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ARM64 doesn't implement find_first_{zero}_bit in arch code and doesn't
enable it in a config. It leads to using find_next_bit() which is less
efficient:
0000000000000000 <find_first_bit>:
0: aa0003e4 mov x4, x0
4: aa0103e0 mov x0, x1
8: b4000181 cbz x1, 38 <find_first_bit+0x38>
c: f9400083 ldr x3, [x4]
10: d2800802 mov x2, #0x40 // #64
14: 91002084 add x4, x4, #0x8
18: b40000c3 cbz x3, 30 <find_first_bit+0x30>
1c: 14000008 b 3c <find_first_bit+0x3c>
20: f8408483 ldr x3, [x4], #8
24: 91010045 add x5, x2, #0x40
28: b50000c3 cbnz x3, 40 <find_first_bit+0x40>
2c: aa0503e2 mov x2, x5
30: eb02001f cmp x0, x2
34: 54ffff68 b.hi 20 <find_first_bit+0x20> // b.pmore
38: d65f03c0 ret
3c: d2800002 mov x2, #0x0 // #0
40: dac00063 rbit x3, x3
44: dac01063 clz x3, x3
48: 8b020062 add x2, x3, x2
4c: eb02001f cmp x0, x2
50: 9a829000 csel x0, x0, x2, ls // ls = plast
54: d65f03c0 ret
...
0000000000000118 <_find_next_bit.constprop.1>:
118: eb02007f cmp x3, x2
11c: 540002e2 b.cs 178 <_find_next_bit.constprop.1+0x60> // b.hs, b.nlast
120: d346fc66 lsr x6, x3, #6
124: f8667805 ldr x5, [x0, x6, lsl #3]
128: b4000061 cbz x1, 134 <_find_next_bit.constprop.1+0x1c>
12c: f8667826 ldr x6, [x1, x6, lsl #3]
130: 8a0600a5 and x5, x5, x6
134: ca0400a6 eor x6, x5, x4
138: 92800005 mov x5, #0xffffffffffffffff // #-1
13c: 9ac320a5 lsl x5, x5, x3
140: 927ae463 and x3, x3, #0xffffffffffffffc0
144: ea0600a5 ands x5, x5, x6
148: 54000120 b.eq 16c <_find_next_bit.constprop.1+0x54> // b.none
14c: 1400000e b 184 <_find_next_bit.constprop.1+0x6c>
150: d346fc66 lsr x6, x3, #6
154: f8667805 ldr x5, [x0, x6, lsl #3]
158: b4000061 cbz x1, 164 <_find_next_bit.constprop.1+0x4c>
15c: f8667826 ldr x6, [x1, x6, lsl #3]
160: 8a0600a5 and x5, x5, x6
164: eb05009f cmp x4, x5
168: 540000c1 b.ne 180 <_find_next_bit.constprop.1+0x68> // b.any
16c: 91010063 add x3, x3, #0x40
170: eb03005f cmp x2, x3
174: 54fffee8 b.hi 150 <_find_next_bit.constprop.1+0x38> // b.pmore
178: aa0203e0 mov x0, x2
17c: d65f03c0 ret
180: ca050085 eor x5, x4, x5
184: dac000a5 rbit x5, x5
188: dac010a5 clz x5, x5
18c: 8b0300a3 add x3, x5, x3
190: eb03005f cmp x2, x3
194: 9a839042 csel x2, x2, x3, ls // ls = plast
198: aa0203e0 mov x0, x2
19c: d65f03c0 ret
...
0000000000000238 <find_next_bit>:
238: a9bf7bfd stp x29, x30, [sp, #-16]!
23c: aa0203e3 mov x3, x2
240: d2800004 mov x4, #0x0 // #0
244: aa0103e2 mov x2, x1
248: 910003fd mov x29, sp
24c: d2800001 mov x1, #0x0 // #0
250: 97ffffb2 bl 118 <_find_next_bit.constprop.1>
254: a8c17bfd ldp x29, x30, [sp], #16
258: d65f03c0 ret
Enabling find_{first,next}_bit() would also benefit for_each_{set,clear}_bit().
On A-53 find_first_bit() is almost twice faster than find_next_bit(), according
to lib/find_bit_benchmark (thanks to Alexey for testing):
GENERIC_FIND_FIRST_BIT=n:
[7126084.948181] find_first_bit: 47389224 ns, 16357 iterations
[7126085.032315] find_first_bit: 19048193 ns, 655 iterations
GENERIC_FIND_FIRST_BIT=y:
[ 84.158068] find_first_bit: 27193319 ns, 16406 iterations
[ 84.233005] find_first_bit: 11082437 ns, 656 iterations
GENERIC_FIND_FIRST_BIT=n bloats the kernel despite that it disables generation
of find_{first,next}_bit():
yury:linux$ scripts/bloat-o-meter vmlinux vmlinux.ffb
add/remove: 4/1 grow/shrink: 19/251 up/down: 564/-1692 (-1128)
...
Overall, GENERIC_FIND_FIRST_BIT=n is harmful both in terms of performance and
code size, and it's better to have GENERIC_FIND_FIRST_BIT enabled.
Tested-by: Alexey Klimov <aklimov@redhat.com>
Signed-off-by: Yury Norov <yury.norov@gmail.com>
Acked-by: Will Deacon <will@kernel.org>
Link: https://lore.kernel.org/r/20210225135700.1381396-2-yury.norov@gmail.com
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
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https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.