In file included from /usr/src/sys/sys/libkern.h:51,
                 from f_ffs.c:33:
/usr/src/sys/sys/systm.h:216:5: warning: conflicting types for built-in function 'log' [-Wbuiltin-declaration-mismatch]
 int log (int, const char *, ...) __printflike(2, 3);
     ^~~
f_ffs.c:39:1: error: redefinition of 'ffs'
 ffs(int mask)
 ^~~
In file included from /usr/src/sys/sys/systm.h:50,
                 from /usr/src/sys/sys/libkern.h:51,
                 from f_ffs.c:33:
kinc/machine/cpufunc.h:269:1: note: previous definition of 'ffs' was here
 ffs(int mask)
 ^~~