Re: [PATCH] Introduce -nolibstdc++ option

[Fangrui Song <maskray@google.com>]

On 2022-06-21, Richard Biener wrote:
>On Tue, Jun 21, 2022 at 9:53 AM Fangrui Song <maskray@google.com> wrote:
>>
>> 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++.
>
>Probably.  Note that we have -static-libstdc++ already so
>-nolibstdc++ matches that.  We also have -nolibc, not -noclib.
>
>Richard.

I think the relation between -static-foo and -nofoo is not that large.
-nostdlib does not have a corresponding -static-stdlib.

Note that gcc has supported -stdlib=libc++ since 2020-12, though the
usage is a bit tricky. Having a C++ standard library agnostic name
helps libc++:)

For -lc, clang has -nolibc.

>> > >
>> > > 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>