linux-kernel - Re: Usage of CXX in tools directory

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Date:   Tue, 6 Apr 2021 11:39:09 -0700
From:   Nick Desaulniers <ndesaulniers@...gle.com>
To:     Sedat Dilek <sedat.dilek@...il.com>
Cc:     Masahiro Yamada <masahiroy@...nel.org>, Yonghong Song <yhs@...com>,
        Arnaldo Carvalho de Melo <arnaldo.melo@...il.com>,
        David Blaikie <dblaikie@...il.com>,
        Bill Wendling <morbo@...gle.com>,
        Nathan Chancellor <nathan@...nel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Mark Rutland <mark.rutland@....com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Ingo Molnar <mingo@...hat.com>, Jiri Olsa <jolsa@...hat.com>,
        Namhyung Kim <namhyung@...nel.org>, dwarves@...r.kernel.org,
        bpf <bpf@...r.kernel.org>, kernel-team@...com,
        Linux Kbuild mailing list <linux-kbuild@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@...nel.org>,
        Clang-Built-Linux ML <clang-built-linux@...glegroups.com>
Subject: Re: Usage of CXX in tools directory

On Sun, Apr 4, 2021 at 8:00 AM Sedat Dilek <sedat.dilek@...il.com> wrote:
>
> [ Please CC me I am not subscribed to all mailing-lists ]
> [ Please CC some more folks if you like ]
>
> 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:

Yes, CXX is not set by LLVM=1 in the top level Makefile.  CXX is not
exported by the top level Makefile.  I suspect that
tools/scripts/Mafefile.include (and possible
testing/selftests/bpf/Makefile) needs to check for LLVM=1 and set
CXX=clang++ explicitly.

>
> 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



-- 
Thanks,
~Nick Desaulniers