csky: bitops: Use the compiler builtin for __fls()

The csky __fls() is a C wrapper, fls(x) - 1, around the ff1 asm
volatile in fls(), so the compiler can neither fold it for a constant
argument nor merge repeated calls. GCC implements __builtin_clzl()
with the same ff1 instruction for CK807, CK810 and CK860; CK610
(abiv1) cannot be built with upstream GCC.

Use the asm-generic builtin __fls() instead. ffs(), __ffs() and fls()
keep their asm: GCC has no ctz pattern for csky, so builtin ffs() and
__ffs() would become calls to __ffssi2() and __ctzsi2(), and a builtin
fls() needs a test for zero that ff1 makes unnecessary by returning 32
for 0.

__fls() now returns unsigned int instead of unsigned long, like the
asm-generic version; both are 32 bits wide on csky. __fls(0) remains
undefined.

With GCC 9 to 16, the text of every defconfig-based vmlinux tested
shrinks by 128 to 320 bytes.

[Yury: below more details from the discussion [1]]

scripts/bloat-o-meter -t on csky defconfig (CK807) vmlinux, GCC 16.2:

add/remove: 0/0 grow/shrink: 0/3 up/down: 0/-196 (-196)
Function                                     old     new   delta
fib6_add                                    1634    1630      -4
build_sched_domains                         3948    3916     -32
skb_try_coalesce                            1766    1606    -160
Total: Before=4621444, After=4621248, chg -0.00%

Run-time code generation: 4 instructions before and after with
GCC 9 to 16; with GCC 11 to 16 both are this sequence:

        unsigned long f(unsigned long x) { return __fls(x); }

        ff1     r0, r0
        movi    r3, 31
        subu    r0, r3, r0
        rts

Constant argument. PP_DMA_INDEX_SHIFT in include/linux/mm.h is
1 + __fls(PP_SIGNATURE - POISON_POINTER_DELTA), which is
1 + __fls(0x40):

        unsigned int f(void) { return 1 + __fls(0x40); }

        before:
        movi    r3, 64
        ff1     r3, r3
        movi    r0, 32
        subu    r0, r3
        rts

        after:
        movi    r0, 7
        rts

Suggested-by: Arnd Bergmann <arnd@arndb.de>
Link: https://lore.kernel.org/all/affe7da1-41aa-4710-bee0-5e3c2b8a8dfc@app.fastmail.com/
Link: https://lore.kernel.org/all/20261027062319.1700461-1-shingekinocore@gmail.com/ [1]
Signed-off-by: Tzu-Ching Yang <shingekinocore@gmail.com>
Signed-off-by: Yury Norov <ynorov@nvidia.com>

0 files changed, 0 insertions, 0 deletions