On Wed, Jul 28, 2021 at 01:27:54AM +0800, kernel test robot wrote:
> tree:   https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git master
> head:   ff1176468d368232b684f75e82563369208bc371
> commit: 7b034d9c1b08b3d06ad712283c1115a7fe39e354 s390/boot: add zstd support
> date:   6 weeks ago
> config: s390-randconfig-r026-20210727 (attached as .config)
...
>    s390-linux-ld: arch/s390/boot/compressed/decompressor.o: in function `HUF_decompress1X2_usingDTable_internal':
> >> decompressor.c:(.text+0x2152): undefined reference to `__clzdi2'

haaaahh.... Not very nice, but that would work:

Vasily Gorbik (1):
  s390/boot: fix zstd build for -march=z900

 arch/s390/boot/compressed/Makefile  | 1 +
 arch/s390/boot/compressed/clz_ctz.c | 2 ++
 2 files changed, 3 insertions(+)
 create mode 100644 arch/s390/boot/compressed/clz_ctz.c

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2.25.4