[llvm-dev] gnu X sysv hash performance

[Rafael Avila de Espindola via llvm-dev]

I got curious how the lld produced gnu hash tables compared to gold. To
test that I timed "perf record ninja check-llvm" (just the lit run) in a
BUILD_SHARED_LIBS build.

The performance was almost identical, so I decided to try sysv versus
gnu (both produced by lld). The results are interesting:

% grep -v '^#' perf-gnu/perf.report-by-dso-sym | head
    38.77%  ld-2.24.so                              [.] do_lookup_x
     8.08%  ld-2.24.so                              [.] strcmp
     2.66%  ld-2.24.so                              [.] _dl_relocate_object
     2.58%  ld-2.24.so                              [.] _dl_lookup_symbol_x
     1.85%  ld-2.24.so                              [.] _dl_name_match_p
     1.46%  [kernel.kallsyms]                       [k] copy_page
     1.38%  ld-2.24.so                              [.] _dl_map_object
     1.30%  [kernel.kallsyms]                       [k] unmap_page_range
     1.28%  [kernel.kallsyms]                       [k]
     filemap_map_pages
     1.26%  libLLVMSupport.so.6.0.0svn              [.] sstep
% grep -v '^#' perf-sysv/perf.report-by-dso-sym | head
    42.18%  ld-2.24.so                              [.] do_lookup_x
    17.73%  ld-2.24.so                              [.] check_match
    14.41%  ld-2.24.so                              [.] strcmp
     1.22%  ld-2.24.so                              [.] _dl_relocate_object
     1.13%  ld-2.24.so                              [.] _dl_lookup_symbol_x
     0.91%  ld-2.24.so                              [.] _dl_name_match_p
     0.67%  ld-2.24.so                              [.] _dl_map_object
     0.65%  [kernel.kallsyms]                       [k] unmap_page_range
     0.63%  [kernel.kallsyms]                       [k] copy_page
     0.59%  libLLVMSupport.so.6.0.0svn              [.] sstep

So the gnu hash table helps a lot, but BUILD_SHARED_LIBS is still crazy
inefficient.

Cheers,
Rafael