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

[Patrick Palka <ppalka@redhat.com>]

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.

>    static ios_base::Init __ioinit;
>  #elif defined(_GLIBCXX_SYMVER_GNU)
>    __extension__ __asm (".globl _ZSt21ios_base_library_initv");
> diff --git a/libstdc++-v3/src/c++98/ios_base_init.h b/libstdc++-v3/src/c++98/ios_base_init.h
> index b600ec3298e..f7edfc84625 100644
> --- a/libstdc++-v3/src/c++98/ios_base_init.h
> +++ b/libstdc++-v3/src/c++98/ios_base_init.h
> @@ -8,6 +8,6 @@
>  // constructor when statically linking with libstdc++.a), instead of
>  // doing so in (each TU that includes) <iostream>.
>  // This needs to be done in the same TU that defines the stream objects.
> -#if __has_attribute(init_priority)
> +#if _GLIBCXX_USE_INIT_PRIORITY_ATTRIBUTE
>  static ios_base::Init __ioinit __attribute__((init_priority(90)));
>  #endif
> -- 
> 2.41.0
> 
>