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

["bkoz at gcc dot gnu.org" <gcc-bugzilla at gcc dot gnu dot org>]

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

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

           What    |Removed                     |Added
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                 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.