[PATCH RFC] bootstrap: Update requirement to C++11.

Richard Biener richard.guenther@gmail.com
Fri May 15 07:14:47 GMT 2020


On Thu, May 14, 2020 at 11:53 PM Jason Merrill via Gcc-patches
<gcc-patches@gcc.gnu.org> wrote:
>
> There seemed to be general agreement last November that we would move to
> allowing C++11 features to be used in GCC 11; this patch implements that
> direction.  Are changes needed anywhere else?
>
> ChangeLog
> 2020-05-14  Jason Merrill  <jason@redhat.com>
>
>         * configure.ac: Update bootstrap dialect to -std=gnu++11.
>
> gcc/ChangeLog
> 2020-05-14  Jason Merrill  <jason@redhat.com>
>
>         * doc/install.texi (Prerequisites): Update boostrap compiler
>         requirement to C++11/GCC 4.8.
> ---
>  gcc/doc/install.texi | 14 ++++++++------
>  configure.ac         |  6 +++---
>  ChangeLog            |  4 ++++
>  configure            |  6 +++---
>  4 files changed, 18 insertions(+), 12 deletions(-)
>
> diff --git a/gcc/doc/install.texi b/gcc/doc/install.texi
> index 876b04f9c45..f47e3c76f73 100644
> --- a/gcc/doc/install.texi
> +++ b/gcc/doc/install.texi
> @@ -238,15 +238,17 @@ described below.
>
>  @heading Tools/packages necessary for building GCC
>  @table @asis
> -@item ISO C++98 compiler
> -Necessary to bootstrap GCC, although versions of GCC prior
> -to 4.8 also allow bootstrapping with a ISO C89 compiler and versions
> -of GCC prior to 3.4 also allow bootstrapping with a traditional
> -(K&R) C compiler.
> +@item ISO C++11 compiler
> +Necessary to bootstrap GCC.
> +
> +Versions of GCC prior to 11 also allow bootstrapping with an ISO C++98
> +compiler, versions of GCC prior to 4.8 also allow bootstrapping with a
> +ISO C89 compiler, and versions of GCC prior to 3.4 also allow
> +bootstrapping with a traditional (K&R) C compiler.
>
>  To build all languages in a cross-compiler or other configuration where
>  3-stage bootstrap is not performed, you need to start with an existing
> -GCC binary (version 3.4 or later) because source code for language
> +GCC binary (version 4.8 or later) because source code for language
>  frontends other than C might use GCC extensions.
>
>  Note that to bootstrap GCC with versions of GCC earlier than 3.4, you
> diff --git a/configure.ac b/configure.ac
> index c78d9cbea62..63d92b73061 100644
> --- a/configure.ac
> +++ b/configure.ac
> @@ -1462,10 +1462,10 @@ case "$have_compiler:$host:$target:$enable_bootstrap" in
>      ;;
>  esac
>
> -# When bootstrapping with GCC, build stage 1 in C++98 mode to ensure that a
> -# C++98 compiler can still start the bootstrap.
> +# When bootstrapping with GCC, build stage 1 in C++11 mode to ensure that a
> +# C++11 compiler can still start the bootstrap.
>  if test "$enable_bootstrap:$GXX" = "yes:yes"; then
> -  CXX="$CXX -std=gnu++98"
> +  CXX="$CXX -std=gnu++11"

So I just spotted this - since we're requiring a ISO C++11 compiler shouldn't
we build stage1 with -std=c++11 rather than gnu++11 (whatever the detailed
differences are here)?  Also not sure what level of -pedantic we'd need to
avoid GNU extensions even with -std=c++11.  Of course there are (I hope)
a lot less GNU extensions for C++ than there were for C and hopefully
no extra in gnu++11 compared to gnu++98 which we checked previously.

Note I think what's missing is some general blurb in our coding conventions
as to how much of C++11 we are supposed to use in non-infrastructure parts
of GCC (I expect things like hash-table.h to use more C++ features than,
say, tree-ssa-alias.c).

There also does not seem to be a configure check which may present
users with a more useful error message than later cryptic fail of build?
I suppose we cannot simply check __cplusplus for this, can we?  Do
other common host compilers need additional options to enable C++11?
Should we try to second guess such flags via configury?  For example
GCC 4.8 defaults to -std=gnu++98 and the above only seems to apply
to the bootstrap case so GCC 4.8 cannot be used to build cross compilers
without adjusting CC and CXX?

Thanks,
Richard.

>  fi
>
>  # Used for setting $lt_cv_objdir
> diff --git a/ChangeLog b/ChangeLog
> index a7fcf77b9b2..1d281855a3e 100644
> --- a/ChangeLog
> +++ b/ChangeLog
> @@ -1,3 +1,7 @@
> +2020-05-14  Jason Merrill  <jason@redhat.com>
> +
> +       * configure.ac: Update bootstrap dialect to -std=gnu++11.
> +
>  2020-04-29  Thomas Schwinge  <thomas@codesourcery.com>
>
>         PR target/92713
> diff --git a/configure b/configure
> index 4cc938ebb7d..9b39035bbcc 100755
> --- a/configure
> +++ b/configure
> @@ -5523,10 +5523,10 @@ $as_echo "$as_me: WARNING: trying to bootstrap a cross compiler" >&2;}
>      ;;
>  esac
>
> -# When bootstrapping with GCC, build stage 1 in C++98 mode to ensure that a
> -# C++98 compiler can still start the bootstrap.
> +# When bootstrapping with GCC, build stage 1 in C++11 mode to ensure that a
> +# C++11 compiler can still start the bootstrap.
>  if test "$enable_bootstrap:$GXX" = "yes:yes"; then
> -  CXX="$CXX -std=gnu++98"
> +  CXX="$CXX -std=gnu++11"
>  fi
>
>  # Used for setting $lt_cv_objdir
>
> base-commit: 4e1592f8e1d6366699e05c0824fc3dc39ca7314b
> --
> 2.18.1
>


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