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

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

[Yonghong Song]

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>
---
 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
+
 #include "pyperf.h"
-- 
2.34.1

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

[Andrii Nakryiko]

On Fri, Nov 10, 2023 at 11:37 AM Yonghong Song <yonghong.song@linux.dev> 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>
> ---
>  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
>
>

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

[Alan Maguire]

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>

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

[Yonghong Song]

On 11/10/23 11:45 AM, Andrii Nakryiko wrote:
> On Fri, Nov 10, 2023 at 11:37 AM Yonghong Song <yonghong.song@linux.dev> 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>
>> ---
>>   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
>
>
> ?

This won't work. For example, using latest upstream clang18, __BPF_CPU_VERSION__ does exist,
and user use cpu v4, in this case we do not want to do unrolling but with the above:
   
(defined(__BPF_CPU_VERSION__) && __BPF_CPU_VERSION__ < 4) is false
__clang_major >= 18 is true

so we do unrolling but we do not need to do since user uses cpu v4.



>
>>   #include "pyperf.h"
>> --
>> 2.34.1
>>
>>

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

[patchwork-bot+netdevbpf]

Hello:

This patch was applied to bpf/bpf-next.git (master)
by Andrii Nakryiko <andrii@kernel.org>:

On Fri, 10 Nov 2023 11:36:44 -0800 you 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
>     ...
> 
> [...]

Here is the summary with links:
  - [bpf-next,v2] selftests/bpf: Fix pyperf180 compilation failure with clang18
    https://git.kernel.org/bpf/bpf-next/c/100888fb6d8a

You are awesome, thank you!
-- 
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