[lldb] Do not use LC_FUNCTION_STARTS data to determine symbol size as symbols are created

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).

Author: bulbazord

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);