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

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Message-ID: <20210502051909.pac2pycninc7fl53@gmail.com>
Date:   Sun, 2 May 2021 08:19:09 +0300
From:   Dan Aloni <dan@...nelim.com>
To:     Nick Desaulniers <ndesaulniers@...gle.com>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Tom Stellard <tstellar@...hat.com>,
        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>,
        Fangrui Song <maskray@...gle.com>,
        Serge Guelton <sguelton@...hat.com>,
        Sylvestre Ledru <sylvestre@...illa.com>
Subject: Re: Very slow clang kernel config ..

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.

-- 
Dan Aloni