Q: clang flag for target bpf?
-----------------------------
-Q: In some cases clang flag ``-target bpf`` is used but in other cases the
+Q: In some cases clang flag ``--target=bpf`` is used but in other cases the
default clang target, which matches the underlying architecture, is used.
What is the difference and when I should use which?
A: Although LLVM IR generation and optimization try to stay architecture
-independent, ``-
target <arch>`` still has some impact on generated code:
+independent, ``-
-target=<arch>`` still has some impact on generated code:
- BPF program may recursively include header file(s) with file scope
inline assembly codes. The default target can handle this well,
The clang option ``-fno-jump-tables`` can be used to disable
switch table generation.
-- For clang ``-target bpf``, it is guaranteed that pointer or long /
+- For clang ``--target=bpf``, it is guaranteed that pointer or long /
unsigned long types will always have a width of 64 bit, no matter
whether underlying clang binary or default target (or kernel) is
32 bit. However, when native clang target is used, then it will
while the BPF LLVM back end still operates in 64 bit. The native
target is mostly needed in tracing for the case of walking ``pt_regs``
or other kernel structures where CPU's register width matters.
- Otherwise, ``clang -target bpf`` is generally recommended.
+ Otherwise, ``clang --target=bpf`` is generally recommended.
You should use default target when:
into these structures is verified by the BPF verifier and may result
in verification failures if the native architecture is not aligned with
the BPF architecture, e.g. 64-bit. An example of this is
- BPF_PROG_TYPE_SK_MSG require ``-target bpf``
+ BPF_PROG_TYPE_SK_MSG require ``--target=bpf``
.. Links
} g2;
int main() { return 0; }
int test() { return 0; }
- -bash-4.4$ clang -c -g -O2 -target bpf t2.c
+ -bash-4.4$ clang -c -g -O2 --target=bpf t2.c
-bash-4.4$ readelf -S t2.o
......
[ 8] .BTF PROGBITS 0000000000000000 00000247
[10] .rel.BTF.ext REL 0000000000000000 000007e0
0000000000000040 0000000000000010 16 9 8
......
- -bash-4.4$ clang -S -g -O2 -target bpf t2.c
+ -bash-4.4$ clang -S -g -O2 --target=bpf t2.c
-bash-4.4$ cat t2.s
......
.section .BTF,"",@progbits
return g1 + g2 + l1 + l2;
}
-Compiled with ``clang -target bpf -O2 -c test.c``, the following is
+Compiled with ``clang --target=bpf -O2 -c test.c``, the following is
the code with ``llvm-objdump -dr test.o``::
0: 18 01 00 00 00 00 00 00 00 00 00 00 00 00 00 00 r1 = 0 ll
return gfunc(a, b) + lfunc(a, b) + global;
}
-Compiled with ``clang -target bpf -O2 -c test.c``, we will have
+Compiled with ``clang --target=bpf -O2 -c test.c``, we will have
following code with `llvm-objdump -dr test.o``::
Disassembly of section .text:
int global() { return 0; }
struct t { void *g; } gbl = { global };
-Compiled with ``clang -target bpf -O2 -g -c test.c``, we will see a
+Compiled with ``clang --target=bpf -O2 -g -c test.c``, we will see a
relocation below in ``.data`` section with command
``llvm-readelf -r test.o``::
$(OUTPUT)/entrypoints.bpf.o: entrypoints.bpf.c $(OUTPUT)/vmlinux.h $(BPFOBJ) | $(OUTPUT)
$(call msg,BPF,$@)
- $(Q)$(CLANG) -g -O2 -target bpf $(INCLUDES) \
+ $(Q)$(CLANG) -g -O2 --target=bpf $(INCLUDES) \
-c $(filter %.c,$^) -o $@ && \
$(LLVM_STRIP) -g $@
$(OUTPUT)/%/iterators.bpf.o: iterators.bpf.c $(BPFOBJ) | $(OUTPUT)
$(call msg,BPF,$@)
$(Q)mkdir -p $(@D)
- $(Q)$(CLANG) -g -O2 -target bpf -m$* $(INCLUDES) \
+ $(Q)$(CLANG) -g -O2 --target=bpf -m$* $(INCLUDES) \
-c $(filter %.c,$^) -o $@ && \
$(LLVM_STRIP) -g $@
BTF_PAHOLE_PROBE := $(shell $(BTF_PAHOLE) --help 2>&1 | grep BTF)
BTF_OBJCOPY_PROBE := $(shell $(LLVM_OBJCOPY) --help 2>&1 | grep -i 'usage.*llvm')
BTF_LLVM_PROBE := $(shell echo "int main() { return 0; }" | \
- $(CLANG) -target bpf -O2 -g -c -x c - -o ./llvm_btf_verify.o; \
+ $(CLANG) --target=bpf -O2 -g -c -x c - -o ./llvm_btf_verify.o; \
$(LLVM_READELF) -S ./llvm_btf_verify.o | grep BTF; \
/bin/rm -f ./llvm_btf_verify.o)
clean-files += vmlinux.h
# Get Clang's default includes on this system, as opposed to those seen by
-# '-target bpf'. This fixes "missing" files on some architectures/distros,
+# '--target=bpf'. This fixes "missing" files on some architectures/distros,
# such as asm/byteorder.h, asm/socket.h, asm/sockios.h, sys/cdefs.h etc.
#
# Use '-idirafter': Don't interfere with include mechanics except where the
$(obj)/%.bpf.o: $(src)/%.bpf.c $(obj)/vmlinux.h $(src)/xdp_sample.bpf.h $(src)/xdp_sample_shared.h
@echo " CLANG-BPF " $@
- $(Q)$(CLANG) -g -O2 -target bpf -D__TARGET_ARCH_$(SRCARCH) \
+ $(Q)$(CLANG) -g -O2 --target=bpf -D__TARGET_ARCH_$(SRCARCH) \
-Wno-compare-distinct-pointer-types -I$(srctree)/include \
-I$(srctree)/samples/bpf -I$(srctree)/tools/include \
-I$(LIBBPF_INCLUDE) $(CLANG_SYS_INCLUDES) \
-/* dummy .h to trick /usr/include/features.h to work with 'clang -target bpf' */
+/* dummy .h to trick /usr/include/features.h to work with 'clang --target=bpf' */
SRC_MAC=$(lookup_mac $VETH0)
DST_IFINDEX=$(cat /sys/class/net/$VETH0/ifindex)
-CLANG_OPTS="-O2 -target bpf -I ../include/"
+CLANG_OPTS="-O2 --target=bpf -I ../include/"
CLANG_OPTS+=" -DSRC_MAC=$SRC_MAC -DDST_MAC=$DST_MAC -DDST_IFINDEX=$DST_IFINDEX"
clang $CLANG_OPTS -c $PROG_SRC -o $BPF_PROG
BTF_PAHOLE_PROBE := $(shell $(BTF_PAHOLE) --help 2>&1 | grep BTF)
BTF_OBJCOPY_PROBE := $(shell $(LLVM_OBJCOPY) --help 2>&1 | grep -i 'usage.*llvm')
BTF_LLVM_PROBE := $(shell echo "int main() { return 0; }" | \
- $(CLANG) -target bpf -O2 -g -c -x c - -o ./llvm_btf_verify.o; \
+ $(CLANG) --target=bpf -O2 -g -c -x c - -o ./llvm_btf_verify.o; \
$(LLVM_READELF) -S ./llvm_btf_verify.o | grep BTF; \
/bin/rm -f ./llvm_btf_verify.o)
clean-files += vmlinux.h
# Get Clang's default includes on this system, as opposed to those seen by
-# '-target bpf'. This fixes "missing" files on some architectures/distros,
+# '--target=bpf'. This fixes "missing" files on some architectures/distros,
# such as asm/byteorder.h, asm/socket.h, asm/sockios.h, sys/cdefs.h etc.
#
# Use '-idirafter': Don't interfere with include mechanics except where the
$(obj)/%.bpf.o: $(src)/%.bpf.c $(EXTRA_BPF_HEADERS_SRC) $(obj)/vmlinux.h
@echo " CLANG-BPF " $@
- $(Q)$(CLANG) -g -O2 -target bpf -D__TARGET_ARCH_$(SRCARCH) \
+ $(Q)$(CLANG) -g -O2 --target=bpf -D__TARGET_ARCH_$(SRCARCH) \
-Wno-compare-distinct-pointer-types -I$(srctree)/include \
-I$(srctree)/samples/bpf -I$(srctree)/tools/include \
-I$(LIBBPF_INCLUDE) $(CLANG_SYS_INCLUDES) \
This is example BPF application with two BPF programs and a mix of BPF maps
and global variables. Source code is split across two source code files.
-**$ clang -target bpf -g example1.bpf.c -o example1.bpf.o**
+**$ clang --target=bpf -g example1.bpf.c -o example1.bpf.o**
-**$ clang -target bpf -g example2.bpf.c -o example2.bpf.o**
+**$ clang --target=bpf -g example2.bpf.c -o example2.bpf.o**
**$ bpftool gen object example.bpf.o example1.bpf.o example2.bpf.o**
-I$(srctree)/tools/include/uapi/ \
-I$(LIBBPF_BOOTSTRAP_INCLUDE) \
-g -O2 -Wall -fno-stack-protector \
- -target bpf -c $< -o $@
+ --target=bpf -c $< -o $@
$(Q)$(LLVM_STRIP) -g $@
$(OUTPUT)%.skel.h: $(OUTPUT)%.bpf.o $(BPFTOOL_BOOTSTRAP)
$(QUIET_GEN)$(BPFTOOL) gen skeleton $< > $@
$(OUTPUT)/%.bpf.o: %.bpf.c $(BPFOBJ) | $(OUTPUT)
- $(QUIET_GEN)$(CLANG) -g -O2 -target bpf $(INCLUDES) \
+ $(QUIET_GEN)$(CLANG) -g -O2 --target=bpf $(INCLUDES) \
-c $(filter %.c,$^) -o $@ && \
$(LLVM_STRIP) -g $@
$(BUILD) -lzstd
$(OUTPUT)test-clang-bpf-co-re.bin:
- $(CLANG) -S -g -target bpf -o - $(patsubst %.bin,%.c,$(@F)) | \
+ $(CLANG) -S -g --target=bpf -o - $(patsubst %.bin,%.c,$(@F)) | \
grep BTF_KIND_VAR
$(OUTPUT)test-file-handle.bin:
OUTPUT=$(HOST_BUILD_DIR)/resolve_btfids/ BPFOBJ=$(HOST_BPFOBJ)
# Get Clang's default includes on this system, as opposed to those seen by
-# '-target bpf'. This fixes "missing" files on some architectures/distros,
+# '--target=bpf'. This fixes "missing" files on some architectures/distros,
# such as asm/byteorder.h, asm/socket.h, asm/sockios.h, sys/cdefs.h etc.
#
# Use '-idirafter': Don't interfere with include mechanics except where the
# $3 - CFLAGS
define CLANG_BPF_BUILD_RULE
$(call msg,CLNG-BPF,$(TRUNNER_BINARY),$2)
- $(Q)$(CLANG) $3 -O2 -target bpf -c $1 -mcpu=v3 -o $2
+ $(Q)$(CLANG) $3 -O2 --target=bpf -c $1 -mcpu=v3 -o $2
endef
# Similar to CLANG_BPF_BUILD_RULE, but with disabled alu32
define CLANG_NOALU32_BPF_BUILD_RULE
$(call msg,CLNG-BPF,$(TRUNNER_BINARY),$2)
- $(Q)$(CLANG) $3 -O2 -target bpf -c $1 -mcpu=v2 -o $2
+ $(Q)$(CLANG) $3 -O2 --target=bpf -c $1 -mcpu=v2 -o $2
endef
# Build BPF object using GCC
define GCC_BPF_BUILD_RULE
-/* dummy .h to trick /usr/include/features.h to work with 'clang -target bpf' */
+/* dummy .h to trick /usr/include/features.h to work with 'clang --target=bpf' */
OUTPUT=$(HOST_BUILD_DIR)/resolve_btfids/ BPFOBJ=$(HOST_BPFOBJ)
# Get Clang's default includes on this system, as opposed to those seen by
-# '-target bpf'. This fixes "missing" files on some architectures/distros,
+# '--target=bpf'. This fixes "missing" files on some architectures/distros,
# such as asm/byteorder.h, asm/socket.h, asm/sockios.h, sys/cdefs.h etc.
#
# Use '-idirafter': Don't interfere with include mechanics except where the
# $3 - CFLAGS
define CLANG_BPF_BUILD_RULE
$(call msg,CLNG-BPF,$(TRUNNER_BINARY),$2)
- $(Q)$(CLANG) $3 -O2 -target bpf -c $1 -mcpu=v3 -o $2
+ $(Q)$(CLANG) $3 -O2 --target=bpf -c $1 -mcpu=v3 -o $2
endef
# Similar to CLANG_BPF_BUILD_RULE, but with disabled alu32
define CLANG_NOALU32_BPF_BUILD_RULE
$(call msg,CLNG-BPF,$(TRUNNER_BINARY),$2)
- $(Q)$(CLANG) $3 -O2 -target bpf -c $1 -mcpu=v2 -o $2
+ $(Q)$(CLANG) $3 -O2 --target=bpf -c $1 -mcpu=v2 -o $2
endef
# Build BPF object using GCC
define GCC_BPF_BUILD_RULE
mkdir -p $@
# Get Clang's default includes on this system, as opposed to those seen by
-# '-target bpf'. This fixes "missing" files on some architectures/distros,
+# '--target=bpf'. This fixes "missing" files on some architectures/distros,
# such as asm/byteorder.h, asm/socket.h, asm/sockios.h, sys/cdefs.h etc.
#
# Use '-idirafter': Don't interfere with include mechanics except where the
CLANG_SYS_INCLUDES = $(call get_sys_includes,$(CLANG),$(CLANG_TARGET_ARCH))
$(OUTPUT)/nat6to4.o: nat6to4.c $(BPFOBJ) | $(MAKE_DIRS)
- $(CLANG) -O2 -target bpf -c $< $(CCINCLUDE) $(CLANG_SYS_INCLUDES) -o $@
+ $(CLANG) -O2 --target=bpf -c $< $(CCINCLUDE) $(CLANG_SYS_INCLUDES) -o $@
$(BPFOBJ): $(wildcard $(BPFDIR)/*.[ch] $(BPFDIR)/Makefile) \
$(APIDIR)/linux/bpf.h \
$(OUTPUT)/%.o: %.c
$(CLANG) $(CLANG_FLAGS) \
- -O2 -target bpf -emit-llvm -c $< -o - | \
+ -O2 --target=bpf -emit-llvm -c $< -o - | \
$(LLC) -march=bpf -mcpu=$(CPU) $(LLC_FLAGS) -filetype=obj -o $@
TEST_PROGS += ./tdc.sh
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