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From:   Sedat Dilek <sedat.dilek@gmail.com>
Date:   Sun, 4 Apr 2021 16:59:33 +0200
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Subject: Usage of CXX in tools directory
To:     Masahiro Yamada <masahiroy@kernel.org>, Yonghong Song <yhs@fb.com>,
        Arnaldo Carvalho de Melo <arnaldo.melo@gmail.com>,
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        Ingo Molnar <mingo@redhat.com>, Jiri Olsa <jolsa@redhat.com>,
        Namhyung Kim <namhyung@kernel.org>
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Hi,

when dealing/experimenting with BPF together with pahole/dwarves and
dwarf-v5 and clang-lto I fell over that there is usage of CXX in tools
directory.
Especially,  I wanted to build and run test_progs from BPF selftests.
One BPF selftest called "test_cpp" used GNU/g++ (and even /usr/bin/ld)
and NOT LLVM/clang++.

For details see the linux-bpf/dwarves thread "[PATCH dwarves]
dwarf_loader: handle DWARF5 DW_OP_addrx properly" in [1].

Lemme check:

$ git grep CXX tools/
tools/build/Build.include:cxx_flags = -Wp,-MD,$(depfile) -Wp,-MT,$@
$(CXXFLAGS) -D"BUILD_STR(s)=\#s" $(CXXFLAGS_$(basetarget).o)
$(CXXFLAGS_$(obj))
tools/build/Makefile.build:quiet_cmd_cxx_o_c = CXX      $@
tools/build/Makefile.build:      cmd_cxx_o_c = $(CXX) $(cxx_flags) -c -o $@ $<
tools/build/Makefile.feature:  feature-$(1) := $(shell $(MAKE)
OUTPUT=$(OUTPUT_FEATURES) CC="$(CC)" CXX="$(CXX)"
CFLAGS="$(EXTRA_CFLAGS) $(FEATURE_CHECK_CFLAGS-$(1))"
CXXFLAGS="$(EXTRA_CXXFLAGS) $(FEATURE_CHECK_CXXFLAGS-$(1))"
LDFLAGS="$(LDFLAGS) $(FEATURE_CHECK_LDFLAGS-$(1))" -C $(feature_dir)
$(OUTPUT_FEATURES)test-$1.bin >/dev/nu
ll 2>/dev/null && echo 1 || echo 0)
tools/build/feature/Makefile:__BUILDXX = $(CXX) $(CXXFLAGS) -MD -Wall
-Werror -o $@ $(patsubst %.bin,%.cpp,$(@F)) $(LDFLAGS)
...
tools/perf/Makefile.config:USE_CXX = 0
tools/perf/Makefile.config:        CXXFLAGS +=
-DHAVE_LIBCLANGLLVM_SUPPORT -I$(shell $(LLVM_CONFIG) --includedir)
tools/perf/Makefile.config:        $(call detected,CONFIG_CXX)
tools/perf/Makefile.config:     USE_CXX = 1
tools/perf/Makefile.perf:export srctree OUTPUT RM CC CXX LD AR CFLAGS
CXXFLAGS V BISON FLEX AWK
tools/perf/Makefile.perf:ifeq ($(USE_CXX), 1)
tools/perf/util/Build:perf-$(CONFIG_CXX) += c++/
...
tools/scripts/Makefile.include:$(call allow-override,CXX,$(CROSS_COMPILE)g++)
...
tools/testing/selftests/bpf/Makefile:CXX ?= $(CROSS_COMPILE)g++
tools/testing/selftests/bpf/Makefile:   $(call msg,CXX,,$@)
tools/testing/selftests/bpf/Makefile:   $(Q)$(CXX) $(CFLAGS) $^ $(LDLIBS) -o $@

The problem is if you pass LLVM=1 there is no clang(++) assigned to
CXX automagically.

[2] says:

LLVM has substitutes for GNU binutils utilities. Kbuild supports
LLVM=1 to enable them.

make LLVM=1
They can be enabled individually. The full list of the parameters:

make CC=clang LD=ld.lld AR=llvm-ar NM=llvm-nm STRIP=llvm-strip \
  OBJCOPY=llvm-objcopy OBJDUMP=llvm-objdump READELF=llvm-readelf \
  HOSTCC=clang HOSTCXX=clang++ HOSTAR=llvm-ar HOSTLD=ld.lld

[ EndOfQuote  ]

So you need to pass CXX=clang++ manually when playing in tools directory:

MAKE="make V=1
MAKE_OPTS="HOSTCC=clang HOSTCXX=clang++ HOSTLD=ld.lld CC=clang
CXX=clang++ LD=ld.lld LLVM=1 LLVM_IAS=1"
MAKE_OPTS="MAKE_OPTS $PAHOLE=/opt/pahole/bin/pahole"

$ LC_ALL=C $MAKE $MAKE_OPTS -C tools/testing/selftests/bpf/ clean
$ LC_ALL=C $MAKE $MAKE_OPTS -C tools/testing/selftests/bpf/

Unsure, if tools needs a special treatment in things of CXX or LLVM=1
needs to be enhanced with CCX=clang++.
If we have HOSTCXX why not have a CXX in toplevel Makefile?

In "tools: Factor Clang, LLC and LLVM utils definitions" (see [3]) I
did some factor-ing.

For the records: Here Linus Git is my base.

Ideas?

Thanks.

Regards,
- Sedat -

P.S.: Just a small note: I know there is less usage of CXX code in the
linux-kernel.

[1] https://lore.kernel.org/bpf/CA+icZUWh6YOkCKG72SndqUbQNwG+iottO4=cPyRRVjaHD2=0qw@mail.gmail.com/T/#m22907f838d2d27be24e8959a53473a62f21cecea
[2] https://www.kernel.org/doc/html/latest/kbuild/llvm.html#llvm-utilities
[3] https://git.kernel.org/linus/211a741cd3e124bffdc13ee82e7e65f204e53f60