On Sun, Apr 4, 2021 at 8:00 AM Sedat Dilek <sedat.dilek@xxxxxxxxx> 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@xxxxxxxxxxxxxx/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
