linux-kernel - Re: Very slow clang kernel config ..

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Date:   Mon, 3 May 2021 12:00:17 -0700
From:   Fangrui Song <maskray@...gle.com>
To:     Tom Stellard <tstellar@...hat.com>
Cc:     Dan Aloni <dan@...nelim.com>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Masahiro Yamada <masahiroy@...nel.org>,
        Nathan Chancellor <nathan@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        clang-built-linux <clang-built-linux@...glegroups.com>,
        Serge Guelton <sguelton@...hat.com>,
        Sylvestre Ledru <sylvestre@...illa.com>
Subject: Re: Very slow clang kernel config ..

On 2021-05-03, Tom Stellard wrote:
>On 5/1/21 10:19 PM, Dan Aloni wrote:
>>On Fri, Apr 30, 2021 at 06:48:11PM -0700, Nick Desaulniers wrote:
>>>On Fri, Apr 30, 2021 at 6:22 PM Linus Torvalds
>>><torvalds@...ux-foundation.org> wrote:
>>>>    0.92%  libLLVM-12.so       llvm::StringMapImpl::LookupBucketFor
>>>
>>>^ wait a minute; notice how in your profile the `Shared Object` is
>>>attributed to `libLLVM-12.so` while mine is `clang-13`?  Clang can be
>>>built as either having libllvm statically linked or dynamically; see
>>>the cmake variables
>>>LLVM_BUILD_LLVM_DYLIB:BOOL
>>>LLVM_LINK_LLVM_DYLIB:BOOL
>>>BUILD_SHARED_LIBS:BOOL
>>>https://llvm.org/docs/CMake.html
>>>
>>>I think those are frowned upon; useful for cutting down on developers
>>>iteration speed due to not having to relink llvm when developing
>>>clang. But shipping that in production? I just checked and it doesn't
>>>look like we do that for AOSP's build of LLVM.
>>
>>There's also `-DLLVM_ENABLE_LTO=Thin` that enables LTO for building LLVM
>>and Clang themselves, considered they can be bootstrapped like this
>>using a previous version of Clang. Combining that with a non-shared
>>library build mode for both Clang and LLVM, the result is possibly the
>>fastest and most optimized build that is achievable.  Unfortunately I
>>see distributions neglecting to enable this in packaging this as well.
>>
>>On a side note, I'm also a Fedora user and agree with Linus about this.
>>I'd like to see an opt-in bypass of the shared library policy via
>>something like `dnf install clang-optimized` that would install the
>>fastest and most optimized Clang build regardless of RPM install size.
>>
>
>I have experimented with creating a static version of clang in the past,
>but never really found a solution I liked enough to upstream into Fedora.
>e.g. This solution[1] that we're using to bootstrap clang in our internal
>clang-as-the-default-cc Fedora buildroots that we use for testing.
>
>If someone could file a bug[2] against the clang package in Fedora (or RHEL even)
>with some data or other information that shows the downsides of the shared
>build of of clang, that would be really helpful.
>
>-Tom
>
>[1] https://src.fedoraproject.org/fork/tstellar/rpms/clang/c/dea2413c6822cc7aa7a08ebe73d10abf8216259f?branch=clang-minimal
>[2] https://bugzilla.redhat.com/
>

I have filed https://bugzilla.redhat.com/show_bug.cgi?id=1956484 with
information from my previous reply https://lore.kernel.org/lkml/20210501235549.vugtjeb7dmd5xell@google.com/

-fpic (.so) -fno-semantic-interposition -Wl,-Bsymbolic-functions is very
close to -fpic/-fpie (.a) in terms of performance.


(
If Fedora is willing to use -fprofile-use (profile guided optimization)
or ThinLTO, that's great as well, with the cost of much longer build time.)