Re: [PATCH] libstdc++: Fix iostream init for Clang on darwin [PR110432]

[Jonathan Wakely <jwakely@redhat.com>]

On Fri, 30 Jun 2023 at 15:29, Patrick Palka <ppalka@redhat.com> wrote:
>
> On Fri, 30 Jun 2023, Jonathan Wakely wrote:
>
> > Tested x86_64-linux. Patrick, PTAL.
> >
> > -- >8 --
> >
> > The __has_attribute(init_priority) check in <iostream> is true for Clang
> > on darwin, which means that user code including <iostream> thinks the
> > library will initialize the global streams. However, when libstdc++ is
> > built by GCC on darwin, the __has_attribute(init_priority) check is
> > false, which means that the library thinks that user code will do the
> > initialization when <iostream> is included. This means that the
> > initialization is never done.
> >
> > Add an autoconf check so that the header and the library both make their
> > decision based on the static properties of GCC at build time, with a
> > consistent outcome.
> >
> > As a belt and braces check, also do the initialization in <iostream> if
> > the compiler including that header doesn't support the attribute (even
> > if the library also containers the initialization). This might result in
> > redundant initialization done in <iostream>, but ensures the
> > initialization happens somewhere if there's any doubt about the
> > attribute working correctly due to missing linker support.
> >
> > libstdc++-v3/ChangeLog:
> >
> >       PR libstdc++/110432
> >       * acinclude.m4 (GLIBCXX_CHECK_INIT_PRIORITY): New.
> >       * config.h.in: Regenerate.
> >       * configure: Regenerate.
> >       * configure.ac: Use GLIBCXX_CHECK_INIT_PRIORITY.
> >       * include/std/iostream:
>
> Missing ChangeLog entry?
>
> >       * src/c++98/ios_base_init.h: Use new autoconf macro instead of
> >       __has_attribute.
> > ---
> >  libstdc++-v3/acinclude.m4              | 27 ++++++++++++++
> >  libstdc++-v3/config.h.in               |  3 ++
> >  libstdc++-v3/configure                 | 51 ++++++++++++++++++++++++++
> >  libstdc++-v3/configure.ac              |  3 ++
> >  libstdc++-v3/include/std/iostream      |  2 +-
> >  libstdc++-v3/src/c++98/ios_base_init.h |  2 +-
> >  6 files changed, 86 insertions(+), 2 deletions(-)
> >
> > diff --git a/libstdc++-v3/acinclude.m4 b/libstdc++-v3/acinclude.m4
> > index 277ae10e031..823832f97d4 100644
> > --- a/libstdc++-v3/acinclude.m4
> > +++ b/libstdc++-v3/acinclude.m4
> > @@ -5680,6 +5680,33 @@ AC_DEFUN([GLIBCXX_CHECK_ALIGNAS_CACHELINE], [
> >    AC_LANG_RESTORE
> >  ])
> >
> > +dnl
> > +dnl Check whether iostream initialization should be done in the library,
> > +dnl using the init_priority attribute.
> > +dnl
> > +dnl Defines:
> > +dnl  _GLIBCXX_USE_INIT_PRIORITY_ATTRIBUTE if GCC supports the init_priority
> > +dnl    attribute for the target.
> > +dnl
> > +AC_DEFUN([GLIBCXX_CHECK_INIT_PRIORITY], [
> > +AC_LANG_SAVE
> > +  AC_LANG_CPLUSPLUS
> > +
> > +  AC_MSG_CHECKING([whether init_priority attribute is supported])
> > +  AC_TRY_COMPILE(, [
> > +  #if ! __has_attribute(init_priority)
> > +  #error init_priority not supported
> > +  #endif
> > +              ], [ac_init_priority=yes], [ac_init_priority=no])
> > +  if test "$ac_init_priority" = yes; then
> > +    AC_DEFINE_UNQUOTED(_GLIBCXX_USE_INIT_PRIORITY_ATTRIBUTE, 1,
> > +      [Define if init_priority should be used for iostream initialization.])
> > +  fi
> > +  AC_MSG_RESULT($ac_init_priority)
> > +
> > +  AC_LANG_RESTORE
> > +])
> > +
> >  # Macros from the top-level gcc directory.
> >  m4_include([../config/gc++filt.m4])
> >  m4_include([../config/tls.m4])
> > diff --git a/libstdc++-v3/configure.ac b/libstdc++-v3/configure.ac
> > index 9770c178767..fc0f2522027 100644
> > --- a/libstdc++-v3/configure.ac
> > +++ b/libstdc++-v3/configure.ac
> > @@ -550,6 +550,9 @@ GLIBCXX_ZONEINFO_DIR
> >  # For src/c++11/shared_ptr.cc alignment.
> >  GLIBCXX_CHECK_ALIGNAS_CACHELINE
> >
> > +# For using init_priority in ios_init.cc
> > +GLIBCXX_CHECK_INIT_PRIORITY
> > +
> >  # Define documentation rules conditionally.
> >
> >  # See if makeinfo has been installed and is modern enough
> > diff --git a/libstdc++-v3/include/std/iostream b/libstdc++-v3/include/std/iostream
> > index cfd124dcf43..ec337cf89dd 100644
> > --- a/libstdc++-v3/include/std/iostream
> > +++ b/libstdc++-v3/include/std/iostream
> > @@ -75,7 +75,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> >    // For construction of filebuffers for cout, cin, cerr, clog et. al.
> >    // When the init_priority attribute is usable, we do this initialization
> >    // in the compiled library instead (src/c++98/globals_io.cc).
> > -#if !__has_attribute(__init_priority__)
> > +#if !(_GLIBCXX_USE_INIT_PRIORITY_ATTRIBUTE && __has_attribute(init_priority))
>
> This should check __init_priority__ since init_priority is a non-reserved
> name I think?  LGTM otherwise.

Thanks, fixed and pushed to trunk. Backport to gcc-13 to follow.