[Bug libstdc++/53579] New: libstdc++ configure atomicity tests use CXXFLAGS instead of CXXFLAGS_FOR_TARGET

[Bug libstdc++/53579] New: libstdc++ configure atomicity tests use CXXFLAGS instead of CXXFLAGS_FOR_TARGET

[olegendo at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53579

             Bug #: 53579
           Summary: libstdc++ configure atomicity tests use CXXFLAGS
                    instead of CXXFLAGS_FOR_TARGET
    Classification: Unclassified
           Product: gcc
           Version: 4.8.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libstdc++
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: olegendo@gcc.gnu.org


When trying to fix PR 29366 by removing config/cpu/sh/atomicity.h I've noticed
that libstdc++'s configure would refuse to pick up
config/cpu/generic/atomicity_builtins.

On the SH target atomicity builtins have to be enabled by either -msoft-atomic
or -mhard-atomic.  For sh-linux -msoft-atomic is enabled by default.  When
building a cross toolchain (top level make) CFLAGS_FOR_TARGET /
CXXFLAGS_FOR_TARGET can be used to enable the atomicity builtins.  However,
libstdc++ configure uses CXXFLAGS when testing for atomicity builtins
availability, so it will default to using config/cpu/generic/atomicity_mutex.

I'm not quite sure where the fix should go, either in the top level
configure/makefile (which fails to set CXXFLAGS to CXXFLAGS_FOR_TARGET for
libstdc++ configure) or libstdc++ itself, because when libstdc++ is actually
compiled CXXFLAGS_FOR_TARGET is passed to xgcc as expected.

[Bug libstdc++/53579] libstdc++ configure atomicity tests use CXXFLAGS instead of CXXFLAGS_FOR_TARGET

[bkoz at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53579

Benjamin Kosnik <bkoz at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |bkoz at gcc dot gnu.org

--- Comment #1 from Benjamin Kosnik <bkoz at gcc dot gnu.org> 2012-08-07 20:53:26 UTC ---

Some notes.

src/Makefile.am does not export CXXFLAGS_FOR_TARGET, only CFLAGS_FOR_TARGET. 

The problem is that configure CXXFLAGS ends up being different than build
CXXFLAGS. And configure flags don't play with CXXFLAGS_FOR_TARGET (but build
flags do.)

gcc/Makefile.in has the pass down for target libs:
EXTRA_TARGET_FLAGS = \
    'AR=$$(AR_FOR_TARGET)' \
    'AS=$(COMPILER_AS_FOR_TARGET)' \
    'CC=$$(CC_FOR_TARGET) $$(XGCC_FLAGS_FOR_TARGET) $$(TFLAGS)' \
    'CFLAGS=$$(CFLAGS_FOR_TARGET)' \
    'CXX=$$(CXX_FOR_TARGET) $$(XGCC_FLAGS_FOR_TARGET) $$(TFLAGS)' \
    'CXXFLAGS=$$(CXXFLAGS_FOR_TARGET)' \

This is what is expected and needed for build, but for configure the situation
is not working in an equivalent manner.

config/mt-gnu:
CXXFLAGS_FOR_TARGET = $(CXXFLAGS) -D_GNU_SOURCE

configure.ac:
if test "x$CXXFLAGS_FOR_TARGET" = x; then
  CXXFLAGS_FOR_TARGET=$CXXFLAGS
  case " $CXXFLAGS " in
    *" -O2 "*) ;;
    *) CXXFLAGS_FOR_TARGET="-O2 $CXXFLAGS_FOR_TARGET" ;;
  esac
  case " $CXXFLAGS " in
    *" -g "* | *" -g3 "*) ;;
    *) CXXFLAGS_FOR_TARGET="-g $CXXFLAGS_FOR_TARGET" ;;
  esac
fi
AC_SUBST(CXXFLAGS_FOR_TARGET)

ie, CXXFLAGS_FOR_TARGET is what should be used, not CXXFLAGS

There are a lot of C++ configure tests in libstdc++ (and other target libs!)
that use CXXFLAGS assuming it's the real thing used to build the library. Not
just atomicity. A consistent solution would be nice here. 

Possible solutions:

1) Setup GLIBCXX_CONFIGURE to export GLIBCXX_CXXFLAGS, which is then used
consistently during configure only, and incorporates CXXFLAGS_FOR_TARGET.
(Makefiles still use CXXFLAGS).

2) use CFLAGS_FOR_TARGET in libstdc++ config

3) like one, but have something like CXXFLAGS_EXTRA_FOR_TARGET which (unlike
CXXFLAGS_FOR_TARGET) is not supposed to replace CXXFLAGS, but instead is always
added to CXXFLAGS, not substituted for it.

[Bug libstdc++/53579] libstdc++ configure use CXXFLAGS instead of CXXFLAGS_FOR_TARGET

[anatol.pomozov at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=53579

--- Comment #3 from Anatol <anatol.pomozov at gmail dot com> ---
I just hit this issue when tried to build cross-tools for arm64.
CFLAGS_FOR_TARGET works as expected and I was assuming that CXXFLAGS_FOR_TARGET
is used instead of CXXFLAGS.

If CXXFLAGS_FOR_TARGET is not honored now, what is recommended workaround for
cross-compilers that need different CXXFLAGS for host and target?