Re: [PATCH v1 1/6] LoongArch: a symmetric multilib subdir layout

[Yujie Yang <yangyujie@loongson.cn>]

On Mon, Aug 14, 2023 at 03:48:53PM +0800, Xi Ruoyao wrote:
> On Mon, 2023-08-14 at 15:37 +0800, Yujie Yang wrote:
> > On Mon, Aug 14, 2023 at 01:38:40PM +0800, Xi Ruoyao wrote:
> > > On Mon, 2023-08-14 at 11:57 +0800, Yang Yujie wrote:
> > > 
> > > > However, for LoongArch, we do not want such a "toplevel" library
> > > > installation since the default ABI may change.  We expect all
> > > > multilib variants of libraries to be installed to their designated
> > > > ABI-specific subdirs (e.g. base/lp64d) of the GCC libdir, so that
> > > > the default ABI can be configured arbitrarily (with --with-abi)
> > > > while the gcc libdir layout stays consistent.  This could be
> > > > helpful for the distribution packaging of GCC libraries.
> > > 
> > > Have you tested a --disable-multilib configuration?  To me with --
> > > disable-configuration everything should be still in the toplevel
> > > directory, not any sub-directory.
> > 
> > That's a good point, sorry I missed --disable-multilib here.
> > 
> > However, you don't really need --disable-multilib since
> > the libraries are only built once in the default ABI configuration
> > as long as --with-multilib-list does not request anything more than
> > that.
> > 
> > Maybe we should force-enabling multilib in all cases.
> 
> I really don't like this.  Why must I always remind my self "hey, this
> is LoongArch, there is a different directory layout" when I don't need
> multilib at all?
> 

AFAIK, the two main uses of the multisubdir layout are in the C++
header directory and the GCC libdir (where libgcc.a resides), respectively.
The GCC libdir is fine since they are private to a user's GCC build.
However, the C++ header directory is shared across the system unless
an alternative sysroot is chosen, so the consisentency of the multilib
layout matters.

So theoretically, the toplevel libraries should have the same ABI under
the the target triplet.  However, for many architectures, the
"--with-abi + MULTILIB_DEFAULT" scheme may cause the toplevel to be
configured to have different meanings.

So I think it's also a reasonable approach that we just simply eliminate
the ambiguous toplevel libraries and use a symmetric layout instead.