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?
Signed-off-by: Yury Norov <[email protected]>
---
arch/arm64/Kconfig | 1 +
1 file changed, 1 insertion(+)
diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
index 1515f6f153a0..2b90ef1f548e 100644
--- a/arch/arm64/Kconfig
+++ b/arch/arm64/Kconfig
@@ -106,6 +106,7 @@ config ARM64
select GENERIC_CPU_AUTOPROBE
select GENERIC_CPU_VULNERABILITIES
select GENERIC_EARLY_IOREMAP
+ select GENERIC_FIND_FIRST_BIT
select GENERIC_IDLE_POLL_SETUP
select GENERIC_IRQ_IPI
select GENERIC_IRQ_MULTI_HANDLER
--
2.25.1
On Sat, Dec 05, 2020 at 08:54:06AM -0800, Yury Norov wrote:
> 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:
[...]
> diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
> index 1515f6f153a0..2b90ef1f548e 100644
> --- a/arch/arm64/Kconfig
> +++ b/arch/arm64/Kconfig
> @@ -106,6 +106,7 @@ config ARM64
> select GENERIC_CPU_AUTOPROBE
> select GENERIC_CPU_VULNERABILITIES
> select GENERIC_EARLY_IOREMAP
> + select GENERIC_FIND_FIRST_BIT
Does this actually make any measurable difference? The disassembly with
or without this is _very_ similar for me (clang 11).
Will
(CC: Alexey Klimov)
On Mon, Dec 7, 2020 at 3:25 AM Will Deacon <[email protected]> wrote:
>
> On Sat, Dec 05, 2020 at 08:54:06AM -0800, Yury Norov wrote:
> > 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:
>
> [...]
>
> > diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
> > index 1515f6f153a0..2b90ef1f548e 100644
> > --- a/arch/arm64/Kconfig
> > +++ b/arch/arm64/Kconfig
> > @@ -106,6 +106,7 @@ config ARM64
> > select GENERIC_CPU_AUTOPROBE
> > select GENERIC_CPU_VULNERABILITIES
> > select GENERIC_EARLY_IOREMAP
> > + select GENERIC_FIND_FIRST_BIT
>
> Does this actually make any measurable difference? The disassembly with
> or without this is _very_ similar for me (clang 11).
>
> Will
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.)
Yury
---
Tested-by: Alexey Klimov <[email protected]>
Start testing find_bit() with random-filled bitmap
[7126084.864616] find_next_bit: 9653351 ns, 164280 iterations
[7126084.881146] find_next_zero_bit: 9591974 ns, 163401 iterations
[7126084.893859] find_last_bit: 5778627 ns, 164280 iterations
[7126084.948181] find_first_bit: 47389224 ns, 16357 iterations
[7126084.958975] find_next_and_bit: 3875849 ns, 73487 iterations
[7126084.965884]
Start testing find_bit() with sparse bitmap
[7126084.973474] find_next_bit: 109879 ns, 655 iterations
[7126084.999365] find_next_zero_bit: 18968440 ns, 327026 iterations
[7126085.006351] find_last_bit: 80503 ns, 655 iterations
[7126085.032315] find_first_bit: 19048193 ns, 655 iterations
[7126085.039303] find_next_and_bit: 82628 ns, 1 iterations
with enabled GENERIC_FIND_FIRST_BIT:
Start testing find_bit() with random-filled bitmap
[ 84.095335] find_next_bit: 9600970 ns, 163770 iterations
[ 84.111695] find_next_zero_bit: 9613137 ns, 163911 iterations
[ 84.124143] find_last_bit: 5713907 ns, 163770 iterations
[ 84.158068] find_first_bit: 27193319 ns, 16406 iterations
[ 84.168663] find_next_and_bit: 3863814 ns, 73671 iterations
[ 84.175392]
Start testing find_bit() with sparse bitmap
[ 84.182660] find_next_bit: 112334 ns, 656 iterations
[ 84.208375] find_next_zero_bit: 18976981 ns, 327025 iterations
[ 84.215184] find_last_bit: 79584 ns, 656 iterations
[ 84.233005] find_first_bit: 11082437 ns, 656 iterations
[ 84.239821] find_next_and_bit: 82209 ns, 1 iterations
root@pine:~# cpupower -c all frequency-info | grep asserted
current CPU frequency: 648 MHz (asserted by call to hardware)
current CPU frequency: 648 MHz (asserted by call to hardware)
current CPU frequency: 648 MHz (asserted by call to hardware)
current CPU frequency: 648 MHz (asserted by call to hardware)
root@pine:~# lscpu
Architecture: aarch64
CPU op-mode(s): 32-bit, 64-bit
Byte Order: Little Endian
CPU(s): 4
On-line CPU(s) list: 0-3
Thread(s) per core: 1
Core(s) per socket: 4
Socket(s): 1
Vendor ID: ARM
Model: 4
Model name: Cortex-A53
Stepping: r0p4
CPU max MHz: 1152.0000
CPU min MHz: 648.0000
BogoMIPS: 48.00
Vulnerability Itlb multihit: Not affected
Vulnerability L1tf: Not affected
Vulnerability Mds: Not affected
Vulnerability Meltdown: Not affected
Vulnerability Spec store bypass: Not affected
Vulnerability Spectre v1: Mitigation; __user pointer sanitization
Vulnerability Spectre v2: Not affected
Vulnerability Srbds: Not affected
Vulnerability Tsx async abort: Not affected
Flags: fp asimd evtstrm aes pmull sha1 sha2
crc32 cpuid
On Mon, Dec 07, 2020 at 05:59:16PM -0800, Yury Norov wrote:
> (CC: Alexey Klimov)
>
> On Mon, Dec 7, 2020 at 3:25 AM Will Deacon <[email protected]> wrote:
> >
> > On Sat, Dec 05, 2020 at 08:54:06AM -0800, Yury Norov wrote:
> > > 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:
> >
> > [...]
> >
> > > diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
> > > index 1515f6f153a0..2b90ef1f548e 100644
> > > --- a/arch/arm64/Kconfig
> > > +++ b/arch/arm64/Kconfig
> > > @@ -106,6 +106,7 @@ config ARM64
> > > select GENERIC_CPU_AUTOPROBE
> > > select GENERIC_CPU_VULNERABILITIES
> > > select GENERIC_EARLY_IOREMAP
> > > + select GENERIC_FIND_FIRST_BIT
> >
> > Does this actually make any measurable difference? The disassembly with
> > or without this is _very_ similar for me (clang 11).
> >
> > Will
>
> 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.)
I guess it's more compiler dependent than anything else, and it's a pity
that find_next_bit() isn't implemented in terms of the generic
find_first_bit() tbh, but if the numbers are as you suggest then I don't
have a problem selecting this on arm64.
Will
On Tue, Dec 08, 2020 at 10:35:50AM +0000, Will Deacon wrote:
> On Mon, Dec 07, 2020 at 05:59:16PM -0800, Yury Norov wrote:
> > (CC: Alexey Klimov)
> >
> > On Mon, Dec 7, 2020 at 3:25 AM Will Deacon <[email protected]> wrote:
> > >
> > > On Sat, Dec 05, 2020 at 08:54:06AM -0800, Yury Norov wrote:
> > > > 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:
> > >
> > > [...]
> > >
> > > > diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
> > > > index 1515f6f153a0..2b90ef1f548e 100644
> > > > --- a/arch/arm64/Kconfig
> > > > +++ b/arch/arm64/Kconfig
> > > > @@ -106,6 +106,7 @@ config ARM64
> > > > select GENERIC_CPU_AUTOPROBE
> > > > select GENERIC_CPU_VULNERABILITIES
> > > > select GENERIC_EARLY_IOREMAP
> > > > + select GENERIC_FIND_FIRST_BIT
> > >
> > > Does this actually make any measurable difference? The disassembly with
> > > or without this is _very_ similar for me (clang 11).
> > >
> > > Will
> >
> > 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.)
>
> I guess it's more compiler dependent than anything else, and it's a pity
> that find_next_bit() isn't implemented in terms of the generic
> find_first_bit() tbh, but if the numbers are as you suggest then I don't
> have a problem selecting this on arm64.
Ping?
On Tue, Feb 23, 2021 at 09:27:44PM -0800, Yury Norov wrote:
> On Tue, Dec 08, 2020 at 10:35:50AM +0000, Will Deacon wrote:
> > On Mon, Dec 07, 2020 at 05:59:16PM -0800, Yury Norov wrote:
> > > (CC: Alexey Klimov)
> > >
> > > On Mon, Dec 7, 2020 at 3:25 AM Will Deacon <[email protected]> wrote:
> > > >
> > > > On Sat, Dec 05, 2020 at 08:54:06AM -0800, Yury Norov wrote:
> > > > > 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:
> > > >
> > > > [...]
> > > >
> > > > > diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
> > > > > index 1515f6f153a0..2b90ef1f548e 100644
> > > > > --- a/arch/arm64/Kconfig
> > > > > +++ b/arch/arm64/Kconfig
> > > > > @@ -106,6 +106,7 @@ config ARM64
> > > > > select GENERIC_CPU_AUTOPROBE
> > > > > select GENERIC_CPU_VULNERABILITIES
> > > > > select GENERIC_EARLY_IOREMAP
> > > > > + select GENERIC_FIND_FIRST_BIT
> > > >
> > > > Does this actually make any measurable difference? The disassembly with
> > > > or without this is _very_ similar for me (clang 11).
> > > >
> > > > Will
> > >
> > > 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.)
> >
> > I guess it's more compiler dependent than anything else, and it's a pity
> > that find_next_bit() isn't implemented in terms of the generic
> > find_first_bit() tbh, but if the numbers are as you suggest then I don't
> > have a problem selecting this on arm64.
>
> Ping?
Not sure what happened to this. Maybe resend at -rc1?
Will
From: Yury Norov <[email protected]>
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.
(otherwise, I'll publish a separate entry for mips-next after 5.12-rc1
release and mention you in "Suggested-by:")
> Signed-off-by: Yury Norov <[email protected]>
>
> ---
> arch/arm64/Kconfig | 1 +
> 1 file changed, 1 insertion(+)
>
> diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
> index 1515f6f153a0..2b90ef1f548e 100644
> --- a/arch/arm64/Kconfig
> +++ b/arch/arm64/Kconfig
> @@ -106,6 +106,7 @@ config ARM64
> select GENERIC_CPU_AUTOPROBE
> select GENERIC_CPU_VULNERABILITIES
> select GENERIC_EARLY_IOREMAP
> + select GENERIC_FIND_FIRST_BIT
> select GENERIC_IDLE_POLL_SETUP
> select GENERIC_IRQ_IPI
> select GENERIC_IRQ_MULTI_HANDLER
> --
> 2.25.1
Thanks,
Al
On Wed, Feb 24, 2021 at 11:52:55AM +0000, Alexander Lobakin wrote:
> From: Yury Norov <[email protected]>
> 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.
From: Yury Norov <[email protected]>
Date: Wed, 24 Feb 2021 07:44:16 -0800
> On Wed, Feb 24, 2021 at 11:52:55AM +0000, Alexander Lobakin wrote:
> > From: Yury Norov <[email protected]>
> > 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)
> ...
Same for MIPS, enabling GENERIC_FIND_FIRST_BIT saves a bunch of .text
memory despite that it introduces new entries.
> 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()
I like that idea. I'm almost sure there'll be no arch that benefits
from CONFIG_GENERIC_FIND_FIRST_BIT=n (and has no arch-optimized
versions).
> > (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.
Lez make a series and see how it goes. I'll send you MIPS part soon.
Al