On Thu, Nov 9, 2023 at 11:55 AM Yonghong Song <yonghong.song@xxxxxxxxx> wrote: > > > On 11/9/23 3:47 AM, Eduard Zingerman wrote: > > On Wed, 2023-11-08 at 21:30 -0800, Yonghong Song wrote: > >> With latest llvm18 (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 llvm18. > >> > >> To workaround the issue, previously all 180 loop iterations are fully unrolled. > >> Now, the fully unrolling count is changed to 90 for llvm18 and later. This reduced > >> some otherwise long branch target distance, and fixed the compilation failure. > >> > >> [1] https://github.com/llvm/llvm-project/commit/1a2e77cf9e11dbf56b5720c607313a566eebb16e > >> > >> Signed-off-by: Yonghong Song <yonghong.song@xxxxxxxxx> > > Can confirm, the issue is present on clang main w/o this patch and > > disappears after this patch. > > > > Yonghong, is there a way to keep original UNROLL_COUNT if cpuv4 is used? > > I thought about this but a little bit lazy so not giving it enough throught. > But since you mentioned this, I think adding a macro to indicate cpu version > by llvm is a good idea. This will give bpf developers some flexibility to > add new features (new cpu variant) or workaround bugs (for a particular cpu variant > but not impacting others if they are fine), etc. > > So here is the llvm patch: https://github.com/llvm/llvm-project/pull/71856 Great idea. Commented on the diff. > With the above llvm patch, the following code change should work: > > diff --git a/tools/testing/selftests/bpf/progs/pyperf180.c b/tools/testing/selftests/bpf/progs/pyperf180.c > index c39f559d3100..2473845d1ee2 100644 > --- a/tools/testing/selftests/bpf/progs/pyperf180.c > +++ b/tools/testing/selftests/bpf/progs/pyperf180.c > @@ -1,4 +1,18 @@ > // SPDX-License-Identifier: GPL-2.0 > // Copyright (c) 2019 Facebook > #define STACK_MAX_LEN 180 > + > +/* llvm upstream commit at llvm18 > + * 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 implemented in llvm18 to specify > + * which cpu version is used for compilation. So we can set a smaller > + * unroll_count if __bpf_cpu_version__ is less than 4, which reduced > + * some branch target distances and resolved the compilation failure. > + */ > +#if defined(__bpf_cpu_version__) && __bpf_cpu_version__ < 4 probably should be combined with __clang_major__ >= 18 check too. > +#define UNROLL_COUNT 90 > +#endif > + > #include "pyperf.h" > > > > > > Tested-by: Eduard Zingerman <eddyz87@xxxxxxxxx>
