Re: [PATCH bpf-next v2] selftests/bpf: Fix pyperf180 compilation failure with clang18

[Alan Maguire <alan.maguire@oracle.com>]

On 10/11/2023 19:36, Yonghong Song wrote:
> With latest clang18 (main branch of llvm-project repo), when building bpf selftests,
>     [~/work/bpf-next (master)]$ make -C tools/testing/selftests/bpf LLVM=1 -j
> 
> The following compilation error happens:
>     fatal error: error in backend: Branch target out of insn range
>     ...
>     Stack dump:
>     0.      Program arguments: clang -g -Wall -Werror -D__TARGET_ARCH_x86 -mlittle-endian
>       -I/home/yhs/work/bpf-next/tools/testing/selftests/bpf/tools/include
>       -I/home/yhs/work/bpf-next/tools/testing/selftests/bpf -I/home/yhs/work/bpf-next/tools/include/uapi
>       -I/home/yhs/work/bpf-next/tools/testing/selftests/usr/include -idirafter
>       /home/yhs/work/llvm-project/llvm/build.18/install/lib/clang/18/include -idirafter /usr/local/include
>       -idirafter /usr/include -Wno-compare-distinct-pointer-types -DENABLE_ATOMICS_TESTS -O2 --target=bpf
>       -c progs/pyperf180.c -mcpu=v3 -o /home/yhs/work/bpf-next/tools/testing/selftests/bpf/pyperf180.bpf.o
>     1.      <eof> parser at end of file
>     2.      Code generation
>     ...
> 
> The compilation failure only happens to cpu=v2 and cpu=v3. cpu=v4 is okay
> since cpu=v4 supports 32-bit branch target offset.
> 
> The above failure is due to upstream llvm patch [1] where some inlining behavior
> are changed in clang18.
> 
> To workaround the issue, previously all 180 loop iterations are fully unrolled.
> The bpf macro __BPF_CPU_VERSION__ (implemented in clang18 recently) is used to avoid
> unrolling changes if cpu=v4. If __BPF_CPU_VERSION__ is not available and the
> compiler is clang18, the unrollng amount is unconditionally reduced.
> 
>   [1] https://github.com/llvm/llvm-project/commit/1a2e77cf9e11dbf56b5720c607313a566eebb16e
> 
> Signed-off-by: Yonghong Song <yonghong.song@linux.dev>

Fixes the issue for me;

Tested-by: Alan Maguire <alan.maguire@oracle.com>