[llvm-dev] gnu X sysv hash performance

[Rui Ueyama via llvm-dev]

On Fri, Dec 1, 2017 at 1:26 PM, Rafael Avila de Espindola <
rafael.espindola at gmail.com> wrote:

>
> 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.


What is "100%" in these numbers? If 100% means all execution time,
ld-2.24.so takes more than 70% of execution time. Is this real?
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