On Sun, Dec 5, 2021 at 6:43 AM Richard Purdie via Gcc-patches
<gcc-patches@gcc.gnu.org> wrote:
>
> On Thu, 2021-12-02 at 20:04 -0700, Jeff Law wrote:
> >
> > On 10/28/2021 10:39 AM, Richard Purdie wrote:
> > > On Thu, 2021-10-28 at 08:51 -0600, Jeff Law wrote:
> > > > On 10/27/2021 2:05 PM, Richard Purdie via Gcc-patches wrote:
> > > > > OpenEmbedded/Yocto Project builds libgcc and the other gcc runtime
> > > > > libraries
> > > > > separately from the compiler and slightly differently to the standard
> > > > > gcc build.
> > > > >
> > > > > In general this works well but in trying to build them separately we
> > > > > run into
> > > > > an issue since we're using our gcc, not xgcc and there is no way to
> > > > > tell configure
> > > > > to use libgcc but not look for libstdc++.
> > > > >
> > > > > This adds such an option allowing such configurations to work.
> > > > >
> > > > > 2021-10-26 Richard Purdie <richard.pur...@linuxfoundation.org>
> > > > >
> > > > > gcc/c-family/ChangeLog:
> > > > >
> > > > > * c.opt: Add --nostdlib++ option
> > > > >
> > > > > gcc/cp/ChangeLog:
> > > > >
> > > > > * g++spec.c (lang_specific_driver): Add --nostdlib++ option
> > > > >
> > > > > gcc/ChangeLog:
> > > > >
> > > > > * doc/invoke.texi: Document --nostdlib++ option
> > > > > * gcc.c: Add --nostdlib++ option
> > > > Couldn't you use -nostdlib then explicitly add -lgcc?
> > > >
> > > > If that works, that would seem better to me compared to adding an option
> > > > to specs processing that is really only useful to one build
> > > > system/procedure.
> > > It sounds great in principle but I've never been able to get it to work.
> > > With
> > > "-nostdinc++ -nostdlib" I miss the startup files so I also tried
> > > "-nostdinc++ -
> > > nodefaultlibs -lgcc". The latter gets further and I can build libstdc++
> > > but the
> > > resulting library doesn't link into applications correctly.
> > Can you be more specific about "doesn't link into applications
> > correctly". I'm still hesitant to add another option if we can
> > reasonably avoid it.
>
> I took a step back and had another look at what our build is doing and why we
> need this. Our build builds the different components separately in many cases
> so
> libstdc++ is built separately since that allows us to tune it to specific
> targets whilst the core gcc is architecture specific.
>
> When we run configure for libstdc++, we have to configure CXX. We can
> configure
> it as:
>
> CXX="${HOST_PREFIX}g++ -nostdinc++"
>
> however that gives errors about a missing libstdc++ during configure tests
> (e.g.
> the atomic ones) since the library isn't present yet and we're trying to build
> it. When I last ran into this I added the nostdlib++ option to mirror the
> stdinc++ one and this solved the problem.
>
> Adding -lgcc doesn't seem to work, binaries using libstdc++ segfault on some
> architectures (mips64 and ppc). I suspect this is a link ordering issue which
> we
> have little control of from the CXX variable but I haven't got deeply into it
> as
> I got the feeling it would be a pain to try and correct, we need the
> compiler's
> automatic linking behaviour which you can't emulate from one commandline.
>
> I did also try -nostdlib and -nodefaultlibs with various libraries added in
> but
> it never seems to work well giving segfaults, again as I suspect the linking
> is
> order specific.
>
> Thinking about the problem from scratch again, I wondered if a dummy libstdc++
> would be enough to make the configure tests work correctly. I've found that I
> can do something like:
>
> mkdir -p XXX/dummylib
> touch XXX/dummylib/libstdc++.so
> export CXX="${CXX} -nostdinc++ -LXXX/dummylib"
>
> and the configure works correctly and the resulting libs don't segfault on
> target.
>
> This is a bit of a hack but probably no worse than some of the other things we
> have to do to build so if you're not keen on the patch we could go with this.
> It
> did seem at least worth discussing our use case though! :)
>
> Cheers,
>
> Richard
>
Drive-by: is -nostdlib++ is implemented, hope its semantics are
similar to clang -nostdlib++ (https://reviews.llvm.org/D35780 since
2017)
