Optimize fake ELF section lookup while parsing symbols in ObjectFileELF

[xiaobai]

Summary

I recently ran into a shared object that had a reasonably large number
of absolute symbols. Parsing all the symbols in the shared object took an
unusually long amount of time, so I looked into it and found that when we
created fake sections for these symbols, we would add them to the module's
section list without inserting it into the cache (section_name_to_section).
While this works, in some cases we perform a lookup of the section in that cache
almost right after we create it and put in the module section list.
Given I had a large amount of symbols that triggered this code path, performance
was abysmal. This change greatly improved performance there.

[zturner]

This could be tested by using lldb-test to dump the section cache and running FileCheck on it to make sure that all expected sections are cached.

[clayborg]

Looks fine to me, but a test would be nice as Zach suggested. I am guessing before we made way too many sections. Should be easy to conjure up a test and verify the same sections is used.

source/Plugins/ObjectFile/ELF/ObjectFileELF.cpp

 }
 
 static uint32_t mipsVariantFromElfFlags (const elf::ELFHeader &header) {
   const uint32_t mips_arch = header.e_flags & llvm::ELF::EF_MIPS_ARCH;
   uint32_t endian = header.e_ident[EI_DATA];
   uint32_t arch_variant = ArchSpec::eMIPSSubType_unknown;
   uint32_t fileclass = header.e_ident[EI_CLASS];
 
-  // If there aren't any elf flags available (e.g core elf file) then return default 
+  // If there aren't any elf flags available (e.g core elf file) then return default
   // 32 or 64 bit arch (without any architecture revision) based on object file's class.
   if (header.e_type == ET_CORE) {
     switch (fileclass) {
     case llvm::ELF::ELFCLASS32:
       return (endian == ELFDATA2LSB) ? ArchSpec::eMIPSSubType_mips32el
                                      : ArchSpec::eMIPSSubType_mips32;
     case llvm::ELF::ELFCLASS64:
       return (endian == ELFDATA2LSB) ? ArchSpec::eMIPSSubType_mips64el
else if ((note.n_type == LLDB_NT_GNU_ABI_TAG) &&
             break;
           }
         }
         if (arch_spec.IsMIPS() &&
             arch_spec.GetTriple().getOS() == llvm::Triple::OSType::UnknownOS)
           // In case of MIPSR6, the LLDB_NT_OWNER_GNU note is missing
           // for some cases (e.g. compile with -nostdlib)
           // Hence set OS to Linux
-          arch_spec.GetTriple().setOS(llvm::Triple::OSType::Linux); 
+          arch_spec.GetTriple().setOS(llvm::Triple::OSType::Linux);
       }
     }
 
     // Calculate the offset of the next note just in case "offset" has been used
     // to poke at the contents of the note data
     offset = note_offset + note.GetByteSize();
   }
 
size_t ObjectFileELF::GetSectionHeaderInfo(SectionHeaderColl &section_headers,
   // Only initialize the arch_spec to okay defaults if they're not already set.
   // We'll refine this with note data as we parse the notes.
   if (arch_spec.GetTriple().getOS() == llvm::Triple::OSType::UnknownOS) {
     llvm::Triple::OSType ostype;
     llvm::Triple::OSType spec_ostype;
     const uint32_t sub_type = subTypeFromElfHeader(header);
     arch_spec.SetArchitecture(eArchTypeELF, header.e_machine, sub_type,
                               header.e_ident[EI_OSABI]);
-    
     // Validate if it is ok to remove GetOsFromOSABI.
     // Note, that now the OS is determined based on EI_OSABI flag and
     // the info extracted from ELF notes (see RefineModuleDetailsFromNote).
     // However in some cases that still might be not enough: for example
     // a shared library might not have any notes at all
     // and have EI_OSABI flag set to System V,
     // as result the OS will be set to UnknownOS.
     GetOsFromOSABI(header.e_ident[EI_OSABI], ostype);
if (symbol_section_sp == nullptr && section_idx == SHN_ABS &&
       ConstString fake_section_name(std::string(".absolute.") + symbol_name);
       symbol_section_sp =
           std::make_shared<Section>(module_sp, this, SHN_ABS, fake_section_name,
                                     eSectionTypeAbsoluteAddress, symbol_value,
                                     symbol.st_size, 0, 0, 0, SHF_ALLOC);
 
       module_section_list->AddSection(symbol_section_sp);
       section_list->AddSection(symbol_section_sp);
+      section_name_to_section.emplace(fake_section_name.GetCString(),
+                                      symbol_section_sp);
     }
 
     if (symbol_section_sp &&
         CalculateType() != ObjectFile::Type::eTypeObjectFile)
       symbol_value -= symbol_section_sp->GetFileAddress();
 
     if (symbol_section_sp && module_section_list &&
         module_section_list != section_list) {