feat(build): Add full C++ build mode

Rather than relying on -Wc++-compat, add a full build flag that uses all
CXXFLAGS and parses the source truly as C++.

.github/workflows/distcheck.yaml

@@ -107,12 +107,14 @@ jobs:
             ./configure --with-mpi=/opt/amazon/openmpi \
                         --with-libfabric=/opt/amazon/efa \
                         --enable-tests \
+                        --enable-cxx=${{ matrix.mode == 'cxx' && 'yes' || 'no' }} \
                         --enable-platform-aws \
                         --enable-neuron
           else
             ./configure --with-mpi=/opt/amazon/openmpi \
                         --with-libfabric=/opt/amazon/efa \
                         --enable-tests \
+                        --enable-cxx=${{ matrix.mode == 'cxx' && 'yes' || 'no' }} \
                         --enable-platform-aws \
                         --with-cuda=/usr/local/cuda
           fi
@@ -139,6 +141,9 @@ jobs:
         cc:
           - gcc
           - clang
+        mode:
+          - cxx
+          - c
         tracing:
           - lttng
           - none
@@ -154,7 +159,7 @@ jobs:
             cc: gcc
             cc-version: 13
 
-    name: u2204/${{ matrix.sdk }}/${{matrix.cc}}-${{matrix.cc-variant}}/build+test
+    name: u2204/${{ matrix.sdk }}/${{matrix.cc}}-${{matrix.cc-variant}}/build+test(${{ matrix.mode }})
     steps:
       - uses: actions/checkout@v4
 
@@ -230,12 +235,14 @@ jobs:
             ./configure --with-mpi=/opt/amazon/openmpi \
                         --with-libfabric=/opt/amazon/efa \
                         --enable-tests \
+                        --enable-cxx=${{ matrix.mode == 'cxx' && 'yes' || 'no' }} \
                         --enable-platform-aws \
                         --with-cuda=/usr/local/cuda/
           else
             ./configure --with-mpi=/opt/amazon/openmpi \
                         --with-libfabric=/opt/amazon/efa \
                         --enable-tests \
+                        --enable-cxx=${{ matrix.mode == 'cxx' && 'yes' || 'no' }} \
                         --enable-platform-aws \
                         --enable-neuron
           fi
@@ -268,7 +275,10 @@ jobs:
         sdk:
           - cuda
           - neuron
-    name: CodeChecker - ${{ matrix.sdk }}
+        mode:
+          - cxx
+          - c
+    name: CodeChecker - ${{ matrix.sdk }} ${{ matrix.mode }}
     steps:
       - uses: actions/checkout@v4
       - uses: actions/setup-python@v5
@@ -309,12 +319,14 @@ jobs:
             ./configure --with-mpi=/opt/amazon/openmpi \
                         --with-libfabric=/opt/amazon/efa \
                         --enable-tests \
+                        --enable-cxx=${{ matrix.mode == 'cxx' && 'yes' || 'no' }} \
                         --enable-platform-aws \
                         --enable-neuron
           else
             ./configure --with-mpi=/opt/amazon/openmpi \
                         --with-libfabric=/opt/amazon/efa \
                         --enable-tests \
+                        --enable-cxx=${{ matrix.mode == 'cxx' && 'yes' || 'no' }} \
                         --enable-platform-aws \
                         --with-cuda=/usr/local/cuda
           fi
@@ -333,7 +345,7 @@ jobs:
       - name: Save CodeChecker HTML output.
         uses: actions/upload-artifact@v4
         with:
-          name: CodeChecker Bug Reports for ${{ matrix.sdk }}
+          name: CodeChecker Bug Reports for ${{ matrix.sdk }} ${{ matrix.mode }}
           path: ${{ steps.codechecker.outputs.result-html-dir }}/*.html
 
       - name: CodeChecker Pass Or Fail?

configure.ac

@@ -46,6 +46,7 @@ NCCL_NET_OFI_DISTCHCK_CONFIGURE_FLAGS=
 # Checks for programs
 AC_PROG_CC
 AM_PROG_CC_C_O
+AC_PROG_CXX
 m4_version_prereq([2.70], [], [AC_PROG_CC_STDC])
 AC_PROG_CXX
 AC_C_INLINE
@@ -56,12 +57,18 @@ dnl between this and AC_PROG_CC.
 AS_IF([test "${ax_enable_debug}" = "no"], [
              dnl Enable O3 optimization
              CFLAGS="${CFLAGS} -O3"
+             CXXFLAGS="${CXXFLAGS} -O3"
+
+             dnl use C++17, disable exceptions, and disable runtime type information
+             CXXFLAGS="-std=c++17 -fno-rtti -fno-exceptions"
 
              dnl dead code elim
              CFLAGS="${CFLAGS} -ffunction-sections -fdata-sections"
+             CXXFLAGS="${CXXFLAGS} -ffunction-sections -fdata-sections"
 
              dnl https://maskray.me/blog/2021-05-09-fno-semantic-interposition
              CFLAGS="${CFLAGS} -fno-semantic-interposition -fvisibility=hidden"
+             CXXFLAGS="${CXXFLAGS} -fno-semantic-interposition -fvisibility=hidden "
              LDFLAGS="${LDFLAGS} -Bsymbolic"
             ])
 CHECK_ENABLE_SANITIZER()
@@ -176,10 +183,15 @@ AS_IF([test "${enable_trace}" = "yes" ],
 AC_DEFINE_UNQUOTED([OFI_NCCL_TRACE], [${trace}], [Defined to 1 unit test output should include TRACE level])
 
 picky_cflags=""
+picky_cxxflags=""
 AC_DEFUN([ADD_PICKY_FLAGS],[
     AC_LANG_PUSH([C])
     AX_CHECK_COMPILE_FLAG([$1], [picky_cflags="${picky_cflags} $1"], [], [-Werror])
     AC_LANG_POP()
+
+    AC_LANG_PUSH([C++])
+    AX_CHECK_COMPILE_FLAG([$1], [picky_cxxflags="${picky_cxxflags} $1"], [], [-Werror -x c++])
+    AC_LANG_POP()
     ])
 
 dnl standard and normal
@@ -221,17 +233,27 @@ AC_ARG_ENABLE([picky-compiler],
 AS_IF([test "${enable_picky_compiler}" != "no"],
       [AC_MSG_NOTICE([Adding ${picky_cflags} to CFLAGS.])
        CFLAGS="${CFLAGS} ${picky_cflags}"
+       CXXFLAGS="${CXXFLAGS} ${picky_cxxflags}"
+       AS_UNSET([picky_cxxflags])
        AS_UNSET([picky_cflags])])
 
 werror_flags="-Werror"
 AC_ARG_ENABLE([werror],
    [AS_HELP_STRING([--enable-werror], [(Developer) Enable setting -Werror.  Off by default, unless building from Git tree.])])
 AS_IF([test -d "${srcdir}/.git" -a -z "${enable_werror}"],
       [AC_MSG_NOTICE([Found .git directory.  Adding ${werror_flags} to CFLAGS.])
-       CFLAGS="${werror_flags} ${CFLAGS}"],
+       CXXFLAGS="${CXXFLAGS} ${werror_flags} "
+       CFLAGS="${CFLAGS} ${werror_flags} "],
       [test "${enable_werror}" = "yes"],
       [AC_MSG_NOTICE([Adding ${werror_flags} to CFLAGS.])
-       CFLAGS="${werror_flags} ${CFLAGS}"])
+       CXXFLAGS="${CXXFLAGS} ${werror_flags} "
+       CFLAGS="${CFLAGS} ${werror_flags} "])
+
+AC_ARG_ENABLE([cxx],
+   [AS_HELP_STRING([--enable-cxx], [(Developer) Treat all C code as C++ source. Off by default.])])
+AS_IF([test "${enable_cxx}" = "yes"],
+      [AC_MSG_NOTICE([Treating all files as CXX files.])
+       CFLAGS="-x c++ ${CXXFLAGS}"])
 
 AC_SUBST([NCCL_NET_OFI_DISTCHCK_CONFIGURE_FLAGS])