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From: Fangrui Song <maskray@google.com>
To: Tom Stellard <tstellar@redhat.com>
Cc: Dan Aloni <dan@kernelim.com>,
	Nick Desaulniers <ndesaulniers@google.com>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	Masahiro Yamada <masahiroy@kernel.org>,
	Nathan Chancellor <nathan@kernel.org>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	clang-built-linux <clang-built-linux@googlegroups.com>,
	Serge Guelton <sguelton@redhat.com>,
	Sylvestre Ledru <sylvestre@mozilla.com>
Subject: Re: Very slow clang kernel config ..
Date: Mon, 3 May 2021 12:00:17 -0700
Message-ID: <20210503190017.3r6mzy7xqx2bfwmk@google.com> (raw)
In-Reply-To: <6c4febd7-f826-9935-4163-138b1fed8278@redhat.com>

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@linux-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.)

  reply index

Thread overview: 47+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-04-29 21:53 Linus Torvalds
2021-04-30  0:19 ` Nick Desaulniers
2021-04-30  2:22   ` Nick Desaulniers
2021-05-01  0:19     ` Nick Desaulniers
2021-05-01  0:23       ` Nick Desaulniers
2021-05-01  0:25         ` Nick Desaulniers
2021-05-01  0:40           ` Nick Desaulniers
2021-05-01  1:22           ` Linus Torvalds
2021-05-01  1:48             ` Nick Desaulniers
2021-05-01  2:16               ` Fangrui Song
2021-05-01  3:32               ` Tom Stellard
2021-05-01 16:32                 ` Linus Torvalds
2021-05-01 19:57                   ` Serge Guelton
2021-05-01 22:39                     ` Linus Torvalds
2021-05-01 23:55                       ` Fangrui Song
2021-05-01 21:58                   ` David Laight
2021-05-02  9:31                   ` Adrian Bunk
2021-05-02 11:35                     ` David Laight
2021-05-02 16:12                     ` Linus Torvalds
2021-05-02 16:45                       ` Adrian Bunk
2021-05-02 16:49                         ` Linus Torvalds
2021-05-02 17:55                           ` Adrian Bunk
2021-05-02 17:59                             ` Linus Torvalds
2021-05-02 21:48                               ` Adrian Bunk
2021-05-04 22:02                                 ` Miguel Ojeda
2021-05-05  0:58                                   ` Theodore Ts'o
2021-05-05 17:21                                     ` Miguel Ojeda
2021-05-04 21:32                     ` Miguel Ojeda
2021-05-05 11:05                       ` David Laight
2021-05-05 13:53                         ` Miguel Ojeda
2021-05-05 14:13                           ` David Laight
2021-05-05 16:06                             ` Miguel Ojeda
2021-05-05 16:25                               ` David Laight
2021-05-05 17:55                                 ` Miguel Ojeda
2021-05-03  1:03                   ` Maciej W. Rozycki
2021-05-03 14:38                     ` Theodore Ts'o
2021-05-03 14:54                       ` Theodore Ts'o
2021-05-03 17:14                         ` Maciej W. Rozycki
2021-05-03 16:09                       ` David Laight
2021-05-04 23:04                       ` Greg Stark
2021-05-05  0:55                         ` Theodore Ts'o
2021-05-01 23:37               ` Mike Hommey
2021-05-02  5:19               ` Dan Aloni
2021-05-03 16:48                 ` Tom Stellard
2021-05-03 19:00                   ` Fangrui Song [this message]
2021-04-30  0:52 ` Nathan Chancellor
2021-04-30  2:21   ` Nick Desaulniers

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