Re: Very slow clang kernel config ..

[Fangrui Song]

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@xxxxxxxxxxxxxxxxxxxx> 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@xxxxxxxxxx/

-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.)