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

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merged 3 commits into from
Nov 12, 2025
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[lldb] Adapt shared cache ObjC class metadata extraction for long term growth #167579

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f3659c8
[lldb] Adapt shared cache ObjC class metadata extraction for long ter…
bulbazord Nov 11, 2025
096ec4b
fixup! [lldb] Adapt shared cache ObjC class metadata extraction for l…
bulbazord Nov 11, 2025
2189bcf
fixup! fixup! [lldb] Adapt shared cache ObjC class metadata extractio…
bulbazord Nov 12, 2025
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260 changes: 156 additions & 104 deletions lldb/source/Plugins/LanguageRuntime/ObjC/AppleObjCRuntime/AppleObjCRuntimeV2.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -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;
Expand All @@ -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;
Expand Down Expand Up @@ -480,7 +482,10 @@ __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)
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);
Expand Down Expand Up @@ -524,59 +529,74 @@ __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]);

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);
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]);

if (classOffsets[i].isDuplicate) {
DEBUG_PRINTF("isDuplicate = true\n");
continue; // duplicate
}
DEBUG_PRINTF ("duplicate_count = %u\n", duplicate_count);
DEBUG_PRINTF ("duplicateClassOffsets = %p\n", duplicateClassOffsets);

if (objectCacheOffset == 0) {
DEBUG_PRINTF("objectCacheOffset == invalidEntryOffset\n");
continue; // invalid offset
}
uint32_t duplicate_start_idx = *start_idx < clsopt->capacity ? 0 : *start_idx - clsopt->capacity;

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 (classOffsets[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. There's an edge case where if there are exactly max_class_infos number of classes,
// start_idx will not get updated and LLDB will enter an infinite loop reading.
if (*start_idx == original_start_idx) {
*start_idx = idx;
}
}
else if (objc_opt->version >= 12 && objc_opt->version <= 15)
Expand Down Expand Up @@ -1959,6 +1979,9 @@ 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;
Expand All @@ -1976,6 +1999,13 @@ 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);
Expand Down Expand Up @@ -2313,10 +2343,7 @@ 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) {
Expand All @@ -2338,22 +2365,40 @@ 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
Expand All @@ -2362,12 +2407,13 @@ 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;

Expand All @@ -2394,78 +2440,84 @@ 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) {
LLDB_LOGF(log, "Error evaluating our find class name function.");
diagnostics.Dump(log);
if (log) {
LLDB_LOGF(log, "Error evaluating our find class name function.");
diagnostics.Dump(log);
break;
}
}
}
} while (num_class_infos_read == max_num_classes_in_buffer);
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@JDevlieghere JDevlieghere Nov 11, 2025

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Interesting way to figure out if we're done reading. I would have (naively) tried to figure out the number of classes to read, but this avoids an additional out argument.

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@bulbazord bulbazord Nov 11, 2025

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clsopt->capacity + clsopt->duplicate_count gives you an upper bound but each entry must be examined to know if it should be counted. It seemed simpler to call the function until the buffer isn't completely filled.

} 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",
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@JDevlieghere JDevlieghere Nov 11, 2025

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Would it be worthwhile (e.g. for performance reasons) to include how many times we had to invoke the utility function to read all the classes?

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@bulbazord bulbazord Nov 11, 2025

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Yes, though this information can be derived from existing logs. I log after every call, this log at the end tells you the total number of classes processed.

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);
Expand Down
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