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s390/bitops: Switch to generic ffs() if supported by compiler

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Use generic ffs() / __builtin_ffs() if supported by the compiler. GCC 16
will have support for __builtin_ffs().
See gcc commit f50cff9766c5 ("s390: Implement clz and ctz for SI mode").

In the distant future when GCC 16 becomes the minimum supported version,
this allows to get rid of the flogr inline assembly.

Kernel image size is reduced by ~500 bytes (gcc 16 beta + defconfig).

Acked-by: Alexander Gordeev <agordeev@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Alexander Gordeev <agordeev@linux.ibm.com>
This commit is contained in:
Heiko Carstens
2025-09-22 13:40:20 +02:00
committed by Alexander Gordeev
parent 7b80a23c0e
commit 6c4e0cb3d8
2 changed files with 15 additions and 0 deletions
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7
arch/s390/Kconfig
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@@ -49,6 +49,13 @@ config KASAN_SHADOW_OFFSET
depends on KASAN
default 0x1C000000000000
config CC_HAS_BUILTIN_FFS
def_bool !(CC_IS_GCC && GCC_VERSION < 160000)
help
GCC versions before 16.0.0 generate library calls to ffs()
for __builtin_ffs() even when __has_builtin(__builtin_ffs)
is true.
config CC_ASM_FLAG_OUTPUT_BROKEN
def_bool CC_IS_GCC && GCC_VERSION < 140200
help