Re: [PATCH] arm64: enable GENERIC_FIND_FIRST_BIT

Yury Norov
2021-02-24T15:57:02Z

On Wed, Feb 24, 2021 at 11:52:55AM +0000, Alexander Lobakin wrote:
> From: Yury Norov <yury.norov@gmail.com>
> Date: Sat, 5 Dec 2020 08:54:06 -0800
>
> Hi,
>
> > ARM64 doesn't implement find_first_{zero}_bit in arch code and doesn't
> > enable it in 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 this functions would also benefit for_each_{set,clear}_bit().
> > Would it make sense to enable this config for all such architectures by
> > default?
>
> I confirm that GENERIC_FIND_FIRST_BIT also produces more optimized and
> fast code on MIPS (32 R2) where there is also no architecture-specific
> bitsearching routines.
> So, if it's okay for other folks, I'd suggest to go for it and enable
> for all similar arches.

As far as I understand the idea of GENERIC_FIND_FIRST_BIT=n, it's
intended to save some space in .text. But in fact it bloats the
kernel:

yury:linux$ scripts/bloat-o-meter vmlinux vmlinux.ffb
add/remove: 4/1 grow/shrink: 19/251 up/down: 564/-1692 (-1128)
...

For the next cycle, I'm going to submit a patch that removes the
GENERIC_FIND_FIRST_BIT completely and forces all architectures to
use find_first{_zero}_bit()

> (otherwise, I'll publish a separate entry for mips-next after 5.12-rc1
> release and mention you in "Suggested-by:")

I think it worth to enable GENERIC_FIND_FIRST_BIT for mips and arm now
and see how it works for people. If there'll be no complains I'll remove
the config entirely. I'm OK if you submit the patch for mips now, or we
can make a series and submit together. Works either way.