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selftests/bpf: Fix pyperf180 compilation failure with clang18 

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] 1a2e77cf9e

Signed-off-by: Yonghong Song <yonghong.song@linux.dev>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Tested-by: Alan Maguire <alan.maguire@oracle.com>
Link: https://lore.kernel.org/bpf/20231110193644.3130906-1-yonghong.song@linux.dev
This commit is contained in:
Yonghong Song 2023-11-10 11:36:44 -08:00 committed by Andrii Nakryiko
parent b8e3a87a62
commit 100888fb6d
1 changed files with 22 additions and 0 deletions
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22
tools/testing/selftests/bpf/progs/pyperf180.c
View File

@ -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
#include "pyperf.h"