perf report of clang with no input

0.sh

perf record -e cycles:pp --call-graph lbr -- clang -Wfoo -c -x c /dev/null

perf report --no-children --sort=dso,symbol --total-cycles

1.perf.txt

+   20.18%  [unknown]            [k] 0xffffffffaba01087                                                                                                                                            ▒
+    9.04%  ld-2.33.so           [.] do_lookup_x                                                                                                                                                   ▒
-    8.47%  libc-2.33.so         [.] __strlen_evex                                                                                                                                                 ▒
   - std::basic_ios<char, std::char_traits<char> >::init@plt                                                                                                                                       ◆
      - 5.48% clang::Builtin::Context::initializeBuiltins                                                                                                                                          ▒
           clang::FrontendAction::BeginSourceFile                                                                                                                                                  ▒
           clang::CompilerInstance::ExecuteAction                                                                                                                                                  ▒
           clang::ExecuteCompilerInvocation                                                                                                                                                        ▒
           cc1_main                                                                                                                                                                                ▒
           ExecuteCC1Tool                                                                                                                                                                          ▒
           std::vector<char const*, std::allocator<char const*> >::_M_assign_aux<char const* const*>                                                                                               ▒
           llvm::CrashRecoveryContext::RunSafely                                                                                                                                                   ▒
           clang::driver::CC1Command::Execute                                                                                                                                                      ▒
           clang::driver::Compilation::ExecuteCommand                                                                                                                                              ▒
           clang::driver::Compilation::ExecuteJobs                                                                                                                                                 ▒
           clang::driver::Driver::ExecuteCompilation                                                                                                                                               ▒
           clang_main                                                                                                                                                                              ▒
           __libc_start_main                                                                                                                                                                       ▒
           _start                                                                                                                                                                                  ▒
      + 1.09% llvm::cl::opt<bool, false, llvm::cl::parser<bool> >::opt<char [20], llvm::cl::OptionHidden, llvm::cl::desc, llvm::cl::initializer<bool> >                                            ▒
      + 1.01% llvm::cl::opt<unsigned long, false, llvm::cl::parser<unsigned long> >::opt<char [30], llvm::cl::desc, llvm::cl::initializer<int>, llvm::cl::OptionHidden>                            ▒
      + 0.89% llvm::cl::opt<bool, false, llvm::cl::parser<bool> >::opt<char [33], llvm::cl::desc, llvm::cl::OptionHidden, llvm::cl::initializer<bool> >                                            ▒
+    6.05%  ld-2.33.so           [.] _dl_lookup_symbol_x                                                                                                                                           ▒
+    4.68%  ld-2.33.so           [.] _dl_relocate_object                                                                                                                                           ▒
-    4.64%  libc-2.33.so         [.] __memcmp_evex_movbe                                                                                                                                           ▒
   - 0x6ad8300                                                                                                                                                                                     ▒
      + 2.40% llvm::ARM::parseArch                                                                                                                                                                 ▒
      + 2.24% matchOption                                                                                                                                                                          ▒
+    2.91%  clang-15             [.] clang::DiagnosticIDs::getNearestOption                                                                                                                        ▒
+    2.72%  clang-15             [.] llvm::opt::OptTable::OptTable                                                                                                                                 ▒
+    2.70%  clang-15             [.] clang::Lexer::LexTokenInternal                                                                                                                                ▒
+    2.68%  clang-15             [.] llvm::opt::StrCmpOptionName                                                                                                                                   ▒
+    2.64%  libc-2.33.so         [.] realloc                                                                                                                                                       ▒
+    2.62%  clang-15             [.] clang::EmitBackendOutput

2.observations.md

1. Didn't disable kptr restrict. Probably kernel calls backing malloc.
2. Lot's of time spent in dynamic loader. Was clang linked with -z now? Can we create a cmake config var to NOT DO THAT?
3. Why waste time initializing builtins?
4. llvm::ARM::parseArch does a lot of work, even when not targeting arm!
5. Why do we start the Lexer for no input? Or EmitBackendOutput? Maybe for default preprocessor defines? Maybe the new -fdriver-only flag could speed things up? https://reviews.llvm.org/rGc12577c61dbf