Re: [PATCH bpf-next v2] selftests/bpf: Drop the need for LLVM's llc

[Yonghong Song <yhs@fb.com>]

On 12/10/20 11:41 AM, Andrew Delgadillo wrote:
> LLC is meant for compiler development and debugging. Consequently, it
> exposes many low level options about its backend. To avoid future bugs
> introduced by using the raw LLC tool, use clang directly so that all
> appropriate options are passed to the back end.
> 
> Additionally, simplify the Makefile by removing the
> CLANG_NATIVE_BPF_BUILD_RULE as it is not being use, stop passing
> dwarfris attr since elfutils/libdw now supports the bpf backend (which
> should work with any recent pahole), and stop passing alu32 since
> -mcpu=v3 implies alu32.
> 
> Signed-off-by: Andrew Delgadillo <adelg@google.com>
> ---
> Changes since v1:
> * do not pass +dwarfris
> * do not pass +alu32 when using -mcpu=v3
> ---
>   tools/testing/selftests/bpf/Makefile | 19 +++----------------
>   1 file changed, 3 insertions(+), 16 deletions(-)
> 
> diff --git a/tools/testing/selftests/bpf/Makefile b/tools/testing/selftests/bpf/Makefile
> index 944ae17a39ed..a96f63dfd8dc 100644
> --- a/tools/testing/selftests/bpf/Makefile
> +++ b/tools/testing/selftests/bpf/Makefile
> @@ -19,7 +19,6 @@ ifneq ($(wildcard $(GENHDR)),)
>   endif
>   
>   CLANG		?= clang
> -LLC		?= llc
>   LLVM_OBJCOPY	?= llvm-objcopy
>   BPF_GCC		?= $(shell command -v bpf-gcc;)
>   SAN_CFLAGS	?=
> @@ -256,24 +255,13 @@ $(OUTPUT)/flow_dissector_load.o: flow_dissector_load.h
>   # $3 - CFLAGS
>   # $4 - LDFLAGS

You can remove TRUNNER_BPF_LDFLAGS completely, so we won't have $4 here.

>   define CLANG_BPF_BUILD_RULE
> -	$(call msg,CLNG-LLC,$(TRUNNER_BINARY),$2)
> -	$(Q)($(CLANG) $3 -O2 -target bpf -emit-llvm			\
> -		-c $1 -o - || echo "BPF obj compilation failed") | 	\
> -	$(LLC) -mattr=dwarfris -march=bpf -mcpu=v3 $4 -filetype=obj -o $2
> +	$(call msg,CLNG-BPF,$(TRUNNER_BINARY),$2)
> +	$(Q)$(CLANG) $3 -O2 -target bpf -c $1 -o $2 -mcpu=v3 $4

and $4 here.

>   endef
>   # Similar to CLANG_BPF_BUILD_RULE, but with disabled alu32
>   define CLANG_NOALU32_BPF_BUILD_RULE
> -	$(call msg,CLNG-LLC,$(TRUNNER_BINARY),$2)
> -	$(Q)($(CLANG) $3 -O2 -target bpf -emit-llvm			\
> -		-c $1 -o - || echo "BPF obj compilation failed") | 	\
> -	$(LLC) -march=bpf -mcpu=v2 $4 -filetype=obj -o $2
> -endef
> -# Similar to CLANG_BPF_BUILD_RULE, but using native Clang and bpf LLC
> -define CLANG_NATIVE_BPF_BUILD_RULE
>   	$(call msg,CLNG-BPF,$(TRUNNER_BINARY),$2)
> -	$(Q)($(CLANG) $3 -O2 -emit-llvm					\
> -		-c $1 -o - || echo "BPF obj compilation failed") | 	\
> -	$(LLC) -march=bpf -mcpu=v3 $4 -filetype=obj -o $2
> +	$(Q)$(CLANG) $3 -O2 -target bpf -c $1 -o $2 -mcpu=v2 $4

and $4 here.

>   endef
>   # Build BPF object using GCC
>   define GCC_BPF_BUILD_RULE
> @@ -402,7 +390,6 @@ TRUNNER_EXTRA_FILES := $(OUTPUT)/urandom_read $(OUTPUT)/bpf_testmod.ko	\
>   		       $(wildcard progs/btf_dump_test_case_*.c)
>   TRUNNER_BPF_BUILD_RULE := CLANG_BPF_BUILD_RULE
>   TRUNNER_BPF_CFLAGS := $(BPF_CFLAGS) $(CLANG_CFLAGS)
> -TRUNNER_BPF_LDFLAGS := -mattr=+alu32
>   $(eval $(call DEFINE_TEST_RUNNER,test_progs))
>   
>   # Define test_progs-no_alu32 test runner.
>