llvmbot wrote:
<!--LLVM PR SUMMARY COMMENT--> @llvm/pr-subscribers-lldb Author: Alex Langford (bulbazord) <details> <summary>Changes</summary> I backed this out due to a problem on one of the bots that myself and others have problems reproducing locally. I'd like to try to land it again, at least to gain more information. Summary: This improves the performance of ObjectFileMacho::ParseSymtab by removing eager and expensive work in favor of doing it later in a less-expensive fashion. Experiment: My goal was to understand LLDB's startup time. First, I produced a Debug build of LLDB (no dSYM) and a Release+NoAsserts build of LLDB. The Release build debugged the Debug build as it debugged a small C++ program. I found that ObjectFileMachO::ParseSymtab accounted for somewhere between 1.2 and 1.3 seconds consistently. After applying this change, I consistently measured a reduction of approximately 100ms, putting the time closer to 1.1s and 1.2s on average. Background: ObjectFileMachO::ParseSymtab will incrementally create symbols by parsing nlist entries from the symtab section of a MachO binary. As it does this, it eagerly tries to determine the size of symbols (e.g. how long a function is) using LC_FUNCTION_STARTS data (or eh_frame if LC_FUNCTION_STARTS is unavailable). Concretely, this is done by performing a binary search on the function starts array and calculating the distance to the next function or the end of the section (whichever is smaller). However, this work is unnecessary for 2 reasons: 1. If you have debug symbol entries (i.e. STABs), the size of a function is usually stored right after the function's entry. Performing this work right before parsing the next entry is unnecessary work. 2. Calculating symbol sizes for symbols of size 0 is already performed in `Symtab::InitAddressIndexes` after all the symbols are added to the Symtab. It also does this more efficiently by walking over a list of symbols sorted by address, so the work to calculate the size per symbol is constant instead of O(log n). --- Full diff: https://github.com/llvm/llvm-project/pull/117079.diff 1 Files Affected: - (modified) lldb/source/Plugins/ObjectFile/Mach-O/ObjectFileMachO.cpp (-63) ``````````diff diff --git a/lldb/source/Plugins/ObjectFile/Mach-O/ObjectFileMachO.cpp b/lldb/source/Plugins/ObjectFile/Mach-O/ObjectFileMachO.cpp index b542e237f023d4..85edf4bd5494ae 100644 --- a/lldb/source/Plugins/ObjectFile/Mach-O/ObjectFileMachO.cpp +++ b/lldb/source/Plugins/ObjectFile/Mach-O/ObjectFileMachO.cpp @@ -3768,7 +3768,6 @@ void ObjectFileMachO::ParseSymtab(Symtab &symtab) { SymbolType type = eSymbolTypeInvalid; SectionSP symbol_section; - lldb::addr_t symbol_byte_size = 0; bool add_nlist = true; bool is_gsym = false; bool demangled_is_synthesized = false; @@ -4354,47 +4353,6 @@ void ObjectFileMachO::ParseSymtab(Symtab &symtab) { if (symbol_section) { const addr_t section_file_addr = symbol_section->GetFileAddress(); - if (symbol_byte_size == 0 && function_starts_count > 0) { - addr_t symbol_lookup_file_addr = nlist.n_value; - // Do an exact address match for non-ARM addresses, else get the - // closest since the symbol might be a thumb symbol which has an - // address with bit zero set. - FunctionStarts::Entry *func_start_entry = - function_starts.FindEntry(symbol_lookup_file_addr, !is_arm); - if (is_arm && func_start_entry) { - // Verify that the function start address is the symbol address - // (ARM) or the symbol address + 1 (thumb). - if (func_start_entry->addr != symbol_lookup_file_addr && - func_start_entry->addr != (symbol_lookup_file_addr + 1)) { - // Not the right entry, NULL it out... - func_start_entry = nullptr; - } - } - if (func_start_entry) { - func_start_entry->data = true; - - addr_t symbol_file_addr = func_start_entry->addr; - if (is_arm) - symbol_file_addr &= THUMB_ADDRESS_BIT_MASK; - - const FunctionStarts::Entry *next_func_start_entry = - function_starts.FindNextEntry(func_start_entry); - const addr_t section_end_file_addr = - section_file_addr + symbol_section->GetByteSize(); - if (next_func_start_entry) { - addr_t next_symbol_file_addr = next_func_start_entry->addr; - // Be sure the clear the Thumb address bit when we calculate the - // size from the current and next address - if (is_arm) - next_symbol_file_addr &= THUMB_ADDRESS_BIT_MASK; - symbol_byte_size = std::min<lldb::addr_t>( - next_symbol_file_addr - symbol_file_addr, - section_end_file_addr - symbol_file_addr); - } else { - symbol_byte_size = section_end_file_addr - symbol_file_addr; - } - } - } symbol_value -= section_file_addr; } @@ -4501,9 +4459,6 @@ void ObjectFileMachO::ParseSymtab(Symtab &symtab) { if (nlist.n_desc & N_WEAK_REF) sym[sym_idx].SetIsWeak(true); - if (symbol_byte_size > 0) - sym[sym_idx].SetByteSize(symbol_byte_size); - if (demangled_is_synthesized) sym[sym_idx].SetDemangledNameIsSynthesized(true); @@ -4622,23 +4577,7 @@ void ObjectFileMachO::ParseSymtab(Symtab &symtab) { Address symbol_addr; if (module_sp->ResolveFileAddress(symbol_file_addr, symbol_addr)) { SectionSP symbol_section(symbol_addr.GetSection()); - uint32_t symbol_byte_size = 0; if (symbol_section) { - const addr_t section_file_addr = symbol_section->GetFileAddress(); - const FunctionStarts::Entry *next_func_start_entry = - function_starts.FindNextEntry(func_start_entry); - const addr_t section_end_file_addr = - section_file_addr + symbol_section->GetByteSize(); - if (next_func_start_entry) { - addr_t next_symbol_file_addr = next_func_start_entry->addr; - if (is_arm) - next_symbol_file_addr &= THUMB_ADDRESS_BIT_MASK; - symbol_byte_size = std::min<lldb::addr_t>( - next_symbol_file_addr - symbol_file_addr, - section_end_file_addr - symbol_file_addr); - } else { - symbol_byte_size = section_end_file_addr - symbol_file_addr; - } sym[sym_idx].SetID(synthetic_sym_id++); // Don't set the name for any synthetic symbols, the Symbol // object will generate one if needed when the name is accessed @@ -4650,8 +4589,6 @@ void ObjectFileMachO::ParseSymtab(Symtab &symtab) { add_symbol_addr(symbol_addr.GetFileAddress()); if (symbol_flags) sym[sym_idx].SetFlags(symbol_flags); - if (symbol_byte_size) - sym[sym_idx].SetByteSize(symbol_byte_size); ++sym_idx; } } `````````` </details> https://github.com/llvm/llvm-project/pull/117079 _______________________________________________ lldb-commits mailing list lldb-commits@lists.llvm.org https://lists.llvm.org/cgi-bin/mailman/listinfo/lldb-commits
