[v3] Set the retain attribute on _elf_set_element if CC supports [BZ #27492]

  So that text_set_element/data_set_element/bss_set_element defined
variables will be retained by the linker.

Note: 'used' and 'retain' are orthogonal: 'used' makes sure the variable
will not be optimized out; 'retain' prevents section garbage collection
if the linker support SHF_GNU_RETAIN.

GNU ld 2.37 and LLD 13 will support -z start-stop-gc which allow C
identifier name sections to be GCed even if there are live
__start_/__stop_ references.

Without the change, there are some static linking problems, e.g.
_IO_cleanup (libio/genops.c) may be discarded by ld --gc-sections, so
stdout is not flushed on exit.

Note: GCC may warning ‘retain’ attribute ignored while __has_attribute(retain) is 1
(https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99587).
---
Changes in v1 -> v2:
* Define attribute_used_retain_section
Changes in v2 -> v3:
* Use attribute_used_retain instead attribute_used_retain_section
---
 config.h.in            |  3 +++
 configure              | 23 +++++++++++++++++++++++
 configure.ac           | 16 ++++++++++++++++
 include/libc-symbols.h | 14 ++++++++++----
 4 files changed, 52 insertions(+), 4 deletions(-)
  

On Mon, Mar 29, 2021 at 4:24 PM Fangrui Song <maskray@google.com> wrote:
>
> So that text_set_element/data_set_element/bss_set_element defined
> variables will be retained by the linker.
>
> Note: 'used' and 'retain' are orthogonal: 'used' makes sure the variable
> will not be optimized out; 'retain' prevents section garbage collection
> if the linker support SHF_GNU_RETAIN.
>
> GNU ld 2.37 and LLD 13 will support -z start-stop-gc which allow C
> identifier name sections to be GCed even if there are live
> __start_/__stop_ references.
>
> Without the change, there are some static linking problems, e.g.
> _IO_cleanup (libio/genops.c) may be discarded by ld --gc-sections, so
> stdout is not flushed on exit.
>
> Note: GCC may warning ‘retain’ attribute ignored while __has_attribute(retain) is 1
> (https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99587).
> ---
> Changes in v1 -> v2:
> * Define attribute_used_retain_section
> Changes in v2 -> v3:
> * Use attribute_used_retain instead attribute_used_retain_section
> ---
>  config.h.in            |  3 +++
>  configure              | 23 +++++++++++++++++++++++
>  configure.ac           | 16 ++++++++++++++++
>  include/libc-symbols.h | 14 ++++++++++----
>  4 files changed, 52 insertions(+), 4 deletions(-)
>
> diff --git a/config.h.in b/config.h.in
> index ca1547ae67..95af1ef229 100644
> --- a/config.h.in
> +++ b/config.h.in
> @@ -187,6 +187,9 @@
>  /* Define if gcc supports attribute ifunc.  */
>  #undef HAVE_GCC_IFUNC
>
> +/* Define if gcc supports attribute retain.  */
> +#undef HAVE_GCC_RETAIN
> +
>  /* Define if the linker defines __ehdr_start.  */
>  #undef HAVE_EHDR_START
>
> diff --git a/configure b/configure
> index fcf43bf7de..6a6b8ac264 100755
> --- a/configure
> +++ b/configure
> @@ -4105,6 +4105,29 @@ fi
>  $as_echo "$libc_cv_textrel_ifunc" >&6; }
>
>
> +# Check if gcc supports attribute ifunc as it is used in libc_ifunc macro.
> +{ $as_echo "$as_me:${as_lineno-$LINENO}: checking for gcc attribute retain support" >&5
> +$as_echo_n "checking for gcc attribute retain support... " >&6; }
> +if ${libc_cv_gcc_retain+:} false; then :
> +  $as_echo_n "(cached) " >&6
> +else
> +  cat > conftest.c <<EOF
> +static int var  __attribute__ ((used, retain, section ("__libc_atexit")));
> +EOF
> +libc_cv_gcc_retain=no
> +if ${CC-cc} -Werror -c conftest.c -o /dev/null 1>&5 \
> +   2>&5 ; then
> +  libc_cv_gcc_retain=yes
> +fi
> +rm -f conftest*
> +fi
> +{ $as_echo "$as_me:${as_lineno-$LINENO}: result: $libc_cv_gcc_retain" >&5
> +$as_echo "$libc_cv_gcc_retain" >&6; }
> +if test $libc_cv_gcc_retain = yes; then
> +  $as_echo "#define HAVE_GCC_RETAIN 1" >>confdefs.h
> +
> +fi
> +
>  # Check if gcc warns about alias for function with incompatible types.
>  { $as_echo "$as_me:${as_lineno-$LINENO}: checking if compiler warns about alias for function with incompatible types" >&5
>  $as_echo_n "checking if compiler warns about alias for function with incompatible types... " >&6; }
> diff --git a/configure.ac b/configure.ac
> index fce967f2c2..031b8c3fb3 100644
> --- a/configure.ac
> +++ b/configure.ac
> @@ -707,6 +707,22 @@ fi
>  rm -f conftest*])
>  AC_SUBST(libc_cv_textrel_ifunc)
>
> +# Check if gcc supports attribute ifunc as it is used in libc_ifunc macro.
> +AC_CACHE_CHECK([for gcc attribute retain support],
> +              libc_cv_gcc_retain, [dnl
> +cat > conftest.c <<EOF
> +static int var  __attribute__ ((used, retain, section ("__libc_atexit")));
> +EOF
> +libc_cv_gcc_retain=no
> +if ${CC-cc} -Werror -c conftest.c -o /dev/null 1>&AS_MESSAGE_LOG_FD \
> +   2>&AS_MESSAGE_LOG_FD ; then
> +  libc_cv_gcc_retain=yes
> +fi
> +rm -f conftest*])
> +if test $libc_cv_gcc_retain = yes; then
> +  AC_DEFINE(HAVE_GCC_RETAIN)
> +fi
> +
>  # Check if gcc warns about alias for function with incompatible types.
>  AC_CACHE_CHECK([if compiler warns about alias for function with incompatible types],
>                libc_cv_gcc_incompatible_alias, [dnl
> diff --git a/include/libc-symbols.h b/include/libc-symbols.h
> index 546fc26a7b..b311c90f0c 100644
> --- a/include/libc-symbols.h
> +++ b/include/libc-symbols.h
> @@ -352,6 +352,12 @@ for linking")
>
>  */
>
> +#ifdef HAVE_GCC_RETAIN
> +# define attribute_used_retain __attribute__ ((__used__, __retain__))
> +#else
> +# define attribute_used_retain __attribute__ ((__used__))
> +#endif
> +
>  /* Symbol set support macros.  */
>
>  /* Make SYMBOL, which is in the text segment, an element of SET.  */
> @@ -367,12 +373,12 @@ for linking")
>  /* When building a shared library, make the set section writable,
>     because it will need to be relocated at run time anyway.  */
>  # define _elf_set_element(set, symbol) \
> -  static const void *__elf_set_##set##_element_##symbol##__ \
> -    __attribute__ ((used, section (#set))) = &(symbol)
> +    static const void *__elf_set_##set##_element_##symbol##__ \
> +      attribute_used_retain __attribute__ ((section (#set))) = &(symbol)
>  #else
>  # define _elf_set_element(set, symbol) \
> -  static const void *const __elf_set_##set##_element_##symbol##__ \
> -    __attribute__ ((used, section (#set))) = &(symbol)
> +    static const void *const __elf_set_##set##_element_##symbol##__ \
> +      attribute_used_retain __attribute__ ((section (#set))) = &(symbol)
>  #endif
>
>  /* Define SET as a symbol set.  This may be required (it is in a.out) to
> --
> 2.31.0.291.g576ba9dcdaf-goog
>

Need gc-sections tests for linkers with and without -z start-stop-gc.
  

On Tue, Mar 30, 2021 at 11:17 AM H.J. Lu <hjl.tools@gmail.com> wrote:
>
> On Mon, Mar 29, 2021 at 4:24 PM Fangrui Song <maskray@google.com> wrote:
> >
> > So that text_set_element/data_set_element/bss_set_element defined
> > variables will be retained by the linker.
> >
> > Note: 'used' and 'retain' are orthogonal: 'used' makes sure the variable
> > will not be optimized out; 'retain' prevents section garbage collection
> > if the linker support SHF_GNU_RETAIN.
> >
> > GNU ld 2.37 and LLD 13 will support -z start-stop-gc which allow C
> > identifier name sections to be GCed even if there are live
> > __start_/__stop_ references.
> >
> > Without the change, there are some static linking problems, e.g.
> > _IO_cleanup (libio/genops.c) may be discarded by ld --gc-sections, so
> > stdout is not flushed on exit.
> >
> > Note: GCC may warning ‘retain’ attribute ignored while __has_attribute(retain) is 1
> > (https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99587).
> > ---
> > Changes in v1 -> v2:
> > * Define attribute_used_retain_section
> > Changes in v2 -> v3:
> > * Use attribute_used_retain instead attribute_used_retain_section
> > ---
> >  config.h.in            |  3 +++
> >  configure              | 23 +++++++++++++++++++++++
> >  configure.ac           | 16 ++++++++++++++++
> >  include/libc-symbols.h | 14 ++++++++++----
> >  4 files changed, 52 insertions(+), 4 deletions(-)
> >
> > diff --git a/config.h.in b/config.h.in
> > index ca1547ae67..95af1ef229 100644
> > --- a/config.h.in
> > +++ b/config.h.in
> > @@ -187,6 +187,9 @@
> >  /* Define if gcc supports attribute ifunc.  */
> >  #undef HAVE_GCC_IFUNC
> >
> > +/* Define if gcc supports attribute retain.  */
> > +#undef HAVE_GCC_RETAIN
> > +
> >  /* Define if the linker defines __ehdr_start.  */
> >  #undef HAVE_EHDR_START
> >
> > diff --git a/configure b/configure
> > index fcf43bf7de..6a6b8ac264 100755
> > --- a/configure
> > +++ b/configure
> > @@ -4105,6 +4105,29 @@ fi
> >  $as_echo "$libc_cv_textrel_ifunc" >&6; }
> >
> >
> > +# Check if gcc supports attribute ifunc as it is used in libc_ifunc macro.
> > +{ $as_echo "$as_me:${as_lineno-$LINENO}: checking for gcc attribute retain support" >&5
> > +$as_echo_n "checking for gcc attribute retain support... " >&6; }
> > +if ${libc_cv_gcc_retain+:} false; then :
> > +  $as_echo_n "(cached) " >&6
> > +else
> > +  cat > conftest.c <<EOF
> > +static int var  __attribute__ ((used, retain, section ("__libc_atexit")));
> > +EOF
> > +libc_cv_gcc_retain=no
> > +if ${CC-cc} -Werror -c conftest.c -o /dev/null 1>&5 \
> > +   2>&5 ; then
> > +  libc_cv_gcc_retain=yes
> > +fi
> > +rm -f conftest*
> > +fi
> > +{ $as_echo "$as_me:${as_lineno-$LINENO}: result: $libc_cv_gcc_retain" >&5
> > +$as_echo "$libc_cv_gcc_retain" >&6; }
> > +if test $libc_cv_gcc_retain = yes; then
> > +  $as_echo "#define HAVE_GCC_RETAIN 1" >>confdefs.h
> > +
> > +fi
> > +
> >  # Check if gcc warns about alias for function with incompatible types.
> >  { $as_echo "$as_me:${as_lineno-$LINENO}: checking if compiler warns about alias for function with incompatible types" >&5
> >  $as_echo_n "checking if compiler warns about alias for function with incompatible types... " >&6; }
> > diff --git a/configure.ac b/configure.ac
> > index fce967f2c2..031b8c3fb3 100644
> > --- a/configure.ac
> > +++ b/configure.ac
> > @@ -707,6 +707,22 @@ fi
> >  rm -f conftest*])
> >  AC_SUBST(libc_cv_textrel_ifunc)
> >
> > +# Check if gcc supports attribute ifunc as it is used in libc_ifunc macro.
> > +AC_CACHE_CHECK([for gcc attribute retain support],
> > +              libc_cv_gcc_retain, [dnl
> > +cat > conftest.c <<EOF
> > +static int var  __attribute__ ((used, retain, section ("__libc_atexit")));
> > +EOF
> > +libc_cv_gcc_retain=no
> > +if ${CC-cc} -Werror -c conftest.c -o /dev/null 1>&AS_MESSAGE_LOG_FD \
> > +   2>&AS_MESSAGE_LOG_FD ; then
> > +  libc_cv_gcc_retain=yes
> > +fi
> > +rm -f conftest*])
> > +if test $libc_cv_gcc_retain = yes; then
> > +  AC_DEFINE(HAVE_GCC_RETAIN)
> > +fi
> > +
> >  # Check if gcc warns about alias for function with incompatible types.
> >  AC_CACHE_CHECK([if compiler warns about alias for function with incompatible types],
> >                libc_cv_gcc_incompatible_alias, [dnl
> > diff --git a/include/libc-symbols.h b/include/libc-symbols.h
> > index 546fc26a7b..b311c90f0c 100644
> > --- a/include/libc-symbols.h
> > +++ b/include/libc-symbols.h
> > @@ -352,6 +352,12 @@ for linking")
> >
> >  */
> >
> > +#ifdef HAVE_GCC_RETAIN
> > +# define attribute_used_retain __attribute__ ((__used__, __retain__))
> > +#else
> > +# define attribute_used_retain __attribute__ ((__used__))
> > +#endif
> > +
> >  /* Symbol set support macros.  */
> >
> >  /* Make SYMBOL, which is in the text segment, an element of SET.  */
> > @@ -367,12 +373,12 @@ for linking")
> >  /* When building a shared library, make the set section writable,
> >     because it will need to be relocated at run time anyway.  */
> >  # define _elf_set_element(set, symbol) \
> > -  static const void *__elf_set_##set##_element_##symbol##__ \
> > -    __attribute__ ((used, section (#set))) = &(symbol)
> > +    static const void *__elf_set_##set##_element_##symbol##__ \
> > +      attribute_used_retain __attribute__ ((section (#set))) = &(symbol)
> >  #else
> >  # define _elf_set_element(set, symbol) \
> > -  static const void *const __elf_set_##set##_element_##symbol##__ \
> > -    __attribute__ ((used, section (#set))) = &(symbol)
> > +    static const void *const __elf_set_##set##_element_##symbol##__ \
> > +      attribute_used_retain __attribute__ ((section (#set))) = &(symbol)
> >  #endif
> >
> >  /* Define SET as a symbol set.  This may be required (it is in a.out) to
> > --
> > 2.31.0.291.g576ba9dcdaf-goog
> >
>
> Need gc-sections tests for linkers with and without -z start-stop-gc.
>
> --
> H.J.

I don't know how to add such a test... It needs to check flush at ext
time but I cannot find a template in libio/tst-* ...
  

On Tue, Mar 30, 2021 at 12:17 PM Fāng-ruì Sòng <maskray@google.com> wrote:
>
> On Tue, Mar 30, 2021 at 11:17 AM H.J. Lu <hjl.tools@gmail.com> wrote:
> >
> > On Mon, Mar 29, 2021 at 4:24 PM Fangrui Song <maskray@google.com> wrote:
> > >
> > > So that text_set_element/data_set_element/bss_set_element defined
> > > variables will be retained by the linker.
> > >
> > > Note: 'used' and 'retain' are orthogonal: 'used' makes sure the variable
> > > will not be optimized out; 'retain' prevents section garbage collection
> > > if the linker support SHF_GNU_RETAIN.
> > >
> > > GNU ld 2.37 and LLD 13 will support -z start-stop-gc which allow C
> > > identifier name sections to be GCed even if there are live
> > > __start_/__stop_ references.
> > >
> > > Without the change, there are some static linking problems, e.g.
> > > _IO_cleanup (libio/genops.c) may be discarded by ld --gc-sections, so
> > > stdout is not flushed on exit.
> > >
> > > Note: GCC may warning ‘retain’ attribute ignored while __has_attribute(retain) is 1
> > > (https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99587).
> > > ---
> > > Changes in v1 -> v2:
> > > * Define attribute_used_retain_section
> > > Changes in v2 -> v3:
> > > * Use attribute_used_retain instead attribute_used_retain_section
> > > ---
> > >  config.h.in            |  3 +++
> > >  configure              | 23 +++++++++++++++++++++++
> > >  configure.ac           | 16 ++++++++++++++++
> > >  include/libc-symbols.h | 14 ++++++++++----
> > >  4 files changed, 52 insertions(+), 4 deletions(-)
> > >
> > > diff --git a/config.h.in b/config.h.in
> > > index ca1547ae67..95af1ef229 100644
> > > --- a/config.h.in
> > > +++ b/config.h.in
> > > @@ -187,6 +187,9 @@
> > >  /* Define if gcc supports attribute ifunc.  */
> > >  #undef HAVE_GCC_IFUNC
> > >
> > > +/* Define if gcc supports attribute retain.  */
> > > +#undef HAVE_GCC_RETAIN
> > > +
> > >  /* Define if the linker defines __ehdr_start.  */
> > >  #undef HAVE_EHDR_START
> > >
> > > diff --git a/configure b/configure
> > > index fcf43bf7de..6a6b8ac264 100755
> > > --- a/configure
> > > +++ b/configure
> > > @@ -4105,6 +4105,29 @@ fi
> > >  $as_echo "$libc_cv_textrel_ifunc" >&6; }
> > >
> > >
> > > +# Check if gcc supports attribute ifunc as it is used in libc_ifunc macro.
> > > +{ $as_echo "$as_me:${as_lineno-$LINENO}: checking for gcc attribute retain support" >&5
> > > +$as_echo_n "checking for gcc attribute retain support... " >&6; }
> > > +if ${libc_cv_gcc_retain+:} false; then :
> > > +  $as_echo_n "(cached) " >&6
> > > +else
> > > +  cat > conftest.c <<EOF
> > > +static int var  __attribute__ ((used, retain, section ("__libc_atexit")));
> > > +EOF
> > > +libc_cv_gcc_retain=no
> > > +if ${CC-cc} -Werror -c conftest.c -o /dev/null 1>&5 \
> > > +   2>&5 ; then
> > > +  libc_cv_gcc_retain=yes
> > > +fi
> > > +rm -f conftest*
> > > +fi
> > > +{ $as_echo "$as_me:${as_lineno-$LINENO}: result: $libc_cv_gcc_retain" >&5
> > > +$as_echo "$libc_cv_gcc_retain" >&6; }
> > > +if test $libc_cv_gcc_retain = yes; then
> > > +  $as_echo "#define HAVE_GCC_RETAIN 1" >>confdefs.h
> > > +
> > > +fi
> > > +
> > >  # Check if gcc warns about alias for function with incompatible types.
> > >  { $as_echo "$as_me:${as_lineno-$LINENO}: checking if compiler warns about alias for function with incompatible types" >&5
> > >  $as_echo_n "checking if compiler warns about alias for function with incompatible types... " >&6; }
> > > diff --git a/configure.ac b/configure.ac
> > > index fce967f2c2..031b8c3fb3 100644
> > > --- a/configure.ac
> > > +++ b/configure.ac
> > > @@ -707,6 +707,22 @@ fi
> > >  rm -f conftest*])
> > >  AC_SUBST(libc_cv_textrel_ifunc)
> > >
> > > +# Check if gcc supports attribute ifunc as it is used in libc_ifunc macro.
> > > +AC_CACHE_CHECK([for gcc attribute retain support],
> > > +              libc_cv_gcc_retain, [dnl
> > > +cat > conftest.c <<EOF
> > > +static int var  __attribute__ ((used, retain, section ("__libc_atexit")));
> > > +EOF
> > > +libc_cv_gcc_retain=no
> > > +if ${CC-cc} -Werror -c conftest.c -o /dev/null 1>&AS_MESSAGE_LOG_FD \
> > > +   2>&AS_MESSAGE_LOG_FD ; then
> > > +  libc_cv_gcc_retain=yes
> > > +fi
> > > +rm -f conftest*])
> > > +if test $libc_cv_gcc_retain = yes; then
> > > +  AC_DEFINE(HAVE_GCC_RETAIN)
> > > +fi
> > > +
> > >  # Check if gcc warns about alias for function with incompatible types.
> > >  AC_CACHE_CHECK([if compiler warns about alias for function with incompatible types],
> > >                libc_cv_gcc_incompatible_alias, [dnl
> > > diff --git a/include/libc-symbols.h b/include/libc-symbols.h
> > > index 546fc26a7b..b311c90f0c 100644
> > > --- a/include/libc-symbols.h
> > > +++ b/include/libc-symbols.h
> > > @@ -352,6 +352,12 @@ for linking")
> > >
> > >  */
> > >
> > > +#ifdef HAVE_GCC_RETAIN
> > > +# define attribute_used_retain __attribute__ ((__used__, __retain__))
> > > +#else
> > > +# define attribute_used_retain __attribute__ ((__used__))
> > > +#endif
> > > +
> > >  /* Symbol set support macros.  */
> > >
> > >  /* Make SYMBOL, which is in the text segment, an element of SET.  */
> > > @@ -367,12 +373,12 @@ for linking")
> > >  /* When building a shared library, make the set section writable,
> > >     because it will need to be relocated at run time anyway.  */
> > >  # define _elf_set_element(set, symbol) \
> > > -  static const void *__elf_set_##set##_element_##symbol##__ \
> > > -    __attribute__ ((used, section (#set))) = &(symbol)
> > > +    static const void *__elf_set_##set##_element_##symbol##__ \
> > > +      attribute_used_retain __attribute__ ((section (#set))) = &(symbol)
> > >  #else
> > >  # define _elf_set_element(set, symbol) \
> > > -  static const void *const __elf_set_##set##_element_##symbol##__ \
> > > -    __attribute__ ((used, section (#set))) = &(symbol)
> > > +    static const void *const __elf_set_##set##_element_##symbol##__ \
> > > +      attribute_used_retain __attribute__ ((section (#set))) = &(symbol)
> > >  #endif
> > >
> > >  /* Define SET as a symbol set.  This may be required (it is in a.out) to
> > > --
> > > 2.31.0.291.g576ba9dcdaf-goog
> > >
> >
> > Need gc-sections tests for linkers with and without -z start-stop-gc.
> >
> > --
> > H.J.
>
> I don't know how to add such a test... It needs to check flush at ext
> time but I cannot find a template in libio/tst-* ...

You can add a special rule to check for linker output with
--print-gc-sections.
  

On 2021-03-30, H.J. Lu wrote:
>On Tue, Mar 30, 2021 at 12:17 PM Fāng-ruì Sòng <maskray@google.com> wrote:
>>
>> On Tue, Mar 30, 2021 at 11:17 AM H.J. Lu <hjl.tools@gmail.com> wrote:
>> >
>> > On Mon, Mar 29, 2021 at 4:24 PM Fangrui Song <maskray@google.com> wrote:
>> > >
>> > > So that text_set_element/data_set_element/bss_set_element defined
>> > > variables will be retained by the linker.
>> > >
>> > > Note: 'used' and 'retain' are orthogonal: 'used' makes sure the variable
>> > > will not be optimized out; 'retain' prevents section garbage collection
>> > > if the linker support SHF_GNU_RETAIN.
>> > >
>> > > GNU ld 2.37 and LLD 13 will support -z start-stop-gc which allow C
>> > > identifier name sections to be GCed even if there are live
>> > > __start_/__stop_ references.
>> > >
>> > > Without the change, there are some static linking problems, e.g.
>> > > _IO_cleanup (libio/genops.c) may be discarded by ld --gc-sections, so
>> > > stdout is not flushed on exit.
>> > >
>> > > Note: GCC may warning ‘retain’ attribute ignored while __has_attribute(retain) is 1
>> > > (https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99587).
>> > > ---
>> > > Changes in v1 -> v2:
>> > > * Define attribute_used_retain_section
>> > > Changes in v2 -> v3:
>> > > * Use attribute_used_retain instead attribute_used_retain_section
>> > > ---
>> > >  config.h.in            |  3 +++
>> > >  configure              | 23 +++++++++++++++++++++++
>> > >  configure.ac           | 16 ++++++++++++++++
>> > >  include/libc-symbols.h | 14 ++++++++++----
>> > >  4 files changed, 52 insertions(+), 4 deletions(-)
>> > >
>> > > diff --git a/config.h.in b/config.h.in
>> > > index ca1547ae67..95af1ef229 100644
>> > > --- a/config.h.in
>> > > +++ b/config.h.in
>> > > @@ -187,6 +187,9 @@
>> > >  /* Define if gcc supports attribute ifunc.  */
>> > >  #undef HAVE_GCC_IFUNC
>> > >
>> > > +/* Define if gcc supports attribute retain.  */
>> > > +#undef HAVE_GCC_RETAIN
>> > > +
>> > >  /* Define if the linker defines __ehdr_start.  */
>> > >  #undef HAVE_EHDR_START
>> > >
>> > > diff --git a/configure b/configure
>> > > index fcf43bf7de..6a6b8ac264 100755
>> > > --- a/configure
>> > > +++ b/configure
>> > > @@ -4105,6 +4105,29 @@ fi
>> > >  $as_echo "$libc_cv_textrel_ifunc" >&6; }
>> > >
>> > >
>> > > +# Check if gcc supports attribute ifunc as it is used in libc_ifunc macro.
>> > > +{ $as_echo "$as_me:${as_lineno-$LINENO}: checking for gcc attribute retain support" >&5
>> > > +$as_echo_n "checking for gcc attribute retain support... " >&6; }
>> > > +if ${libc_cv_gcc_retain+:} false; then :
>> > > +  $as_echo_n "(cached) " >&6
>> > > +else
>> > > +  cat > conftest.c <<EOF
>> > > +static int var  __attribute__ ((used, retain, section ("__libc_atexit")));
>> > > +EOF
>> > > +libc_cv_gcc_retain=no
>> > > +if ${CC-cc} -Werror -c conftest.c -o /dev/null 1>&5 \
>> > > +   2>&5 ; then
>> > > +  libc_cv_gcc_retain=yes
>> > > +fi
>> > > +rm -f conftest*
>> > > +fi
>> > > +{ $as_echo "$as_me:${as_lineno-$LINENO}: result: $libc_cv_gcc_retain" >&5
>> > > +$as_echo "$libc_cv_gcc_retain" >&6; }
>> > > +if test $libc_cv_gcc_retain = yes; then
>> > > +  $as_echo "#define HAVE_GCC_RETAIN 1" >>confdefs.h
>> > > +
>> > > +fi
>> > > +
>> > >  # Check if gcc warns about alias for function with incompatible types.
>> > >  { $as_echo "$as_me:${as_lineno-$LINENO}: checking if compiler warns about alias for function with incompatible types" >&5
>> > >  $as_echo_n "checking if compiler warns about alias for function with incompatible types... " >&6; }
>> > > diff --git a/configure.ac b/configure.ac
>> > > index fce967f2c2..031b8c3fb3 100644
>> > > --- a/configure.ac
>> > > +++ b/configure.ac
>> > > @@ -707,6 +707,22 @@ fi
>> > >  rm -f conftest*])
>> > >  AC_SUBST(libc_cv_textrel_ifunc)
>> > >
>> > > +# Check if gcc supports attribute ifunc as it is used in libc_ifunc macro.
>> > > +AC_CACHE_CHECK([for gcc attribute retain support],
>> > > +              libc_cv_gcc_retain, [dnl
>> > > +cat > conftest.c <<EOF
>> > > +static int var  __attribute__ ((used, retain, section ("__libc_atexit")));
>> > > +EOF
>> > > +libc_cv_gcc_retain=no
>> > > +if ${CC-cc} -Werror -c conftest.c -o /dev/null 1>&AS_MESSAGE_LOG_FD \
>> > > +   2>&AS_MESSAGE_LOG_FD ; then
>> > > +  libc_cv_gcc_retain=yes
>> > > +fi
>> > > +rm -f conftest*])
>> > > +if test $libc_cv_gcc_retain = yes; then
>> > > +  AC_DEFINE(HAVE_GCC_RETAIN)
>> > > +fi
>> > > +
>> > >  # Check if gcc warns about alias for function with incompatible types.
>> > >  AC_CACHE_CHECK([if compiler warns about alias for function with incompatible types],
>> > >                libc_cv_gcc_incompatible_alias, [dnl
>> > > diff --git a/include/libc-symbols.h b/include/libc-symbols.h
>> > > index 546fc26a7b..b311c90f0c 100644
>> > > --- a/include/libc-symbols.h
>> > > +++ b/include/libc-symbols.h
>> > > @@ -352,6 +352,12 @@ for linking")
>> > >
>> > >  */
>> > >
>> > > +#ifdef HAVE_GCC_RETAIN
>> > > +# define attribute_used_retain __attribute__ ((__used__, __retain__))
>> > > +#else
>> > > +# define attribute_used_retain __attribute__ ((__used__))
>> > > +#endif
>> > > +
>> > >  /* Symbol set support macros.  */
>> > >
>> > >  /* Make SYMBOL, which is in the text segment, an element of SET.  */
>> > > @@ -367,12 +373,12 @@ for linking")
>> > >  /* When building a shared library, make the set section writable,
>> > >     because it will need to be relocated at run time anyway.  */
>> > >  # define _elf_set_element(set, symbol) \
>> > > -  static const void *__elf_set_##set##_element_##symbol##__ \
>> > > -    __attribute__ ((used, section (#set))) = &(symbol)
>> > > +    static const void *__elf_set_##set##_element_##symbol##__ \
>> > > +      attribute_used_retain __attribute__ ((section (#set))) = &(symbol)
>> > >  #else
>> > >  # define _elf_set_element(set, symbol) \
>> > > -  static const void *const __elf_set_##set##_element_##symbol##__ \
>> > > -    __attribute__ ((used, section (#set))) = &(symbol)
>> > > +    static const void *const __elf_set_##set##_element_##symbol##__ \
>> > > +      attribute_used_retain __attribute__ ((section (#set))) = &(symbol)
>> > >  #endif
>> > >
>> > >  /* Define SET as a symbol set.  This may be required (it is in a.out) to
>> > > --
>> > > 2.31.0.291.g576ba9dcdaf-goog
>> > >
>> >
>> > Need gc-sections tests for linkers with and without -z start-stop-gc.
>> >
>> > --
>> > H.J.
>>
>> I don't know how to add such a test... It needs to check flush at ext
>> time but I cannot find a template in libio/tst-* ...
>
>You can add a special rule to check for linker output with
>--print-gc-sections.

I can come up runtime tests. ( https://github.com/MaskRay/glibc/tree/retain)
Runtime tests are direct.
(Unsure how to test --print-gc-sections with tests-static...)


diff --git a/libio/Makefile b/libio/Makefile
index 12ce41038f..5e0e7f00cd 100644
--- a/libio/Makefile
+++ b/libio/Makefile
@@ -195,6 +195,13 @@ ifeq (yes,$(build-shared))
  tests-special += $(objpfx)tst-fopenloc-cmp.out $(objpfx)tst-fopenloc-mem.out \
  		 $(objpfx)tst-bz24228-mem.out
  endif
+
+tests += tst-cleanup-start-stop-gc tst-cleanup-nostart-stop-gc
+tests-static += tst-cleanup-start-stop-gc tst-cleanup-nostart-stop-gc
+tests-special += $(objpfx)tst-cleanup-start-stop-gc.out \
+		$(objpfx)tst-cleanup-nostart-stop-gc.out
+LDFLAGS-tst-cleanup-start-stop-gc = -Wl,--gc-sections,-z,start-stop-gc
+LDFLAGS-tst-cleanup-nostart-stop-gc = -Wl,--gc-sections,-z,nostart-stop-gc
  endif
  
  include ../Rules
@@ -224,6 +231,20 @@ $(objpfx)tst_wprintf2.out: $(gen-locales)
  $(objpfx)tst-wfile-sync.out: $(gen-locales)
  endif
  
+$(objpfx)tst-cleanup-start-stop-gc.out: $(objpfx)tst-cleanup-start-stop-gc
+	$< > $@T
+	grep hello $@T; $(evaluate-test)
+	rm -f $@T
+$(objpfx)tst-cleanup-start-stop-gc.o: tst-cleanup.c
+	$(compile.c) -o $@
+
+$(objpfx)tst-cleanup-nostart-stop-gc.out: $(objpfx)tst-cleanup-nostart-stop-gc
+	$< > $@T
+	grep hello $@T; $(evaluate-test)
+	rm -f $@T
+$(objpfx)tst-cleanup-nostart-stop-gc.o: tst-cleanup.c
+	$(compile.c) -o $@
+
  $(objpfx)test-freopen.out: test-freopen.sh $(objpfx)test-freopen
  	$(SHELL) $< $(common-objpfx) '$(test-program-prefix)'	\
  	$(common-objpfx)libio/; \
diff --git a/libio/tst-cleanup.c b/libio/tst-cleanup.c
new file mode 100644
index 0000000000..de8130a482
--- /dev/null
+++ b/libio/tst-cleanup.c
@@ -0,0 +1,14 @@
+#include <stdio.h>
+#include <stdlib.h>
+
+void
+hook (void)
+{
+  puts ("hello");
+}
+
+int
+main (void)
+{
+  atexit (hook);
+}
  

On Tue, Mar 30, 2021 at 9:34 PM Fāng-ruì Sòng <maskray@google.com> wrote:
>
> On 2021-03-30, H.J. Lu wrote:
> >On Tue, Mar 30, 2021 at 12:17 PM Fāng-ruì Sòng <maskray@google.com> wrote:
> >>
> >> On Tue, Mar 30, 2021 at 11:17 AM H.J. Lu <hjl.tools@gmail.com> wrote:
> >> >
> >> > On Mon, Mar 29, 2021 at 4:24 PM Fangrui Song <maskray@google.com> wrote:
> >> > >
> >> > > So that text_set_element/data_set_element/bss_set_element defined
> >> > > variables will be retained by the linker.
> >> > >
> >> > > Note: 'used' and 'retain' are orthogonal: 'used' makes sure the variable
> >> > > will not be optimized out; 'retain' prevents section garbage collection
> >> > > if the linker support SHF_GNU_RETAIN.
> >> > >
> >> > > GNU ld 2.37 and LLD 13 will support -z start-stop-gc which allow C
> >> > > identifier name sections to be GCed even if there are live
> >> > > __start_/__stop_ references.
> >> > >
> >> > > Without the change, there are some static linking problems, e.g.
> >> > > _IO_cleanup (libio/genops.c) may be discarded by ld --gc-sections, so
> >> > > stdout is not flushed on exit.
> >> > >
> >> > > Note: GCC may warning ‘retain’ attribute ignored while __has_attribute(retain) is 1
> >> > > (https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99587).
> >> > > ---
> >> > > Changes in v1 -> v2:
> >> > > * Define attribute_used_retain_section
> >> > > Changes in v2 -> v3:
> >> > > * Use attribute_used_retain instead attribute_used_retain_section
> >> > > ---
> >> > >  config.h.in            |  3 +++
> >> > >  configure              | 23 +++++++++++++++++++++++
> >> > >  configure.ac           | 16 ++++++++++++++++
> >> > >  include/libc-symbols.h | 14 ++++++++++----
> >> > >  4 files changed, 52 insertions(+), 4 deletions(-)
> >> > >
> >> > > diff --git a/config.h.in b/config.h.in
> >> > > index ca1547ae67..95af1ef229 100644
> >> > > --- a/config.h.in
> >> > > +++ b/config.h.in
> >> > > @@ -187,6 +187,9 @@
> >> > >  /* Define if gcc supports attribute ifunc.  */
> >> > >  #undef HAVE_GCC_IFUNC
> >> > >
> >> > > +/* Define if gcc supports attribute retain.  */
> >> > > +#undef HAVE_GCC_RETAIN
> >> > > +
> >> > >  /* Define if the linker defines __ehdr_start.  */
> >> > >  #undef HAVE_EHDR_START
> >> > >
> >> > > diff --git a/configure b/configure
> >> > > index fcf43bf7de..6a6b8ac264 100755
> >> > > --- a/configure
> >> > > +++ b/configure
> >> > > @@ -4105,6 +4105,29 @@ fi
> >> > >  $as_echo "$libc_cv_textrel_ifunc" >&6; }
> >> > >
> >> > >
> >> > > +# Check if gcc supports attribute ifunc as it is used in libc_ifunc macro.
> >> > > +{ $as_echo "$as_me:${as_lineno-$LINENO}: checking for gcc attribute retain support" >&5
> >> > > +$as_echo_n "checking for gcc attribute retain support... " >&6; }
> >> > > +if ${libc_cv_gcc_retain+:} false; then :
> >> > > +  $as_echo_n "(cached) " >&6
> >> > > +else
> >> > > +  cat > conftest.c <<EOF
> >> > > +static int var  __attribute__ ((used, retain, section ("__libc_atexit")));
> >> > > +EOF
> >> > > +libc_cv_gcc_retain=no
> >> > > +if ${CC-cc} -Werror -c conftest.c -o /dev/null 1>&5 \
> >> > > +   2>&5 ; then
> >> > > +  libc_cv_gcc_retain=yes
> >> > > +fi
> >> > > +rm -f conftest*
> >> > > +fi
> >> > > +{ $as_echo "$as_me:${as_lineno-$LINENO}: result: $libc_cv_gcc_retain" >&5
> >> > > +$as_echo "$libc_cv_gcc_retain" >&6; }
> >> > > +if test $libc_cv_gcc_retain = yes; then
> >> > > +  $as_echo "#define HAVE_GCC_RETAIN 1" >>confdefs.h
> >> > > +
> >> > > +fi
> >> > > +
> >> > >  # Check if gcc warns about alias for function with incompatible types.
> >> > >  { $as_echo "$as_me:${as_lineno-$LINENO}: checking if compiler warns about alias for function with incompatible types" >&5
> >> > >  $as_echo_n "checking if compiler warns about alias for function with incompatible types... " >&6; }
> >> > > diff --git a/configure.ac b/configure.ac
> >> > > index fce967f2c2..031b8c3fb3 100644
> >> > > --- a/configure.ac
> >> > > +++ b/configure.ac
> >> > > @@ -707,6 +707,22 @@ fi
> >> > >  rm -f conftest*])
> >> > >  AC_SUBST(libc_cv_textrel_ifunc)
> >> > >
> >> > > +# Check if gcc supports attribute ifunc as it is used in libc_ifunc macro.
> >> > > +AC_CACHE_CHECK([for gcc attribute retain support],
> >> > > +              libc_cv_gcc_retain, [dnl
> >> > > +cat > conftest.c <<EOF
> >> > > +static int var  __attribute__ ((used, retain, section ("__libc_atexit")));
> >> > > +EOF
> >> > > +libc_cv_gcc_retain=no
> >> > > +if ${CC-cc} -Werror -c conftest.c -o /dev/null 1>&AS_MESSAGE_LOG_FD \
> >> > > +   2>&AS_MESSAGE_LOG_FD ; then
> >> > > +  libc_cv_gcc_retain=yes
> >> > > +fi
> >> > > +rm -f conftest*])
> >> > > +if test $libc_cv_gcc_retain = yes; then
> >> > > +  AC_DEFINE(HAVE_GCC_RETAIN)
> >> > > +fi
> >> > > +
> >> > >  # Check if gcc warns about alias for function with incompatible types.
> >> > >  AC_CACHE_CHECK([if compiler warns about alias for function with incompatible types],
> >> > >                libc_cv_gcc_incompatible_alias, [dnl
> >> > > diff --git a/include/libc-symbols.h b/include/libc-symbols.h
> >> > > index 546fc26a7b..b311c90f0c 100644
> >> > > --- a/include/libc-symbols.h
> >> > > +++ b/include/libc-symbols.h
> >> > > @@ -352,6 +352,12 @@ for linking")
> >> > >
> >> > >  */
> >> > >
> >> > > +#ifdef HAVE_GCC_RETAIN
> >> > > +# define attribute_used_retain __attribute__ ((__used__, __retain__))
> >> > > +#else
> >> > > +# define attribute_used_retain __attribute__ ((__used__))
> >> > > +#endif
> >> > > +
> >> > >  /* Symbol set support macros.  */
> >> > >
> >> > >  /* Make SYMBOL, which is in the text segment, an element of SET.  */
> >> > > @@ -367,12 +373,12 @@ for linking")
> >> > >  /* When building a shared library, make the set section writable,
> >> > >     because it will need to be relocated at run time anyway.  */
> >> > >  # define _elf_set_element(set, symbol) \
> >> > > -  static const void *__elf_set_##set##_element_##symbol##__ \
> >> > > -    __attribute__ ((used, section (#set))) = &(symbol)
> >> > > +    static const void *__elf_set_##set##_element_##symbol##__ \
> >> > > +      attribute_used_retain __attribute__ ((section (#set))) = &(symbol)
> >> > >  #else
> >> > >  # define _elf_set_element(set, symbol) \
> >> > > -  static const void *const __elf_set_##set##_element_##symbol##__ \
> >> > > -    __attribute__ ((used, section (#set))) = &(symbol)
> >> > > +    static const void *const __elf_set_##set##_element_##symbol##__ \
> >> > > +      attribute_used_retain __attribute__ ((section (#set))) = &(symbol)
> >> > >  #endif
> >> > >
> >> > >  /* Define SET as a symbol set.  This may be required (it is in a.out) to
> >> > > --
> >> > > 2.31.0.291.g576ba9dcdaf-goog
> >> > >
> >> >
> >> > Need gc-sections tests for linkers with and without -z start-stop-gc.
> >> >
> >> > --
> >> > H.J.
> >>
> >> I don't know how to add such a test... It needs to check flush at ext
> >> time but I cannot find a template in libio/tst-* ...
> >
> >You can add a special rule to check for linker output with
> >--print-gc-sections.
>
> I can come up runtime tests. ( https://github.com/MaskRay/glibc/tree/retain)
> Runtime tests are direct.
> (Unsure how to test --print-gc-sections with tests-static...)
>
>
> diff --git a/libio/Makefile b/libio/Makefile
> index 12ce41038f..5e0e7f00cd 100644
> --- a/libio/Makefile
> +++ b/libio/Makefile
> @@ -195,6 +195,13 @@ ifeq (yes,$(build-shared))
>   tests-special += $(objpfx)tst-fopenloc-cmp.out $(objpfx)tst-fopenloc-mem.out \
>                  $(objpfx)tst-bz24228-mem.out
>   endif
> +
> +tests += tst-cleanup-start-stop-gc tst-cleanup-nostart-stop-gc
> +tests-static += tst-cleanup-start-stop-gc tst-cleanup-nostart-stop-gc
> +tests-special += $(objpfx)tst-cleanup-start-stop-gc.out \
> +               $(objpfx)tst-cleanup-nostart-stop-gc.out
> +LDFLAGS-tst-cleanup-start-stop-gc = -Wl,--gc-sections,-z,start-stop-gc
> +LDFLAGS-tst-cleanup-nostart-stop-gc = -Wl,--gc-sections,-z,nostart-stop-gc

1. Need to check if linker supports -z start-stop-gc.
2. Need to test with only -Wl,--gc-sections.

>   endif
>
>   include ../Rules
> @@ -224,6 +231,20 @@ $(objpfx)tst_wprintf2.out: $(gen-locales)
>   $(objpfx)tst-wfile-sync.out: $(gen-locales)
>   endif
>
> +$(objpfx)tst-cleanup-start-stop-gc.out: $(objpfx)tst-cleanup-start-stop-gc
> +       $< > $@T
> +       grep hello $@T; $(evaluate-test)
> +       rm -f $@T

tst-cleanup-start-stop-gc.out isn't generated.  You should add a ".exp" file
and compare the .out file against it.


> +$(objpfx)tst-cleanup-start-stop-gc.o: tst-cleanup.c
> +       $(compile.c) -o $@
> +
> +$(objpfx)tst-cleanup-nostart-stop-gc.out: $(objpfx)tst-cleanup-nostart-stop-gc
> +       $< > $@T
> +       grep hello $@T; $(evaluate-test)
> +       rm -f $@T
> +$(objpfx)tst-cleanup-nostart-stop-gc.o: tst-cleanup.c
> +       $(compile.c) -o $@
> +
>   $(objpfx)test-freopen.out: test-freopen.sh $(objpfx)test-freopen
>         $(SHELL) $< $(common-objpfx) '$(test-program-prefix)'   \
>         $(common-objpfx)libio/; \
> diff --git a/libio/tst-cleanup.c b/libio/tst-cleanup.c
> new file mode 100644
> index 0000000000..de8130a482
> --- /dev/null
> +++ b/libio/tst-cleanup.c
> @@ -0,0 +1,14 @@

Missing comments and copyright notice.

> +#include <stdio.h>
> +#include <stdlib.h>
> +
> +void
> +hook (void)
> +{
> +  puts ("hello");
> +}
> +
> +int
> +main (void)
> +{
> +  atexit (hook);
> +}



--
H.J.
  

On 2021-03-31, H.J. Lu wrote:
>On Tue, Mar 30, 2021 at 9:34 PM Fāng-ruì Sòng <maskray@google.com> wrote:
>>
>> On 2021-03-30, H.J. Lu wrote:
>> >On Tue, Mar 30, 2021 at 12:17 PM Fāng-ruì Sòng <maskray@google.com> wrote:
>> >>
>> >> On Tue, Mar 30, 2021 at 11:17 AM H.J. Lu <hjl.tools@gmail.com> wrote:
>> >> >
>> >> > On Mon, Mar 29, 2021 at 4:24 PM Fangrui Song <maskray@google.com> wrote:
>> >> > >
>> >> > > So that text_set_element/data_set_element/bss_set_element defined
>> >> > > variables will be retained by the linker.
>> >> > >
>> >> > > Note: 'used' and 'retain' are orthogonal: 'used' makes sure the variable
>> >> > > will not be optimized out; 'retain' prevents section garbage collection
>> >> > > if the linker support SHF_GNU_RETAIN.
>> >> > >
>> >> > > GNU ld 2.37 and LLD 13 will support -z start-stop-gc which allow C
>> >> > > identifier name sections to be GCed even if there are live
>> >> > > __start_/__stop_ references.
>> >> > >
>> >> > > Without the change, there are some static linking problems, e.g.
>> >> > > _IO_cleanup (libio/genops.c) may be discarded by ld --gc-sections, so
>> >> > > stdout is not flushed on exit.
>> >> > >
>> >> > > Note: GCC may warning ‘retain’ attribute ignored while __has_attribute(retain) is 1
>> >> > > (https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99587).
>> >> > > ---
>> >> > > Changes in v1 -> v2:
>> >> > > * Define attribute_used_retain_section
>> >> > > Changes in v2 -> v3:
>> >> > > * Use attribute_used_retain instead attribute_used_retain_section
>> >> > > ---
>> >> > >  config.h.in            |  3 +++
>> >> > >  configure              | 23 +++++++++++++++++++++++
>> >> > >  configure.ac           | 16 ++++++++++++++++
>> >> > >  include/libc-symbols.h | 14 ++++++++++----
>> >> > >  4 files changed, 52 insertions(+), 4 deletions(-)
>> >> > >
>> >> > > diff --git a/config.h.in b/config.h.in
>> >> > > index ca1547ae67..95af1ef229 100644
>> >> > > --- a/config.h.in
>> >> > > +++ b/config.h.in
>> >> > > @@ -187,6 +187,9 @@
>> >> > >  /* Define if gcc supports attribute ifunc.  */
>> >> > >  #undef HAVE_GCC_IFUNC
>> >> > >
>> >> > > +/* Define if gcc supports attribute retain.  */
>> >> > > +#undef HAVE_GCC_RETAIN
>> >> > > +
>> >> > >  /* Define if the linker defines __ehdr_start.  */
>> >> > >  #undef HAVE_EHDR_START
>> >> > >
>> >> > > diff --git a/configure b/configure
>> >> > > index fcf43bf7de..6a6b8ac264 100755
>> >> > > --- a/configure
>> >> > > +++ b/configure
>> >> > > @@ -4105,6 +4105,29 @@ fi
>> >> > >  $as_echo "$libc_cv_textrel_ifunc" >&6; }
>> >> > >
>> >> > >
>> >> > > +# Check if gcc supports attribute ifunc as it is used in libc_ifunc macro.
>> >> > > +{ $as_echo "$as_me:${as_lineno-$LINENO}: checking for gcc attribute retain support" >&5
>> >> > > +$as_echo_n "checking for gcc attribute retain support... " >&6; }
>> >> > > +if ${libc_cv_gcc_retain+:} false; then :
>> >> > > +  $as_echo_n "(cached) " >&6
>> >> > > +else
>> >> > > +  cat > conftest.c <<EOF
>> >> > > +static int var  __attribute__ ((used, retain, section ("__libc_atexit")));
>> >> > > +EOF
>> >> > > +libc_cv_gcc_retain=no
>> >> > > +if ${CC-cc} -Werror -c conftest.c -o /dev/null 1>&5 \
>> >> > > +   2>&5 ; then
>> >> > > +  libc_cv_gcc_retain=yes
>> >> > > +fi
>> >> > > +rm -f conftest*
>> >> > > +fi
>> >> > > +{ $as_echo "$as_me:${as_lineno-$LINENO}: result: $libc_cv_gcc_retain" >&5
>> >> > > +$as_echo "$libc_cv_gcc_retain" >&6; }
>> >> > > +if test $libc_cv_gcc_retain = yes; then
>> >> > > +  $as_echo "#define HAVE_GCC_RETAIN 1" >>confdefs.h
>> >> > > +
>> >> > > +fi
>> >> > > +
>> >> > >  # Check if gcc warns about alias for function with incompatible types.
>> >> > >  { $as_echo "$as_me:${as_lineno-$LINENO}: checking if compiler warns about alias for function with incompatible types" >&5
>> >> > >  $as_echo_n "checking if compiler warns about alias for function with incompatible types... " >&6; }
>> >> > > diff --git a/configure.ac b/configure.ac
>> >> > > index fce967f2c2..031b8c3fb3 100644
>> >> > > --- a/configure.ac
>> >> > > +++ b/configure.ac
>> >> > > @@ -707,6 +707,22 @@ fi
>> >> > >  rm -f conftest*])
>> >> > >  AC_SUBST(libc_cv_textrel_ifunc)
>> >> > >
>> >> > > +# Check if gcc supports attribute ifunc as it is used in libc_ifunc macro.
>> >> > > +AC_CACHE_CHECK([for gcc attribute retain support],
>> >> > > +              libc_cv_gcc_retain, [dnl
>> >> > > +cat > conftest.c <<EOF
>> >> > > +static int var  __attribute__ ((used, retain, section ("__libc_atexit")));
>> >> > > +EOF
>> >> > > +libc_cv_gcc_retain=no
>> >> > > +if ${CC-cc} -Werror -c conftest.c -o /dev/null 1>&AS_MESSAGE_LOG_FD \
>> >> > > +   2>&AS_MESSAGE_LOG_FD ; then
>> >> > > +  libc_cv_gcc_retain=yes
>> >> > > +fi
>> >> > > +rm -f conftest*])
>> >> > > +if test $libc_cv_gcc_retain = yes; then
>> >> > > +  AC_DEFINE(HAVE_GCC_RETAIN)
>> >> > > +fi
>> >> > > +
>> >> > >  # Check if gcc warns about alias for function with incompatible types.
>> >> > >  AC_CACHE_CHECK([if compiler warns about alias for function with incompatible types],
>> >> > >                libc_cv_gcc_incompatible_alias, [dnl
>> >> > > diff --git a/include/libc-symbols.h b/include/libc-symbols.h
>> >> > > index 546fc26a7b..b311c90f0c 100644
>> >> > > --- a/include/libc-symbols.h
>> >> > > +++ b/include/libc-symbols.h
>> >> > > @@ -352,6 +352,12 @@ for linking")
>> >> > >
>> >> > >  */
>> >> > >
>> >> > > +#ifdef HAVE_GCC_RETAIN
>> >> > > +# define attribute_used_retain __attribute__ ((__used__, __retain__))
>> >> > > +#else
>> >> > > +# define attribute_used_retain __attribute__ ((__used__))
>> >> > > +#endif
>> >> > > +
>> >> > >  /* Symbol set support macros.  */
>> >> > >
>> >> > >  /* Make SYMBOL, which is in the text segment, an element of SET.  */
>> >> > > @@ -367,12 +373,12 @@ for linking")
>> >> > >  /* When building a shared library, make the set section writable,
>> >> > >     because it will need to be relocated at run time anyway.  */
>> >> > >  # define _elf_set_element(set, symbol) \
>> >> > > -  static const void *__elf_set_##set##_element_##symbol##__ \
>> >> > > -    __attribute__ ((used, section (#set))) = &(symbol)
>> >> > > +    static const void *__elf_set_##set##_element_##symbol##__ \
>> >> > > +      attribute_used_retain __attribute__ ((section (#set))) = &(symbol)
>> >> > >  #else
>> >> > >  # define _elf_set_element(set, symbol) \
>> >> > > -  static const void *const __elf_set_##set##_element_##symbol##__ \
>> >> > > -    __attribute__ ((used, section (#set))) = &(symbol)
>> >> > > +    static const void *const __elf_set_##set##_element_##symbol##__ \
>> >> > > +      attribute_used_retain __attribute__ ((section (#set))) = &(symbol)
>> >> > >  #endif
>> >> > >
>> >> > >  /* Define SET as a symbol set.  This may be required (it is in a.out) to
>> >> > > --
>> >> > > 2.31.0.291.g576ba9dcdaf-goog
>> >> > >
>> >> >
>> >> > Need gc-sections tests for linkers with and without -z start-stop-gc.
>> >> >
>> >> > --
>> >> > H.J.
>> >>
>> >> I don't know how to add such a test... It needs to check flush at ext
>> >> time but I cannot find a template in libio/tst-* ...
>> >
>> >You can add a special rule to check for linker output with
>> >--print-gc-sections.
>>
>> I can come up runtime tests. ( https://github.com/MaskRay/glibc/tree/retain)
>> Runtime tests are direct.
>> (Unsure how to test --print-gc-sections with tests-static...)
>>
>>
>> diff --git a/libio/Makefile b/libio/Makefile
>> index 12ce41038f..5e0e7f00cd 100644
>> --- a/libio/Makefile
>> +++ b/libio/Makefile
>> @@ -195,6 +195,13 @@ ifeq (yes,$(build-shared))
>>   tests-special += $(objpfx)tst-fopenloc-cmp.out $(objpfx)tst-fopenloc-mem.out \
>>                  $(objpfx)tst-bz24228-mem.out
>>   endif
>> +
>> +tests += tst-cleanup-start-stop-gc tst-cleanup-nostart-stop-gc
>> +tests-static += tst-cleanup-start-stop-gc tst-cleanup-nostart-stop-gc
>> +tests-special += $(objpfx)tst-cleanup-start-stop-gc.out \
>> +               $(objpfx)tst-cleanup-nostart-stop-gc.out
>> +LDFLAGS-tst-cleanup-start-stop-gc = -Wl,--gc-sections,-z,start-stop-gc
>> +LDFLAGS-tst-cleanup-nostart-stop-gc = -Wl,--gc-sections,-z,nostart-stop-gc
>
>1. Need to check if linker supports -z start-stop-gc.

This seems difficult to do with Makefile... Second, do we need to check it?
ld warns instead of errors for an unknown -z option, so not checking it
may be fine. (GNU ld warns even with --fatal-warnings.)

>2. Need to test with only -Wl,--gc-sections.

OK, removed -z,nostart-stop-gc

-LDFLAGS-tst-cleanup-nostart-stop-gc = -Wl,--gc-sections,-z,nostart-stop-gc
+LDFLAGS-tst-cleanup-nostart-stop-gc = -Wl,--gc-sections

>>   endif
>>
>>   include ../Rules
>> @@ -224,6 +231,20 @@ $(objpfx)tst_wprintf2.out: $(gen-locales)
>>   $(objpfx)tst-wfile-sync.out: $(gen-locales)
>>   endif
>>
>> +$(objpfx)tst-cleanup-start-stop-gc.out: $(objpfx)tst-cleanup-start-stop-gc
>> +       $< > $@T
>> +       grep hello $@T; $(evaluate-test)
>> +       rm -f $@T
>
>tst-cleanup-start-stop-gc.out isn't generated.  You should add a ".exp" file
>and compare the .out file against it.

Thanks for the tip. Added tst-cleanup.exp with the content "hello\n" (see the usage below).
>
>> +$(objpfx)tst-cleanup-start-stop-gc.o: tst-cleanup.c
>> +       $(compile.c) -o $@
>> +
>> +$(objpfx)tst-cleanup-nostart-stop-gc.out: $(objpfx)tst-cleanup-nostart-stop-gc
>> +       $< > $@T
>> +       grep hello $@T; $(evaluate-test)
>> +       rm -f $@T
>> +$(objpfx)tst-cleanup-nostart-stop-gc.o: tst-cleanup.c
>> +       $(compile.c) -o $@
>> +
>>   $(objpfx)test-freopen.out: test-freopen.sh $(objpfx)test-freopen
>>         $(SHELL) $< $(common-objpfx) '$(test-program-prefix)'   \
>>         $(common-objpfx)libio/; \
>> diff --git a/libio/tst-cleanup.c b/libio/tst-cleanup.c
>> new file mode 100644
>> index 0000000000..de8130a482
>> --- /dev/null
>> +++ b/libio/tst-cleanup.c
>> @@ -0,0 +1,14 @@
>
>Missing comments and copyright notice.

OK. I find that many tst* files don't have a copyright notice..
Added.

>> +#include <stdio.h>
>> +#include <stdlib.h>
>> +
>> +void
>> +hook (void)
>> +{
>> +  puts ("hello");
>> +}
>> +
>> +int
>> +main (void)
>> +{
>> +  atexit (hook);
>> +}


diff --git a/libio/Makefile b/libio/Makefile
index 12ce41038f..472a1e064c 100644
--- a/libio/Makefile
+++ b/libio/Makefile
@@ -195,6 +195,13 @@ ifeq (yes,$(build-shared))
  tests-special += $(objpfx)tst-fopenloc-cmp.out $(objpfx)tst-fopenloc-mem.out \
  		 $(objpfx)tst-bz24228-mem.out
  endif
+
+tests += tst-cleanup-start-stop-gc tst-cleanup-nostart-stop-gc
+tests-static += tst-cleanup-start-stop-gc tst-cleanup-nostart-stop-gc
+tests-special += $(objpfx)tst-cleanup-start-stop-gc-cmp.out \
+		$(objpfx)tst-cleanup-nostart-stop-gc-cmp.out
+LDFLAGS-tst-cleanup-start-stop-gc = -Wl,--gc-sections,-z,start-stop-gc
+LDFLAGS-tst-cleanup-nostart-stop-gc = -Wl,--gc-sections,-z,nostart-stop-gc
  endif
  
  include ../Rules
@@ -224,6 +231,18 @@ $(objpfx)tst_wprintf2.out: $(gen-locales)
  $(objpfx)tst-wfile-sync.out: $(gen-locales)
  endif
  
+$(objpfx)tst-cleanup-start-stop-gc-cmp.out: tst-cleanup.exp $(objpfx)tst-cleanup-start-stop-gc.out
+	cmp $^ > $@; \
+	$(evaluate-test)
+$(objpfx)tst-cleanup-start-stop-gc.o: tst-cleanup.c
+	$(compile.c) -o $@
+
+$(objpfx)tst-cleanup-nostart-stop-gc-cmp.out: tst-cleanup.exp $(objpfx)tst-cleanup-nostart-stop-gc.out
+	cmp $^ > $@; \
+	$(evaluate-test)
+$(objpfx)tst-cleanup-nostart-stop-gc.o: tst-cleanup.c
+	$(compile.c) -o $@
+
  $(objpfx)test-freopen.out: test-freopen.sh $(objpfx)test-freopen
  	$(SHELL) $< $(common-objpfx) '$(test-program-prefix)'	\
  	$(common-objpfx)libio/; \
diff --git a/libio/tst-cleanup.c b/libio/tst-cleanup.c
new file mode 100644
index 0000000000..7f0a34a91e
--- /dev/null
+++ b/libio/tst-cleanup.c
@@ -0,0 +1,33 @@
+/* Copyright (C) 2021 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <https://www.gnu.org/licenses/>.  */
+
+/* Test that stdout is flushed after atexit callbacks were run.  */
+
+#include <stdio.h>
+#include <stdlib.h>
+
+void
+hook (void)
+{
+  puts ("hello");
+}
+
+int
+main (void)
+{
+  atexit (hook);
+}
  

On Wed, Mar 31, 2021 at 11:09 AM Fāng-ruì Sòng <maskray@google.com> wrote:
>
>
> On 2021-03-31, H.J. Lu wrote:
> >On Tue, Mar 30, 2021 at 9:34 PM Fāng-ruì Sòng <maskray@google.com> wrote:
> >>
> >> On 2021-03-30, H.J. Lu wrote:
> >> >On Tue, Mar 30, 2021 at 12:17 PM Fāng-ruì Sòng <maskray@google.com> wrote:
> >> >>
> >> >> On Tue, Mar 30, 2021 at 11:17 AM H.J. Lu <hjl.tools@gmail.com> wrote:
> >> >> >
> >> >> > On Mon, Mar 29, 2021 at 4:24 PM Fangrui Song <maskray@google.com> wrote:
> >> >> > >
> >> >> > > So that text_set_element/data_set_element/bss_set_element defined
> >> >> > > variables will be retained by the linker.
> >> >> > >
> >> >> > > Note: 'used' and 'retain' are orthogonal: 'used' makes sure the variable
> >> >> > > will not be optimized out; 'retain' prevents section garbage collection
> >> >> > > if the linker support SHF_GNU_RETAIN.
> >> >> > >
> >> >> > > GNU ld 2.37 and LLD 13 will support -z start-stop-gc which allow C
> >> >> > > identifier name sections to be GCed even if there are live
> >> >> > > __start_/__stop_ references.
> >> >> > >
> >> >> > > Without the change, there are some static linking problems, e.g.
> >> >> > > _IO_cleanup (libio/genops.c) may be discarded by ld --gc-sections, so
> >> >> > > stdout is not flushed on exit.
> >> >> > >
> >> >> > > Note: GCC may warning ‘retain’ attribute ignored while __has_attribute(retain) is 1
> >> >> > > (https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99587).
> >> >> > > ---
> >> >> > > Changes in v1 -> v2:
> >> >> > > * Define attribute_used_retain_section
> >> >> > > Changes in v2 -> v3:
> >> >> > > * Use attribute_used_retain instead attribute_used_retain_section
> >> >> > > ---
> >> >> > >  config.h.in            |  3 +++
> >> >> > >  configure              | 23 +++++++++++++++++++++++
> >> >> > >  configure.ac           | 16 ++++++++++++++++
> >> >> > >  include/libc-symbols.h | 14 ++++++++++----
> >> >> > >  4 files changed, 52 insertions(+), 4 deletions(-)
> >> >> > >
> >> >> > > diff --git a/config.h.in b/config.h.in
> >> >> > > index ca1547ae67..95af1ef229 100644
> >> >> > > --- a/config.h.in
> >> >> > > +++ b/config.h.in
> >> >> > > @@ -187,6 +187,9 @@
> >> >> > >  /* Define if gcc supports attribute ifunc.  */
> >> >> > >  #undef HAVE_GCC_IFUNC
> >> >> > >
> >> >> > > +/* Define if gcc supports attribute retain.  */
> >> >> > > +#undef HAVE_GCC_RETAIN
> >> >> > > +
> >> >> > >  /* Define if the linker defines __ehdr_start.  */
> >> >> > >  #undef HAVE_EHDR_START
> >> >> > >
> >> >> > > diff --git a/configure b/configure
> >> >> > > index fcf43bf7de..6a6b8ac264 100755
> >> >> > > --- a/configure
> >> >> > > +++ b/configure
> >> >> > > @@ -4105,6 +4105,29 @@ fi
> >> >> > >  $as_echo "$libc_cv_textrel_ifunc" >&6; }
> >> >> > >
> >> >> > >
> >> >> > > +# Check if gcc supports attribute ifunc as it is used in libc_ifunc macro.
> >> >> > > +{ $as_echo "$as_me:${as_lineno-$LINENO}: checking for gcc attribute retain support" >&5
> >> >> > > +$as_echo_n "checking for gcc attribute retain support... " >&6; }
> >> >> > > +if ${libc_cv_gcc_retain+:} false; then :
> >> >> > > +  $as_echo_n "(cached) " >&6
> >> >> > > +else
> >> >> > > +  cat > conftest.c <<EOF
> >> >> > > +static int var  __attribute__ ((used, retain, section ("__libc_atexit")));
> >> >> > > +EOF
> >> >> > > +libc_cv_gcc_retain=no
> >> >> > > +if ${CC-cc} -Werror -c conftest.c -o /dev/null 1>&5 \
> >> >> > > +   2>&5 ; then
> >> >> > > +  libc_cv_gcc_retain=yes
> >> >> > > +fi
> >> >> > > +rm -f conftest*
> >> >> > > +fi
> >> >> > > +{ $as_echo "$as_me:${as_lineno-$LINENO}: result: $libc_cv_gcc_retain" >&5
> >> >> > > +$as_echo "$libc_cv_gcc_retain" >&6; }
> >> >> > > +if test $libc_cv_gcc_retain = yes; then
> >> >> > > +  $as_echo "#define HAVE_GCC_RETAIN 1" >>confdefs.h
> >> >> > > +
> >> >> > > +fi
> >> >> > > +
> >> >> > >  # Check if gcc warns about alias for function with incompatible types.
> >> >> > >  { $as_echo "$as_me:${as_lineno-$LINENO}: checking if compiler warns about alias for function with incompatible types" >&5
> >> >> > >  $as_echo_n "checking if compiler warns about alias for function with incompatible types... " >&6; }
> >> >> > > diff --git a/configure.ac b/configure.ac
> >> >> > > index fce967f2c2..031b8c3fb3 100644
> >> >> > > --- a/configure.ac
> >> >> > > +++ b/configure.ac
> >> >> > > @@ -707,6 +707,22 @@ fi
> >> >> > >  rm -f conftest*])
> >> >> > >  AC_SUBST(libc_cv_textrel_ifunc)
> >> >> > >
> >> >> > > +# Check if gcc supports attribute ifunc as it is used in libc_ifunc macro.
> >> >> > > +AC_CACHE_CHECK([for gcc attribute retain support],
> >> >> > > +              libc_cv_gcc_retain, [dnl
> >> >> > > +cat > conftest.c <<EOF
> >> >> > > +static int var  __attribute__ ((used, retain, section ("__libc_atexit")));
> >> >> > > +EOF
> >> >> > > +libc_cv_gcc_retain=no
> >> >> > > +if ${CC-cc} -Werror -c conftest.c -o /dev/null 1>&AS_MESSAGE_LOG_FD \
> >> >> > > +   2>&AS_MESSAGE_LOG_FD ; then
> >> >> > > +  libc_cv_gcc_retain=yes
> >> >> > > +fi
> >> >> > > +rm -f conftest*])
> >> >> > > +if test $libc_cv_gcc_retain = yes; then
> >> >> > > +  AC_DEFINE(HAVE_GCC_RETAIN)
> >> >> > > +fi
> >> >> > > +
> >> >> > >  # Check if gcc warns about alias for function with incompatible types.
> >> >> > >  AC_CACHE_CHECK([if compiler warns about alias for function with incompatible types],
> >> >> > >                libc_cv_gcc_incompatible_alias, [dnl
> >> >> > > diff --git a/include/libc-symbols.h b/include/libc-symbols.h
> >> >> > > index 546fc26a7b..b311c90f0c 100644
> >> >> > > --- a/include/libc-symbols.h
> >> >> > > +++ b/include/libc-symbols.h
> >> >> > > @@ -352,6 +352,12 @@ for linking")
> >> >> > >
> >> >> > >  */
> >> >> > >
> >> >> > > +#ifdef HAVE_GCC_RETAIN
> >> >> > > +# define attribute_used_retain __attribute__ ((__used__, __retain__))
> >> >> > > +#else
> >> >> > > +# define attribute_used_retain __attribute__ ((__used__))
> >> >> > > +#endif
> >> >> > > +
> >> >> > >  /* Symbol set support macros.  */
> >> >> > >
> >> >> > >  /* Make SYMBOL, which is in the text segment, an element of SET.  */
> >> >> > > @@ -367,12 +373,12 @@ for linking")
> >> >> > >  /* When building a shared library, make the set section writable,
> >> >> > >     because it will need to be relocated at run time anyway.  */
> >> >> > >  # define _elf_set_element(set, symbol) \
> >> >> > > -  static const void *__elf_set_##set##_element_##symbol##__ \
> >> >> > > -    __attribute__ ((used, section (#set))) = &(symbol)
> >> >> > > +    static const void *__elf_set_##set##_element_##symbol##__ \
> >> >> > > +      attribute_used_retain __attribute__ ((section (#set))) = &(symbol)
> >> >> > >  #else
> >> >> > >  # define _elf_set_element(set, symbol) \
> >> >> > > -  static const void *const __elf_set_##set##_element_##symbol##__ \
> >> >> > > -    __attribute__ ((used, section (#set))) = &(symbol)
> >> >> > > +    static const void *const __elf_set_##set##_element_##symbol##__ \
> >> >> > > +      attribute_used_retain __attribute__ ((section (#set))) = &(symbol)
> >> >> > >  #endif
> >> >> > >
> >> >> > >  /* Define SET as a symbol set.  This may be required (it is in a.out) to
> >> >> > > --
> >> >> > > 2.31.0.291.g576ba9dcdaf-goog
> >> >> > >
> >> >> >
> >> >> > Need gc-sections tests for linkers with and without -z start-stop-gc.
> >> >> >
> >> >> > --
> >> >> > H.J.
> >> >>
> >> >> I don't know how to add such a test... It needs to check flush at ext
> >> >> time but I cannot find a template in libio/tst-* ...
> >> >
> >> >You can add a special rule to check for linker output with
> >> >--print-gc-sections.
> >>
> >> I can come up runtime tests. ( https://github.com/MaskRay/glibc/tree/retain)
> >> Runtime tests are direct.
> >> (Unsure how to test --print-gc-sections with tests-static...)
> >>
> >>
> >> diff --git a/libio/Makefile b/libio/Makefile
> >> index 12ce41038f..5e0e7f00cd 100644
> >> --- a/libio/Makefile
> >> +++ b/libio/Makefile
> >> @@ -195,6 +195,13 @@ ifeq (yes,$(build-shared))
> >>   tests-special += $(objpfx)tst-fopenloc-cmp.out $(objpfx)tst-fopenloc-mem.out \
> >>                  $(objpfx)tst-bz24228-mem.out
> >>   endif
> >> +
> >> +tests += tst-cleanup-start-stop-gc tst-cleanup-nostart-stop-gc
> >> +tests-static += tst-cleanup-start-stop-gc tst-cleanup-nostart-stop-gc
> >> +tests-special += $(objpfx)tst-cleanup-start-stop-gc.out \
> >> +               $(objpfx)tst-cleanup-nostart-stop-gc.out
> >> +LDFLAGS-tst-cleanup-start-stop-gc = -Wl,--gc-sections,-z,start-stop-gc
> >> +LDFLAGS-tst-cleanup-nostart-stop-gc = -Wl,--gc-sections,-z,nostart-stop-gc
> >
> >1. Need to check if linker supports -z start-stop-gc.
>
> This seems difficult to do with Makefile... Second, do we need to check it?
> ld warns instead of errors for an unknown -z option, so not checking it
> may be fine. (GNU ld warns even with --fatal-warnings.)

Please use LIBC_LINKER_FEATURE and LIBC_CONFIG_VAR to
detect and check linker features.

> >2. Need to test with only -Wl,--gc-sections.
>
> OK, removed -z,nostart-stop-gc

We need all 3 tests with -Wl,--gc-sections.

> -LDFLAGS-tst-cleanup-nostart-stop-gc = -Wl,--gc-sections,-z,nostart-stop-gc
> +LDFLAGS-tst-cleanup-nostart-stop-gc = -Wl,--gc-sections
>
> >>   endif
> >>
> >>   include ../Rules
> >> @@ -224,6 +231,20 @@ $(objpfx)tst_wprintf2.out: $(gen-locales)
> >>   $(objpfx)tst-wfile-sync.out: $(gen-locales)
> >>   endif
> >>
> >> +$(objpfx)tst-cleanup-start-stop-gc.out: $(objpfx)tst-cleanup-start-stop-gc
> >> +       $< > $@T
> >> +       grep hello $@T; $(evaluate-test)
> >> +       rm -f $@T
> >
> >tst-cleanup-start-stop-gc.out isn't generated.  You should add a ".exp" file
> >and compare the .out file against it.
>
> Thanks for the tip. Added tst-cleanup.exp with the content "hello\n" (see the usage below).
> >
> >> +$(objpfx)tst-cleanup-start-stop-gc.o: tst-cleanup.c
> >> +       $(compile.c) -o $@
> >> +
> >> +$(objpfx)tst-cleanup-nostart-stop-gc.out: $(objpfx)tst-cleanup-nostart-stop-gc
> >> +       $< > $@T
> >> +       grep hello $@T; $(evaluate-test)
> >> +       rm -f $@T
> >> +$(objpfx)tst-cleanup-nostart-stop-gc.o: tst-cleanup.c
> >> +       $(compile.c) -o $@
> >> +
> >>   $(objpfx)test-freopen.out: test-freopen.sh $(objpfx)test-freopen
> >>         $(SHELL) $< $(common-objpfx) '$(test-program-prefix)'   \
> >>         $(common-objpfx)libio/; \
> >> diff --git a/libio/tst-cleanup.c b/libio/tst-cleanup.c
> >> new file mode 100644
> >> index 0000000000..de8130a482
> >> --- /dev/null
> >> +++ b/libio/tst-cleanup.c
> >> @@ -0,0 +1,14 @@
> >
> >Missing comments and copyright notice.
>
> OK. I find that many tst* files don't have a copyright notice..
> Added.
>
> >> +#include <stdio.h>
> >> +#include <stdlib.h>
> >> +
> >> +void
> >> +hook (void)
> >> +{
> >> +  puts ("hello");
> >> +}
> >> +
> >> +int
> >> +main (void)
> >> +{
> >> +  atexit (hook);
> >> +}
>
>
> diff --git a/libio/Makefile b/libio/Makefile
> index 12ce41038f..472a1e064c 100644
> --- a/libio/Makefile
> +++ b/libio/Makefile
> @@ -195,6 +195,13 @@ ifeq (yes,$(build-shared))
>   tests-special += $(objpfx)tst-fopenloc-cmp.out $(objpfx)tst-fopenloc-mem.out \
>                  $(objpfx)tst-bz24228-mem.out
>   endif
> +
> +tests += tst-cleanup-start-stop-gc tst-cleanup-nostart-stop-gc
> +tests-static += tst-cleanup-start-stop-gc tst-cleanup-nostart-stop-gc
> +tests-special += $(objpfx)tst-cleanup-start-stop-gc-cmp.out \
> +               $(objpfx)tst-cleanup-nostart-stop-gc-cmp.out
> +LDFLAGS-tst-cleanup-start-stop-gc = -Wl,--gc-sections,-z,start-stop-gc
> +LDFLAGS-tst-cleanup-nostart-stop-gc = -Wl,--gc-sections,-z,nostart-stop-gc
>   endif
>
>   include ../Rules
> @@ -224,6 +231,18 @@ $(objpfx)tst_wprintf2.out: $(gen-locales)
>   $(objpfx)tst-wfile-sync.out: $(gen-locales)
>   endif
>
> +$(objpfx)tst-cleanup-start-stop-gc-cmp.out: tst-cleanup.exp $(objpfx)tst-cleanup-start-stop-gc.out
> +       cmp $^ > $@; \
> +       $(evaluate-test)
> +$(objpfx)tst-cleanup-start-stop-gc.o: tst-cleanup.c
> +       $(compile.c) -o $@
> +
> +$(objpfx)tst-cleanup-nostart-stop-gc-cmp.out: tst-cleanup.exp $(objpfx)tst-cleanup-nostart-stop-gc.out
> +       cmp $^ > $@; \
> +       $(evaluate-test)
> +$(objpfx)tst-cleanup-nostart-stop-gc.o: tst-cleanup.c
> +       $(compile.c) -o $@
> +
>   $(objpfx)test-freopen.out: test-freopen.sh $(objpfx)test-freopen
>         $(SHELL) $< $(common-objpfx) '$(test-program-prefix)'   \
>         $(common-objpfx)libio/; \
> diff --git a/libio/tst-cleanup.c b/libio/tst-cleanup.c
> new file mode 100644
> index 0000000000..7f0a34a91e
> --- /dev/null
> +++ b/libio/tst-cleanup.c
> @@ -0,0 +1,33 @@
> +/* Copyright (C) 2021 Free Software Foundation, Inc.
> +   This file is part of the GNU C Library.
> +
> +   The GNU C Library is free software; you can redistribute it and/or
> +   modify it under the terms of the GNU Lesser General Public
> +   License as published by the Free Software Foundation; either
> +   version 2.1 of the License, or (at your option) any later version.
> +
> +   The GNU C Library is distributed in the hope that it will be useful,
> +   but WITHOUT ANY WARRANTY; without even the implied warranty of
> +   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
> +   Lesser General Public License for more details.
> +
> +   You should have received a copy of the GNU Lesser General Public
> +   License along with the GNU C Library; if not, see
> +   <https://www.gnu.org/licenses/>.  */
> +
> +/* Test that stdout is flushed after atexit callbacks were run.  */
> +
> +#include <stdio.h>
> +#include <stdlib.h>
> +
> +void
> +hook (void)
> +{
> +  puts ("hello");
> +}
> +
> +int
> +main (void)
> +{
> +  atexit (hook);
> +}