If x is not 0, __ffs(x) is equivalent to:
  (unsigned long)__builtin_ctzl(x)
And if x is not ~0UL, ffz(x) is equivalent to:
  (unsigned long)__builtin_ctzl(~x)
Because __builting_ctzl() returns an int, a cast to (unsigned long) is
necessary to avoid potential warnings on implicit casts.

Concerning the edge cases, __builtin_ctzl(0) is always undefined,
whereas __ffs(0) and ffz(~0UL) may or may not be defined, depending on
the processor. Regardless, for both functions, developers are asked to
check against 0 or ~0UL so replacing __ffs() or ffz() by
__builting_ctzl() is safe.

For x86_64, the current __ffs() and ffz() implementations do not
produce optimized code when called with a constant expression. On the
contrary, the __builtin_ctzl() folds into a single instruction.

However, for non constant expressions, the __ffs() and ffz() asm
versions of the kernel remains slightly better than the code produced
by GCC (it produces a useless instruction to clear eax).

Use __builtin_constant_p() to select between the kernel's
__ffs()/ffz() and the __builtin_ctzl() depending on whether the
argument is constant or not.

** Statistics **

On a allyesconfig, before...:

  $ objdump -d vmlinux.o | grep tzcnt | wc -l
  3607

...and after:

  $ objdump -d vmlinux.o | grep tzcnt | wc -l
  2600

So, roughly 27.9% of the calls to either __ffs() or ffz() were using
constant expressions and could be optimized out.

(tests done on linux v5.18-rc5 x86_64 using GCC 11.2.1)

Note: on x86_64, the BSF instruction produces TZCNT when used with the
REP prefix (which explain the use of `grep tzcnt' instead of `grep bsf'
in above benchmark). c.f. [1]

[1] e26a44a2 ("x86: Use REP BSF unconditionally")

  [ bp: Massage commit message. ]
Signed-off-by: Vincent Mailhol <mailhol.vincent@wanadoo.fr>
Signed-off-by: Borislav Petkov <bp@suse.de>
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
Reviewed-by: Yury Norov <yury.norov@gmail.com>
Link: https://lore.kernel.org/r/20220511160319.1045812-1-mailhol.vincent@wanadoo.fr