On Fri, Nov 10, 2023 at 11:37 AM Yonghong Song <yonghong.song@xxxxxxxxx> 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@xxxxxxxxx> > --- > tools/testing/selftests/bpf/progs/pyperf180.c | 22 +++++++++++++++++++ > 1 file changed, 22 insertions(+) > > diff --git a/tools/testing/selftests/bpf/progs/pyperf180.c b/tools/testing/selftests/bpf/progs/pyperf180.c > index c39f559d3100..42c4a8b62e36 100644 > --- a/tools/testing/selftests/bpf/progs/pyperf180.c > +++ b/tools/testing/selftests/bpf/progs/pyperf180.c > @@ -1,4 +1,26 @@ > // SPDX-License-Identifier: GPL-2.0 > // Copyright (c) 2019 Facebook > #define STACK_MAX_LEN 180 > + > +/* llvm upstream commit at clang18 > + * https://github.com/llvm/llvm-project/commit/1a2e77cf9e11dbf56b5720c607313a566eebb16e > + * changed inlining behavior and caused compilation failure as some branch > + * target distance exceeded 16bit representation which is the maximum for > + * cpu v1/v2/v3. Macro __BPF_CPU_VERSION__ is later implemented in clang18 > + * to specify which cpu version is used for compilation. So a smaller > + * unroll_count can be set if __BPF_CPU_VERSION__ is less than 4, which > + * reduced some branch target distances and resolved the compilation failure. > + * > + * To capture the case where a developer/ci uses clang18 but the corresponding > + * repo checkpoint does not have __BPF_CPU_VERSION__, a smaller unroll_count > + * will be set as well to prevent potential compilation failures. > + */ > +#ifdef __BPF_CPU_VERSION__ > +#if __BPF_CPU_VERSION__ < 4 > +#define UNROLL_COUNT 90 > +#endif > +#elif __clang_major__ == 18 > +#define UNROLL_COUNT 90 > +#endif > + can it be written as one if? #if (defined(__BPF_CPU_VERSION__) && __BPF_CPU_VERSION__ < 4) || __clang_major >= 18 ? > #include "pyperf.h" > -- > 2.34.1 > >
