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gdb: Require C++11

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Use AX_CXX_COMPILE_STDCXX to detect if the compiler supports C++11,
and if -std=xxx switches are necessary to enable C++11.

We need to tweak AX_CXX_COMPILE_STDCXX a bit though.  Pristine
upstream AX_CXX_COMPILE_STDCXX appends -std=gnu++11 to CXX directly.
That doesn't work for us, because the top level Makefile passes CXX
down to subdirs, and that overrides whatever gdb/Makefile may set CXX
to.  The result would be that a make invocation from the build/gdb/
directory would use "g++ -std=gnu++11" as expected, while a make
invocation at the top level would not.

So instead of having AX_CXX_COMPILE_STDCXX set CXX directly, tweak it
to AC_SUBST a separate variable -- CXX_DIALECT -- and use '$(CXX)
(CXX_DIALECT)' to compile/link.

Confirmed that this enables C++11 starting with gcc 4.8, the first gcc
release with full C++11 support.

Also confirmed that configure errors out gracefully with older GCC
releases:

  checking whether /opt/gcc-4.7/bin/g++ supports C++11 features by default... no
  checking whether /opt/gcc-4.7/bin/g++ supports C++11 features with -std=gnu++11... no
  checking whether /opt/gcc-4.7/bin/g++ supports C++11 features with -std=gnu++0x... no
  checking whether /opt/gcc-4.7/bin/g++ supports C++11 features with -std=c++11... no
  checking whether /opt/gcc-4.7/bin/g++ supports C++11 features with -std=c++0x... no
  checking whether /opt/gcc-4.7/bin/g++ supports C++11 features with +std=c++11... no
  checking whether /opt/gcc-4.7/bin/g++ supports C++11 features with -h std=c++11... no
  configure: error: *** A compiler with support for C++11 language features is required.
  Makefile:9451: recipe for target 'configure-gdb' failed
  make[1]: *** [configure-gdb] Error 1
  make[1]: Leaving directory '/home/pedro/brno/pedro/gdb/mygit/cxx-convertion/build-gcc-4.7'

If we need to revert back to making C++11 optional, all that's
necessary is to change the "mandatory" to "optional" in configure.ac
and regenerate configure (both gdb and gdbserver).

gdb/ChangeLog:
2016-10-28  Pedro Alves  <palves@redhat.com>

	* Makefile.in (CXX_DIALECT): Get from configure.
	(COMPILE.pre, CC_LD): Append $(CXX_DIALECT).
	(FLAGS_TO_PASS): Pass CXX_DIALECT.
	* acinclude.m4: Include ax_cxx_compile_stdcxx.m4.
	* ax_cxx_compile_stdcxx.m4: Add FSF copyright header.  Set and
	AC_SUBST CXX_DIALECT instead of changing CXX/CXXCPP.
	* configure.ac: Call AX_CXX_COMPILE_STDCXX.
	* config.in: Regenerate.
	* configure: Regenerate.

gdb/gdbserver/ChangeLog:
2016-10-28  Pedro Alves  <palves@redhat.com>

	* Makefile.in (CXX_DIALECT): Get from configure.
	(COMPILE.pre, CC_LD): Append $(CXX_DIALECT).
	* acinclude.m4: Include ../ax_cxx_compile_stdcxx.m4.
	* configure.ac: Call AX_CXX_COMPILE_STDCXX.
	* config.in: Regenerate.
	* configure: Regenerate.
This commit is contained in:
Pedro Alves
2016-10-13 00:27:45 +01:00
parent 87106a7b0a
commit 0bcda68539
13 changed files with 2019 additions and 14 deletions
Download Patch File Download Diff File Expand all files Collapse all files

21
gdb/ax_cxx_compile_stdcxx.m4
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@@ -1,3 +1,12 @@
# Copyright (c) 2016 Free Software Foundation, Inc.
#
# Originally based on the AX_CXX_COMPILE_STDCXX macro found at the url
# below.
#
# Local GDB customizations:
#
# - AC_SUBST CXX_DIALECT instead of changing CXX/CXXCPP.
#
# ===========================================================================
# http://www.gnu.org/software/autoconf-archive/ax_cxx_compile_stdcxx.html
# ===========================================================================
@@ -58,6 +67,7 @@ AC_DEFUN([AX_CXX_COMPILE_STDCXX], [dnl
[$3], [optional], [ax_cxx_compile_cxx$1_required=false],
[m4_fatal([invalid third argument `$3' to AX_CXX_COMPILE_STDCXX])])
AC_LANG_PUSH([C++])dnl
CXX_DIALECT=""
ac_success=no
AC_CACHE_CHECK(whether $CXX supports C++$1 features by default,
ax_cv_cxx_compile_cxx$1,
@@ -81,10 +91,7 @@ AC_DEFUN([AX_CXX_COMPILE_STDCXX], [dnl
[eval $cachevar=no])
CXX="$ac_save_CXX"])
if eval test x\$$cachevar = xyes; then
CXX="$CXX $switch"
if test -n "$CXXCPP" ; then
CXXCPP="$CXXCPP $switch"
fi
CXX_DIALECT="$switch"
ac_success=yes
break
fi
@@ -107,10 +114,7 @@ AC_DEFUN([AX_CXX_COMPILE_STDCXX], [dnl
[eval $cachevar=no])
CXX="$ac_save_CXX"])
if eval test x\$$cachevar = xyes; then
CXX="$CXX $switch"
if test -n "$CXXCPP" ; then
CXXCPP="$CXXCPP $switch"
fi
CXX_DIALECT="$switch"
ac_success=yes
break
fi
@@ -131,6 +135,7 @@ AC_DEFUN([AX_CXX_COMPILE_STDCXX], [dnl
[define if the compiler supports basic C++$1 syntax])
fi
AC_SUBST(HAVE_CXX$1)
AC_SUBST(CXX_DIALECT)
])