Re: libstdc++ search path while building GCC

[Jonathan Wakely <jwakely.gcc@gmail.com>]

On Tue, 28 Jul 2020 at 15:15, Ludovic Courtès <ludovic.courtes@inria.fr> wrote:
>
> Hi,
>
> Jonathan Wakely <jwakely.gcc@gmail.com> skribis:
>
> > On Wed, 22 Jul 2020 at 21:39, Ludovic Courtès <ludovic.courtes@inria.fr> wrote:
> >>
> >> Hello,
> >>
> >> While building GCC on a non-FHS distro, we do something like¹:
> >>
> >>   export CPLUS_INCLUDE_PATH=/path/to/host/gcc/include/c++:/path/to/host/gcc/include
> >
> > That looks wrong. Why are you doing this? Doesn't your host GCC
> > already know where its headers are?
>
> It does.  With Guix, CPLUS_INCLUDE_PATH is automatically populated with
> all the include/ and include/c++/ directories of dependencies, which
> includes GCC’s own libstdc++.  We could override that, but I’m trying to
> understand what’s going on.
>
> >> This ensures the host GCC’s C++ headers are found by ‘xgcc’ during
> >> build.
> >
> > Why do you want that to happen?
> >
> > It's wrong in general, but it's specifically wrong in your case
> > because the host compiler is a different version to the one you're
> > trying to build. There is absolutely no guarantee that GCC 10.1 can
> > compile the libstdc++ headers from GCC 7.5, and trying to do so is not
> > supported.
>
> Is it wrong?  I’m talking about the first time where
> ‘libstdc++-v3/configure’ runs, which itself checks for C++ features.
> It’s checking for C++ features of the host compiler/C++ library, isn’t it?

No. Building libstdc++ is done with the GCC that's just been built,
not with the host compiler.

You can see that for yourself, because te error you showed was running
/tmp/guix-build-gcc-10.1.0.drv-0/build/./gcc/xgcc and that's not the
host compiler, that's the one that's just been built.

I think the problem is that CPLUS_INCLUDE_PATH affects both the host
compiler, and the newly-built one. I don't see how you can use an
environment variable to set include paths to C++ headers that are
valid for both compilers at once. Because no set of libstdc++ headers
is valid for both GCC 7.5 and 10.1 at the same time.