[cfe-dev] RFC: New clang option to force linking libclang_rt.a

[Petr Hosek via cfe-dev]

On Wed, Feb 6, 2019 at 2:57 AM Peter Smith via cfe-dev <
cfe-dev at lists.llvm.org> wrote:

> Hello Amara,
>
> Given the amount of possible names, sometimes target dependent, that
> libclang_rt can take I think the current situation is unfriendly
> enough to do something about it.
>
> The GCC equivalent solution to this is -print-libgcc-file-name
> From the man page:
> "-print-libgcc-file-name
>            Same as -print-file-name=libgcc.a.
>
>            This is useful when you use -nostdlib or -nodefaultlibs but
> you do want to link with libgcc.a.  You can do:
>
>                    gcc -nostdlib <files>... `gcc -print-libgcc-file-name`
> "
>
> Would it be worth coming up with a similar solution in clang?
>

This is already supported in Clang, we also use this solution in the CMake
build for libunwind, libc++abi, libc++ and compiler-rt to find out the
location of builtins (since all these libraries are being built with
-nodefaultlibs), see for example
https://github.com/llvm/llvm-project/blob/master/compiler-rt/cmake/Modules/HandleCompilerRT.cmake#L3


> Some other questions:
> - On Linux there is not a single libclang_rt.a, but many separate ones
> like ./clang/9.0.0/lib/linux/libclang_rt.builtins-x86_64.a would this
> option be just the builtins?
> - Would this extend to libgcc on linux? I don't know enough about
> Windows to know about an equivalent?
> - A possible alternative is that the presence of --rtlib=<library> is
> sufficient to add the -l even if --nodefaultlib etc. are added.
>
> Peter
>
> On Tue, 5 Feb 2019 at 22:53, Duncan Exon Smith via cfe-dev
> <cfe-dev at lists.llvm.org> wrote:
> >
> > +cfe-dev, bcc: llvm-dev, since you're talking about changes to Clang.
> >
> > > On 2019 Feb  5, at 14:43, Amara Emerson <aemerson at apple.com> wrote:
> > >
> > > Hi all,
> > >
> > > As far as I’m aware, there’s no way to specify to the clang driver at
> link time that you want to link in libclang_rt.a even in the presence of
> -nostdlib or -nodefaultlib. This behavior can be desirable in some cases as
> users may want to avoid linking libc++ or libSystem.dylib for example, but
> still want to have the compiler’s own builtin routines available. Since we
> view compiler-rt as an intricately linked (no pun intended) part of the
> overall compiler toolchain, I think this is a reasonable request.
> > >
> > > I’m proposing a new driver option to trigger this behaviour:
> -flink-rtlib
> > >
> > > Thoughts?
> > >
> > > Thanks,
> > > Amara
> >
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