Re: [PATCH] Introduce -nolibstdc++ option

[Fangrui Song <maskray@google.com>]

On Tue, Jun 21, 2022 at 1:43 AM Richard Biener via Gcc-patches
<gcc-patches@gcc.gnu.org> wrote:
>
> On Tue, Jun 21, 2022 at 7:56 AM Alexandre Oliva via Gcc-patches
> <gcc-patches@gcc.gnu.org> wrote:
> >
> >
> > Using g++ to link without libstdc++, as in g++.dg/abi/pure-virtual1.C,
> > is error prone, because there's no way to tell g++ to drop libstdc++
> > without also dropping libc and any other libraries that the target
> > implicitly links in.
> >
> > This has often led to the need for manual adjustments to this
> > testcase.
> >
> > I figured adding support for -nolibstdc++, even though redundant,
> > makes some sense.  One could presumably use gcc rather than g++ for
> > linking, for the same effect, but sometimes changing the link command
> > is harder than adding an option, as in our testsuite.
> >
> > Regstrapped on x86_64-linux-gnu, also tested with a cross to
> > aarch64-rtems6.  Ok to install?
>
> OK in case nobody objects in 24h.
>
> Richard.

Is this similar to clang -nostdlib++ ?
When libstdc++ is selected, clang -nostdlib++ removes -lstdc++.

> >
> > for  gcc/ChangeLog
> >
> >         * common.opt (nolibstdc++): New.
> >         * doc/invoke.texi (-nolibstdc++): Document it.
> >
> > for  gcc/cp/ChangeLog
> >
> >         * g++spec.c (lang_specific_driver): Implement -nolibstdc++.
> >
> > for  gcc/testsuite/ChangeLog
> >
> >         * g++.dg/abi/pure-virtual1.C: Use -nolibstdc++.
> > ---
> >  gcc/common.opt                           |    3 +++
> >  gcc/cp/g++spec.cc                        |    1 +
> >  gcc/doc/invoke.texi                      |    6 +++++-
> >  gcc/testsuite/g++.dg/abi/pure-virtual1.C |    2 +-
> >  4 files changed, 10 insertions(+), 2 deletions(-)
> >
> > diff --git a/gcc/common.opt b/gcc/common.opt
> > index 32917aafcaec1..e00c6fc2fb098 100644
> > --- a/gcc/common.opt
> > +++ b/gcc/common.opt
> > @@ -3456,6 +3456,9 @@ Driver
> >  nolibc
> >  Driver
> >
> > +nolibstdc++
> > +Driver
> > +
> >  nostdlib
> >  Driver
> >
> > diff --git a/gcc/cp/g++spec.cc b/gcc/cp/g++spec.cc
> > index 8174d652776b1..539e6ca089d85 100644
> > --- a/gcc/cp/g++spec.cc
> > +++ b/gcc/cp/g++spec.cc
> > @@ -160,6 +160,7 @@ lang_specific_driver (struct cl_decoded_option **in_decoded_options,
> >         {
> >         case OPT_nostdlib:
> >         case OPT_nodefaultlibs:
> > +       case OPT_nolibstdc__:
> >           library = -1;
> >           break;
> >
> > diff --git a/gcc/doc/invoke.texi b/gcc/doc/invoke.texi
> > index 50f57877477bc..469b6d97e0dfa 100644
> > --- a/gcc/doc/invoke.texi
> > +++ b/gcc/doc/invoke.texi
> > @@ -652,7 +652,7 @@ Objective-C and Objective-C++ Dialects}.
> >  @item Linker Options
> >  @xref{Link Options,,Options for Linking}.
> >  @gccoptlist{@var{object-file-name}  -fuse-ld=@var{linker}  -l@var{library} @gol
> > --nostartfiles  -nodefaultlibs  -nolibc  -nostdlib @gol
> > +-nostartfiles  -nodefaultlibs  -nolibc  -nolibstdc++  -nostdlib @gol
> >  -e @var{entry}  --entry=@var{entry} @gol
> >  -pie  -pthread  -r  -rdynamic @gol
> >  -s  -static  -static-pie  -static-libgcc  -static-libstdc++ @gol
> > @@ -16787,6 +16787,10 @@ absence of a C library is assumed, for example @option{-lpthread} or
> >  @option{-lm} in some configurations.  This is intended for bare-board
> >  targets when there is indeed no C library available.
> >
> > +@item -nolibstdc++
> > +@opindex nolibstdc++
> > +Do not link with standard C++ libraries implicitly.
> > +
> >  @item -nostdlib
> >  @opindex nostdlib
> >  Do not use the standard system startup files or libraries when linking.
> > diff --git a/gcc/testsuite/g++.dg/abi/pure-virtual1.C b/gcc/testsuite/g++.dg/abi/pure-virtual1.C
> > index 538e2cb097a0d..889c33e4952f4 100644
> > --- a/gcc/testsuite/g++.dg/abi/pure-virtual1.C
> > +++ b/gcc/testsuite/g++.dg/abi/pure-virtual1.C
> > @@ -1,7 +1,7 @@
> >  // Test that we don't need libsupc++ just for __cxa_pure_virtual.
> >  // { dg-do link }
> >  // { dg-require-weak }
> > -// { dg-additional-options "-fno-rtti -nodefaultlibs -lc" }
> > +// { dg-additional-options "-fno-rtti -nolibstdc++" }
> >  // { dg-additional-options "-Wl,-undefined,dynamic_lookup" { target *-*-darwin* } }
> >  // { dg-xfail-if "AIX weak" { powerpc-ibm-aix* } }
> >
> >
> > --
> > Alexandre Oliva, happy hacker                https://FSFLA.org/blogs/lxo/
> >    Free Software Activist                       GNU Toolchain Engineer
> > Disinformation flourishes because many people care deeply about injustice
> > but very few check the facts.  Ask me about <https://stallmansupport.org>