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

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>

Author: hcahca

arch/s390/Kconfig

@@ -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

arch/s390/include/asm/bitops.h

@@ -122,6 +122,8 @@ static inline bool test_bit_inv(unsigned long nr,
 	return test_bit(nr ^ (BITS_PER_LONG - 1), ptr);
 }
 
+#ifndef CONFIG_CC_HAS_BUILTIN_FFS
+
 /**
  * __flogr - find leftmost one
  * @word - The word to search
@@ -194,6 +196,12 @@ static __always_inline __flatten int ffs(int word)
 	return BITS_PER_LONG - __flogr(-val & val);
 }
 
+#else /* CONFIG_CC_HAS_BUILTIN_FFS */
+
+#include <asm-generic/bitops/builtin-ffs.h>
+
+#endif /* CONFIG_CC_HAS_BUILTIN_FFS */
+
 #include <asm-generic/bitops/builtin-__ffs.h>
 #include <asm-generic/bitops/ffz.h>
 #include <asm-generic/bitops/builtin-__fls.h>