_infos_ptr = %p\n", class_infos_ptr);
     DEBUG_PRINTF ("class_infos_byte_size = %u (%llu class infos)\n", class_infos_byte_size, (uint64_t)(class_infos_byte_size/sizeof(ClassInfo)));
+    DEBUG_PRINTF ("start_idx = %u\n", *start_idx);
     if (objc_opt_ro_ptr)
     {
         const objc_opt_t *objc_opt = (objc_opt_t *)objc_opt_ro_ptr;
@@ -480,7 +482,11 @@ __lldb_apple_objc_v2_get_shared_cache_class_info (void *objc_opt_ro_ptr,
             DEBUG_PRINTF ("clsopt->mask = 0x%8.8x\n", clsopt->mask);
             DEBUG_PRINTF ("classOffsets = %p\n", classOffsets);
 
-            for (uint32_t i=0; i<clsopt->capacity; ++i)
+            const uint32_t original_start_idx = *start_idx;
+
+            // Always start at the start_idx here. If it's greater than the capacity,
+            // it will skip the loop entirely and go to the duplicate handling below.
+            for (uint32_t i=*start_idx; i<clsopt->capacity; ++i)
             {
                 const uint64_t objectCacheOffset = classOffsets[i].objectCacheOffset;
                 DEBUG_PRINTF("objectCacheOffset[%u] = %u\n", i, objectCacheOffset);
@@ -524,59 +530,77 @@ __lldb_apple_objc_v2_get_shared_cache_class_info (void *objc_opt_ro_ptr,
                 else
                 {
                     DEBUG_PRINTF("not(class_infos && idx < max_class_infos)\n");
+                    *start_idx = i;
+                    break;
                 }
                 ++idx;
             }
 
-            const uint32_t *duplicate_count_ptr = (uint32_t *)&classOffsets[clsopt->capacity];
-            const uint32_t duplicate_count = *duplicate_count_ptr;
-            const objc_classheader_v16_t *duplicateClassOffsets = (const objc_classheader_v16_t *)(&duplicate_count_ptr[1]);
+            if (idx < max_class_infos) {
+                const uint32_t *duplicate_count_ptr = (uint32_t *)&classOffsets[clsopt->capacity];
+                const uint32_t duplicate_count = *duplicate_count_ptr;
+                const objc_classheader_v16_t *duplicateClassOffsets = (const objc_classheader_v16_t *)(&duplicate_count_ptr[1]);
 
-            DEBUG_PRINTF ("duplicate_count = %u\n", duplicate_count);
-            DEBUG_PRINTF ("duplicateClassOffsets = %p\n", duplicateClassOffsets);
+                DEBUG_PRINTF ("duplicate_count = %u\n", duplicate_count);
+                DEBUG_PRINTF ("duplicateClassOffsets = %p\n", duplicateClassOffsets);
 
-            for (uint32_t i=0; i<duplicate_count; ++i)
-            {
-                const uint64_t objectCacheOffset = duplicateClassOffsets[i].objectCacheOffset;
-                DEBUG_PRINTF("objectCacheOffset[%u] = %u\n", i, objectCacheOffset);
+                const uint32_t duplicate_start_idx =
+                      *start_idx < clsopt->capacity ?
+                      0 :
+                      *start_idx - clsopt->capacity;
 
-                if (classOffsets[i].isDuplicate) {
-                    DEBUG_PRINTF("isDuplicate = true\n");
-                    continue; // duplicate
-                }
-
-                if (objectCacheOffset == 0) {
-                    DEBUG_PRINTF("objectCacheOffset == invalidEntryOffset\n");
-                    continue; // invalid offset
-                }
-
-                if (class_infos && idx < max_class_infos)
+                for (uint32_t i=duplicate_start_idx; i<duplicate_count; ++i)
                 {
-                    class_infos[idx].isa = (Class)((uint8_t *)shared_cache_base_ptr + objectCacheOffset);
+                    const uint64_t objectCacheOffset = duplicateClassOffsets[i].objectCacheOffset;
+                    DEBUG_PRINTF("objectCacheOffset[%u] = %u\n", i, objectCacheOffset);
 
-                    // Lookup the class name.
-                    const char *name = class_name_lookup_func(class_infos[idx].isa);
-                    DEBUG_PRINTF("[%u] isa = %8p %s\n", idx, class_infos[idx].isa, name);
+                    if (duplicateClassOffsets[i].isDuplicate) {
+                        DEBUG_PRINTF("isDuplicate = true\n");
+                        continue; // duplicate
+                    }
 
-                    // Hash the class name so we don't have to read it.
-                    const char *s = name;
-                    uint32_t h = 5381;
-                    for (unsigned char c = *s; c; c = *++s)
+                    if (objectCacheOffset == 0) {
+                        DEBUG_PRINTF("objectCacheOffset == invalidEntryOffset\n");
+                        continue; // invalid offset
+                    }
+
+                    if (class_infos && idx < max_class_infos)
                     {
-                        // class_getName demangles swift names and the hash must
-                        // be calculated on the mangled name.  hash==0 means lldb
-                        // will fetch the mangled name and compute the hash in
-                        // ParseClassInfoArray.
-                        if (c == '.')
+                        class_infos[idx].isa = (Class)((uint8_t *)shared_cache_base_ptr + objectCacheOffset);
+
+                        // Lookup the class name.
+                        const char *name = class_name_lookup_func(class_infos[idx].isa);
+                        DEBUG_PRINTF("[%u] isa = %8p %s\n", idx, class_infos[idx].isa, name);
+
+                        // Hash the class name so we don't have to read it.
+                        const char *s = name;
+                        uint32_t h = 5381;
+                        for (unsigned char c = *s; c; c = *++s)
                         {
-                            h = 0;
-                            break;
+                            // class_getName demangles swift names and the hash must
+                            // be calculated on the mangled name.  hash==0 means lldb
+                            // will fetch the mangled name and compute the hash in
+                            // ParseClassInfoArray.
+                            if (c == '.')
+                            {
+                                h = 0;
+                                break;
+                            }
+                            h = ((h << 5) + h) + c;
                         }
-                        h = ((h << 5) + h) + c;
+                        class_infos[idx].hash = h;
+                    } else {
+                        DEBUG_PRINTF("not(class_infos && idx < max_class_infos)\n");
+                        *start_idx = i;
+                        break;
                     }
-                    class_infos[idx].hash = h;
+                    ++idx;
                 }
-                ++idx;
+            }
+            // Always make sure start_idx gets updated. Otherwise we have an infinite
+            // loop if there are exactly max_class_infos number of classes.
+            if (*start_idx == original_start_idx) {
+              *start_idx = idx;
             }
         }
         else if (objc_opt->version >= 12 && objc_opt->version <= 15)
@@ -1959,6 +1983,9 @@ AppleObjCRuntimeV2::SharedCacheClassInfoExtractor::
   CompilerType clang_uint64_t_pointer_type =
       scratch_ts_sp->GetBuiltinTypeForEncodingAndBitSize(eEncodingUint, 64)
           .GetPointerType();
+  CompilerType clang_uint32_t_pointer_type =
+      scratch_ts_sp->GetBuiltinTypeForEncodingAndBitSize(eEncodingUint, 32)
+          .GetPointerType();
 
   // Next make the function caller for our implementation utility function.
   ValueList arguments;
@@ -1976,6 +2003,13 @@ AppleObjCRuntimeV2::SharedCacheClassInfoExtractor::
   value.SetValueType(Value::ValueType::Scalar);
   value.SetCompilerType(clang_uint32_t_type);
   arguments.PushValue(value);
+
+  value.SetValueType(Value::ValueType::Scalar);
+  value.SetCompilerType(clang_uint32_t_pointer_type);
+  arguments.PushValue(value);
+
+  value.SetValueType(Value::ValueType::Scalar);
+  value.SetCompilerType(clang_uint32_t_type);
   arguments.PushValue(value);
 
   std::unique_ptr<UtilityFunction> utility_fn = std::move(*utility_fn_or_error);
@@ -2313,10 +2347,7 @@ AppleObjCRuntimeV2::SharedCacheClassInfoExtractor::UpdateISAToDescriptorMap() {
 
   // The number of entries to pre-allocate room for.
   // Each entry is (addrsize + 4) bytes
-  // FIXME: It is not sustainable to continue incrementing this value every time
-  // the shared cache grows. This is because it requires allocating memory in
-  // the inferior process and some inferior processes have small memory limits.
-  const uint32_t max_num_classes = 212992;
+  const uint32_t max_num_classes_in_buffer = 212992;
 
   UtilityFunction *get_class_info_code = GetClassInfoUtilityFunction(exe_ctx);
   if (!get_class_info_code) {
@@ -2338,22 +2369,40 @@ AppleObjCRuntimeV2::SharedCacheClassInfoExtractor::UpdateISAToDescriptorMap() {
   DiagnosticManager diagnostics;
 
   const uint32_t class_info_byte_size = addr_size + 4;
-  const uint32_t class_infos_byte_size = max_num_classes * class_info_byte_size;
+  const uint32_t class_infos_byte_size =
+      max_num_classes_in_buffer * 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);
+  constexpr uint32_t class_info_start_idx_byte_size = sizeof(uint32_t);
+  lldb::addr_t class_info_start_idx_addr =
+      process->AllocateMemory(class_info_start_idx_byte_size,
+                              ePermissionsReadable | ePermissionsWritable, err);
 
-  if (class_infos_addr == LLDB_INVALID_ADDRESS) {
+  if (class_infos_addr == LLDB_INVALID_ADDRESS ||
+      relative_selector_offset_addr == LLDB_INVALID_ADDRESS ||
+      class_info_start_idx_addr == LLDB_INVALID_ADDRESS) {
     LLDB_LOGF(log,
               "unable to allocate %" PRIu32
               " bytes in process for shared cache read",
               class_infos_byte_size);
     return DescriptorMapUpdateResult::Fail();
   }
 
+  const uint32_t start_idx_init_value = 0;
+  size_t bytes_written = process->WriteMemory(
+      class_info_start_idx_addr, &start_idx_init_value, sizeof(uint32_t), err);
+  if (bytes_written != sizeof(uint32_t)) {
+    LLDB_LOGF(log,
+              "unable to write %" PRIu32
+              " bytes in process for shared cache read",
+              class_infos_byte_size);
+    return DescriptorMapUpdateResult::Fail();
+  }
+
   std::lock_guard<std::mutex> guard(m_mutex);
 
   // Fill in our function argument values
@@ -2362,12 +2411,13 @@ AppleObjCRuntimeV2::SharedCacheClassInfoExtractor::UpdateISAToDescriptorMap() {
   arguments.GetValueAtIndex(2)->GetScalar() = class_infos_addr;
   arguments.GetValueAtIndex(3)->GetScalar() = relative_selector_offset_addr;
   arguments.GetValueAtIndex(4)->GetScalar() = class_infos_byte_size;
+  arguments.GetValueAtIndex(5)->GetScalar() = class_info_start_idx_addr;
   // Only dump the runtime classes from the expression evaluation if the log is
   // verbose:
   Log *type_log = GetLog(LLDBLog::Types);
   bool dump_log = type_log && type_log->GetVerbose();
 
-  arguments.GetValueAtIndex(5)->GetScalar() = dump_log ? 1 : 0;
+  arguments.GetValueAtIndex(6)->GetScalar() = dump_log ? 1 : 0;
 
   bool success = false;
 
@@ -2394,78 +2444,80 @@ AppleObjCRuntimeV2::SharedCacheClassInfoExtractor::UpdateISAToDescriptorMap() {
 
     diagnostics.Clear();
 
-    // Run the function
-    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 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 > max_num_classes) {
-          num_class_infos = max_num_classes;
-
-          success = false;
-        } else {
+    uint32_t num_class_infos_read = 0;
+    bool already_read_relative_selector_offset = false;
+
+    do {
+      // Run the function.
+      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_read = return_value.GetScalar().ULong();
+        num_class_infos += num_class_infos_read;
+        LLDB_LOG(log, "Discovered {0} Objective-C classes in the shared cache",
+                 num_class_infos_read);
+        if (num_class_infos_read > 0) {
           success = true;
-        }
 
-        // Read the relative selector offset.
-        DataBufferHeap relative_selector_offset_buffer(64, 0);
-        if (process->ReadMemory(relative_selector_offset_addr,
-                                relative_selector_offset_buffer.GetBytes(),
-                                relative_selector_offset_buffer.GetByteSize(),
-                                err) ==
-            relative_selector_offset_buffer.GetByteSize()) {
-          DataExtractor relative_selector_offset_data(
-              relative_selector_offset_buffer.GetBytes(),
-              relative_selector_offset_buffer.GetByteSize(),
-              process->GetByteOrder(), addr_size);
-          lldb::offset_t offset = 0;
-          uint64_t relative_selector_offset =
-              relative_selector_offset_data.GetU64(&offset);
-          if (relative_selector_offset > 0) {
-            // The offset is relative to the objc_opt struct.
-            m_runtime.SetRelativeSelectorBaseAddr(objc_opt_ptr +
-                                                  relative_selector_offset);
+          // Read the relative selector offset. This only needs to occur once no
+          // matter how many times the function is called.
+          if (!already_read_relative_selector_offset) {
+            DataBufferHeap relative_selector_offset_buffer(64, 0);
+            if (process->ReadMemory(
+                    relative_selector_offset_addr,
+                    relative_selector_offset_buffer.GetBytes(),
+                    relative_selector_offset_buffer.GetByteSize(),
+                    err) == relative_selector_offset_buffer.GetByteSize()) {
+              DataExtractor relative_selector_offset_data(
+                  relative_selector_offset_buffer.GetBytes(),
+                  relative_selector_offset_buffer.GetByteSize(),
+                  process->GetByteOrder(), addr_size);
+              lldb::offset_t offset = 0;
+              uint64_t relative_selector_offset =
+                  relative_selector_offset_data.GetU64(&offset);
+              if (relative_selector_offset > 0) {
+                // The offset is relative to the objc_opt struct.
+                m_runtime.SetRelativeSelectorBaseAddr(objc_opt_ptr +
+                                                      relative_selector_offset);
+              }
+            }
+            already_read_relative_selector_offset = true;
           }
-        }
-
-        // Read the ClassInfo structures
-        DataBufferHeap class_infos_buffer(
-            num_class_infos * class_info_byte_size, 0);
-        if (process->ReadMemory(class_infos_addr, class_infos_buffer.GetBytes(),
-                                class_infos_buffer.GetByteSize(),
-                                err) == class_infos_buffer.GetByteSize()) {
-          DataExtractor class_infos_data(class_infos_buffer.GetBytes(),
-                                         class_infos_buffer.GetByteSize(),
-                                         process->GetByteOrder(), addr_size);
 
-          m_runtime.ParseClassInfoArray(class_infos_data, num_class_infos);
+          // Read the ClassInfo structures
+          DataBufferHeap class_infos_buffer(
+              num_class_infos_read * class_info_byte_size, 0);
+          if (process->ReadMemory(class_infos_addr,
+                                  class_infos_buffer.GetBytes(),
+                                  class_infos_buffer.GetByteSize(),
+                                  err) == class_infos_buffer.GetByteSize()) {
+            DataExtractor class_infos_data(class_infos_buffer.GetBytes(),
+                                           class_infos_buffer.GetByteSize(),
+                                           process->GetByteOrder(), addr_size);
+
+            m_runtime.ParseClassInfoArray(class_infos_data,
+                                          num_class_infos_read);
+          }
         }
-      } else {
-        success = true;
-      }
-    } else {
-      if (log) {
+      } else if (log) {
         LLDB_LOGF(log, "Error evaluating our find class name function.");
         diagnostics.Dump(log);
+        break;
       }
-    }
-  } else {
-    if (log) {
-      LLDB_LOGF(log, "Error writing function arguments.");
-      diagnostics.Dump(log);
-    }
+    } while (num_class_infos_read == max_num_classes_in_buffer);
+  } else if (log) {
+    LLDB_LOGF(log, "Error writing function arguments.");
+    diagnostics.Dump(log);
   }
 
-  // Deallocate the memory we allocated for the ClassInfo array
+  LLDB_LOG(log, "Processed {0} Objective-C classes total from the shared cache",
+           num_class_infos);
+  // Cleanup memory we allocated in the process.
+  process->DeallocateMemory(relative_selector_offset_addr);
+  process->DeallocateMemory(class_info_start_idx_addr);
   process->DeallocateMemory(class_infos_addr);
 
   return DescriptorMapUpdateResult(success, false, num_class_infos);
