Re: [PATCH] csky: bitops: Use the compiler builtin for __fls()

[Tzu-Ching Yang <shingekinocore@gmail.com>]

On Fri, Oct 23, 2026 at 11:45:35AM -0400, Yury Norov wrote:
> While waiting happy, can you print the bloat-o-meter results, and a
> couple listings of the code generation, including compile- and runtime
> cases.

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%

The same with GCC 12.5:

add/remove: 0/0 grow/shrink: 7/6 up/down: 94/-330 (-236)
Function                                     old     new   delta
__netdev_alloc_skb                           304     338     +34
skb_ts_finish                                 34      54     +20
skb_copy_bits                                608     628     +20
skb_release_data                             354     362      +8
kfree_skb_list_reason                        280     286      +6
fib6_add                                    1632    1636      +4
ipv6_get_saddr_eval                          424     426      +2
_find_last_bit                                86      84      -2
build_sched_domains                         4292    4280     -12
skb_store_bits                               622     606     -16
skb_abort_seq_read                            54      34     -20
__build_skb                                  116      72     -44
skb_try_coalesce                            1560    1324    -236
Total: Before=4333762, After=4333526, chg -0.01%

In both builds only skb_try_coalesce and build_sched_domains change
in instruction count (GCC 16: 524 -> 466 and 1240 -> 1228; GCC 12:
476 -> 413 and 1333 -> 1327). The other lines are not new code. Most
are literal-pool words (the constants that lrw loads) and their
2-byte alignment that moved into or out of a function's range:
__build_skb has 24 instructions and 11 pool words before, 24 and 0
after. fib6_add, ipv6_get_saddr_eval and _find_last_bit each call
__fls() once at run time; they keep their instruction count, and one
instruction changes between its 16- and 32-bit encoding (fib6_add
also gains 2 bytes of pool alignment).

Over the 35 defconfig-based builds (CK807, CK810, CK860, some with
KUnit or SMP, GCC 9 to 16) the bloat-o-meter total is between -118
and -318 bytes in every one. The 128 to 320 bytes in the changelog
are the .text change, which also counts the padding between
functions.

Listings: a standalone file with the old and the new helpers from
arch/csky/include/asm/bitops.h, built by GCC 16.2 with the kernel's
flags, -O2 and -mcpu=ck807, plus -fno-ipa-icf so that GCC keeps the
old and the new copies apart; objdump -d with the addresses dropped.
The constant listing is the same with GCC 9 to 16.

Run-time argument: 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

(GCC 9 and 10 put the movi first for the builtin; GCC 9 also writes
the asm version as ff1/nor/addi.)

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

This is where skb_try_coalesce shrinks: netmem_is_pp(), inlined into
it through skb_pp_frag_ref(), uses that shift, and the function goes
from 12 ff1 to 2 with GCC 12 and 16. The 2 that stay are
__ffs(PAGE_OFFSET) from the same macros; __ffs() keeps its asm
(brev; ff1). build_sched_domains also has one ff1 fewer.

Thanks,
Tzu-Ching