https://github.com/felipepiovezan created https://github.com/llvm/llvm-project/pull/201284
This reduces the number of memory reads performed when reading Objective C classes metadata. Note: these addresses are indeed sequential (with a small offset between them), but there are so many of them that they would not fit into a single Process::ReadMemory cache line, so this is still a win. >From 36543a6cafa90d6be50849bf2b9a10a634f3bf6d Mon Sep 17 00:00:00 2001 From: Felipe de Azevedo Piovezan <[email protected]> Date: Thu, 28 May 2026 11:15:39 +0100 Subject: [PATCH] [lldb] Use batched memory reads in ClassDescriptorV2::relative_list_entry_t This reduces the number of memory reads performed when reading Objective C classes metadata. --- .../AppleObjCClassDescriptorV2.cpp | 65 ++++++++++--------- .../AppleObjCClassDescriptorV2.h | 7 +- 2 files changed, 38 insertions(+), 34 deletions(-) diff --git a/lldb/source/Plugins/LanguageRuntime/ObjC/AppleObjCRuntime/AppleObjCClassDescriptorV2.cpp b/lldb/source/Plugins/LanguageRuntime/ObjC/AppleObjCRuntime/AppleObjCClassDescriptorV2.cpp index 38481ff9e6cfd..1b8992d5e3f5d 100644 --- a/lldb/source/Plugins/LanguageRuntime/ObjC/AppleObjCRuntime/AppleObjCClassDescriptorV2.cpp +++ b/lldb/source/Plugins/LanguageRuntime/ObjC/AppleObjCRuntime/AppleObjCClassDescriptorV2.cpp @@ -393,29 +393,27 @@ ClassDescriptorV2::ivar_t::Read(Process *process, lldb::addr_t addr) { return result; } -llvm::Expected<ClassDescriptorV2::relative_list_entry_t> -ClassDescriptorV2::relative_list_entry_t::Read(Process *process, - lldb::addr_t addr) { +llvm::Expected<llvm::SmallVector<ClassDescriptorV2::relative_list_entry_t>> +ClassDescriptorV2::ReadRelativeListEntries(Process &process, + llvm::ArrayRef<lldb::addr_t> addrs) { size_t size = sizeof(uint64_t); // m_image_index : 16 // m_list_offset : 48 - DataBufferHeap buffer(size, '\0'); - Status error; - - process->ReadMemory(addr, buffer.GetBytes(), size, error); - if (error.Fail()) - return llvm::joinErrors( - error.takeError(), - llvm::createStringErrorV( - "Failed to read relative_list_entry_t at address {0:x}", addr)); - - DataExtractor extractor(buffer.GetBytes(), size, process->GetByteOrder(), - process->GetAddressByteSize()); - lldb::offset_t cursor = 0; - uint64_t raw_entry = extractor.GetU64_unchecked(&cursor); - uint16_t image_index = raw_entry & 0xFFFF; - int64_t list_offset = llvm::SignExtend64<48>(raw_entry >> 16); - return relative_list_entry_t{image_index, list_offset}; + llvm::SmallVector<std::optional<uint64_t>> raw_entries = + process.ReadUnsignedIntegersFromMemory(addrs, size); + + llvm::SmallVector<relative_list_entry_t> results; + results.reserve(addrs.size()); + for (auto [addr, maybe_raw] : llvm::zip(addrs, raw_entries)) { + if (!maybe_raw) + return llvm::createStringErrorV( + "Failed to read relative_list_entry_t at address {0:x}", addr); + uint64_t raw = *maybe_raw; + uint16_t image_index = raw & 0xFFFF; + int64_t list_offset = llvm::SignExtend64<48>(raw >> 16); + results.push_back(relative_list_entry_t{image_index, list_offset}); + } + return results; } llvm::Expected<ClassDescriptorV2::relative_list_list_t> @@ -505,18 +503,23 @@ llvm::Error ClassDescriptorV2::ProcessRelativeMethodLists( if (!relative_method_lists) return relative_method_lists.takeError(); - for (uint32_t i = 0; i < relative_method_lists->m_count; i++) { - // 2. Extract the image index and the list offset from the - // relative_list_entry_t - const lldb::addr_t entry_addr = relative_method_lists->m_first_ptr + - (i * relative_method_lists->m_entsize); - auto entry = relative_list_entry_t::Read(process, entry_addr); - if (!entry) - return entry.takeError(); + // 2. Compute the address of every relative_list_entry_t and read them all in + // a single batched memory read. + auto to_entry_addr = [&](uint64_t idx) { + return relative_method_lists->m_first_ptr + + (idx * relative_method_lists->m_entsize); + }; + auto entry_addrs = llvm::to_vector(llvm::map_range( + llvm::seq<uint64_t>(relative_method_lists->m_count), to_entry_addr)); + + auto entries = ReadRelativeListEntries(*process, entry_addrs); + if (!entries) + return entries.takeError(); + for (auto [entry_addr, entry] : llvm::zip(entry_addrs, *entries)) { // 3. Calculate the pointer to the method_list_t from the // relative_list_entry_t - const lldb::addr_t method_list_addr = entry_addr + entry->m_list_offset; + const lldb::addr_t method_list_addr = entry_addr + entry.m_list_offset; // 4. Get the method_list_t from the pointer llvm::Expected<method_list_t> method_list = @@ -525,10 +528,10 @@ llvm::Error ClassDescriptorV2::ProcessRelativeMethodLists( return method_list.takeError(); // 5. Cache the result so we don't need to reconstruct it later. - m_image_to_method_lists[entry->m_image_index].emplace_back(*method_list); + m_image_to_method_lists[entry.m_image_index].emplace_back(*method_list); // 6. If the relevant image is loaded, add the methods to the Decl - if (!m_runtime.IsSharedCacheImageLoaded(entry->m_image_index)) + if (!m_runtime.IsSharedCacheImageLoaded(entry.m_image_index)) continue; ProcessMethodList(instance_method_func, *method_list); diff --git a/lldb/source/Plugins/LanguageRuntime/ObjC/AppleObjCRuntime/AppleObjCClassDescriptorV2.h b/lldb/source/Plugins/LanguageRuntime/ObjC/AppleObjCRuntime/AppleObjCClassDescriptorV2.h index e861203e7f7d1..e62eedf92c579 100644 --- a/lldb/source/Plugins/LanguageRuntime/ObjC/AppleObjCRuntime/AppleObjCClassDescriptorV2.h +++ b/lldb/source/Plugins/LanguageRuntime/ObjC/AppleObjCRuntime/AppleObjCClassDescriptorV2.h @@ -210,11 +210,12 @@ class ClassDescriptorV2 : public ObjCLanguageRuntime::ClassDescriptor { struct relative_list_entry_t { uint16_t m_image_index; int64_t m_list_offset; - - static llvm::Expected<relative_list_entry_t> Read(Process *process, - lldb::addr_t addr); }; + static llvm::Expected< + llvm::SmallVector<ClassDescriptorV2::relative_list_entry_t>> + ReadRelativeListEntries(Process &process, llvm::ArrayRef<lldb::addr_t> addrs); + struct relative_list_list_t { uint32_t m_entsize; uint32_t m_count; _______________________________________________ lldb-commits mailing list [email protected] https://lists.llvm.org/cgi-bin/mailman/listinfo/lldb-commits
