[llvm-dev] How to compile glibc with clang/llvm?

Kristina Brooks via llvm-dev llvm-dev at lists.llvm.org
Sun Dec 23 12:24:43 PST 2018

Such is the case for most compilers, Clang supports extensions such as blocks for example, which IIRC date
back to before C++11 lambdas or around the same time. There's a lot of other implementation specific differences
for example in attributes.

Anyway, skipping that point, Glibc upstream isn't actively trying to work with Clang compliant unlike mainline Linux,
as you may guess from the name Glibc is a GNU libc, aimed to work and be built by the GNU Compiler Collection aka
GCC. If we look at a non-GNU libc such as, say, Bionic, currently it's built with a Clang based toolchain.

If you want a starting point for building Glibc forks with Clang, I mentioned one already, if the documentation
is correct, it has decent Clang support but YMMV.

On 23/12/2018 18:53, blubee blubeeme wrote:
> GNU likes to use non standard extensions for convenience while LLVM/Clang would like stick to the standards.
> Unless GNU adopts a more standards compliant behavior OR LLVM decides to relax their adherence to the standards
> the tools will always be somewhat incompatible w/o major work.
> For example recent changes to the Linux Kernel adopted GNU goto asm; which means that Clang will not compile
> the kernel; 
> I am not sure how far LLVM is willing to bend over to please the GNU nor am I certain GNU will become more
> standards complaint anytime soon.
> You might try the route of patching glibc but that might not be the best use of your time.
> There are other c implementations out there.
> Best
> On Mon, Dec 24, 2018 at 12:01 AM Alberto Barbaro via llvm-dev <llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org>>
> wrote:
>     Hi,
>     Could you keep us posted, or at least me? I'm interested to compile glibc as well and obtain the bitcode.
>     Would you consider to use https://www.musl-libc.org in the meantime?
>     Thanks
>     On Sun, Dec 23, 2018, 12:00 Kristina Brooks via llvm-dev <llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org>
>     wrote:
>         Hi.
>         Actually from reading the README, it seems to imply that it can be built
>         with Clang 6.0.0 and above now, though it does incorporate a lot of patches not
>         specific to Clang building so you will end up with them as well:
>         ----------------------------------------------------------------------------
>         GRTE v5 and later can also be built with clang and (optionally) lld.
>         LLVM support for GNU source code continues to evolve (as of June
>         2018), so the process is less straightforward, and likely to change
>         from what is documented here.  There are a number of glibc patches
>         that make this work, including additional configure options mentioned
>         below.
>         The minimum version of clang is 6.0.0.  If lld is to be used for linking,
>         it needs to be newer than 6.0.0.
>         Configure:
>           CC=path-to-llvm/clang CXX=path-to-llvm/clang++ \
>             ../glibc/configure --disable-werror --with-clang --disable-float128 \
>               --with-lld --with-default-link --disable-multi-arch --prefix=/something
>         ------------------------------------------------------------------------------
>         On 23/12/2018 02:51, Kristina Brooks wrote:
>         > Hi.
>         >
>         > I've managed to do it before, with a lot of patches to Glibc, as well as ld.so and getting basic programs to
>         start with
>         > it. However, a lot of tests did not pass and in general I abandoned it due to the fact that Glibc source code
>         is not
>         > very easy to work with.
>         >
>         > I can upload it somewhere, not sure if there is any general interest in this kind of thing. I think current
>         GRTE may
>         > also be build-able with Clang (my fork was based on older GRTE which had unfinished patches to support Clang),
>         though I
>         > haven't had the time to look at that recently.
>         >
>         > I should add a huge disclaimer: this won't be easy and requires understanding the macro hell in glibc, GRTE
>         relies on
>         > some hacky uses of `_Pragma` to supplement for some macros otherwise not possible with Clang. Even if you do
>         get it to
>         > build I think it may be a waste of time to try and just implement the missing symbols in your case (or figure
>         out why
>         > they're not being exported).
>         >
>         > Thanks.
>         >
>         > On 22/12/2018 09:29, xuruobin via llvm-dev wrote:
>         >> To whom it may concern,
>         >>
>         >>      Is there a way to build glibc with clang/llvm? I’m working on enabling llvm-cov for my compiler which is
>         a totally new arch with a libc.a built from newlib. I successfully built compiler-rt but when I typed the
>         command ` clang++ --target=xxx -fprofile-instr-generate -fcoverage-mapping foo.cc -o foo`, the linker failed
>         because of undefined reference to `mmap'/`ftruncate'/`mkdir'. I found these functions are supported by glibc but
>         I cannot bulid glibc with clang/llvm(configure gave me an error “These critical programs are missing or too old:
>         compiler”). So I want to know how can I compile glibc with clang/llvm or are there some WIP patches to help me
>         with this requirement?
>         >>
>         >> Thanks,
>         >> Ruobin.
>         >> _______________________________________________
>         >> LLVM Developers mailing list
>         >> llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org>
>         >> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>         >>
>         >
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