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From:   Nick Desaulniers <ndesaulniers@google.com>
Date:   Fri, 30 Apr 2021 17:40:56 -0700
Message-ID: <CAKwvOd=56=XJ9UPKVYJ=pTRGMNzdQe5k-rsx=66XQhWtrwtEqQ@mail.gmail.com>
Subject: Re: Very slow clang kernel config ..
To:     Linus Torvalds <torvalds@linux-foundation.org>,
        Masahiro Yamada <masahiroy@kernel.org>
Cc:     Nathan Chancellor <nathan@kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        clang-built-linux <clang-built-linux@googlegroups.com>
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On Fri, Apr 30, 2021 at 5:25 PM Nick Desaulniers
<ndesaulniers@google.com> wrote:
>
> On Fri, Apr 30, 2021 at 5:23 PM Nick Desaulniers
> <ndesaulniers@google.com> wrote:
> >
> > On Fri, Apr 30, 2021 at 5:19 PM Nick Desaulniers
> > <ndesaulniers@google.com> wrote:
> > >
> > > A quick:
> > > $ perf record -e cycles:pp --call-graph lbr make LLVM=1 LLVM_IAS=1
> > > -j72 allmodconfig
> > > $ perf report --no-children --sort=dso,symbol
> > > shows:
> > >      2.35%  [unknown]                [k] 0xffffffffabc00fc7
> > > +    2.29%  libc-2.31.so             [.] _int_malloc
> > >      1.24%  libc-2.31.so             [.] _int_free
> > > +    1.23%  ld-2.31.so               [.] do_lookup_x
> >
> > ^ bfd
>
>
> > > +    0.63%  ld-2.31.so               [.] _dl_lookup_symbol_x
> >
> > ^ bfd
>
> Ah, no, sorry, these are the runtime link editor/loader. So probably
> spending quite some time resolving symbols in large binaries.

I don't see NOW in:
$ llvm-readelf -Wd `which clang`
so I don't think clang was linked as `-Wl,-z,now`. I also see both
.hash and .gnu.hash in
$ llvm-readelf -S `which clang`.

The presence of NOW or lack of .gnu.hash would have been my guess for
symbol lookup issues. Perhaps the length of C++ mangled symbols
doesn't help.
-- 
Thanks,
~Nick Desaulniers