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From:   Nick Desaulniers <ndesaulniers@google.com>
Date:   Fri, 30 Apr 2021 17:23:40 -0700
Message-ID: <CAKwvOdm3D=dqKw=kx46PLaiqfHOZJL3QFKGc8kxqJqpwdFFWqw@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 5:19 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

> +    1.14%  libc-2.31.so             [.] __strlen_avx2
> +    1.06%  libc-2.31.so             [.] malloc
> +    1.03%  clang-13                 [.] llvm::StringMapImpl::LookupBucketFor
>      1.01%  libc-2.31.so             [.] __memmove_avx_unaligned_erms
> +    0.76%  conf                     [.] yylex

^ kconfig

> +    0.68%  clang-13                 [.] llvm::Instruction::getNumSuccessors
> +    0.63%  libbfd-2.35.2-system.so  [.] bfd_hash_lookup

^ bfd

> +    0.63%  clang-13                 [.] llvm::PMDataManager::findAnalysisPass

^ this is another suspect one to me, though perhaps I should isolate
out the HOSTCC invocations vs cc-option checks. Should retest with
just CC=clang rather than HOSTCC=clang set via LLVM=1.

> +    0.63%  ld-2.31.so               [.] _dl_lookup_symbol_x

^ bfd

>      0.62%  libc-2.31.so             [.] __memcmp_avx2_movbe
>      0.60%  libc-2.31.so             [.] __strcmp_avx2
> +    0.56%  clang-13                 [.] llvm::ValueHandleBase::AddToUseList
> +    0.56%  clang-13                 [.]
> llvm::operator==<llvm::DenseMap<llvm::BasicBlock const*, unsigned int,
> llvm::DenseMapInfo<llvm::BasicBlock const*>, llvm::detail::Dense
>      0.53%  clang-13                 [.]
> llvm::SmallPtrSetImplBase::insert_imp_big


-- 
Thanks,
~Nick Desaulniers