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Date: Fri, 28 Oct 2022 18:45:50 +0100
From: "Russell King (Oracle)" <linux@...linux.org.uk>
To: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Yury Norov <yury.norov@...il.com>,
Catalin Marinas <catalin.marinas@....com>,
Mark Rutland <mark.rutland@....com>,
Will Deacon <will@...nel.org>,
Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH 1/5] ARM: findbit: document ARMv5 bit offset calculation
On Fri, Oct 28, 2022 at 10:05:29AM -0700, Linus Torvalds wrote:
> On Fri, Oct 28, 2022 at 9:47 AM Russell King (Oracle)
> <rmk+kernel@...linux.org.uk> wrote:
> >
> > Document the ARMv5 bit offset calculation code.
>
> Hmm. Don't the generic bitop functions end up using this? We do have a
> comment in the code that says
>
> * On ARMv5 and above, the gcc built-ins may rely on the clz instruction
> * and produce optimal inlined code in all cases. On ARMv7 it is even
> * better by also using the rbit instruction.
It's true that the generic code also makes use of the rbit and clz
instructions - but in terms of the speed of the functions these only
get used once we've found a word that is interesting to locate the
bit we want in.
> but that 'may' makes me wonder...
>
> IOW, what is it in the hand-written code that doesn't get done by the
> generic code these days?
For the _find_first_bit, there isn't much difference in the number
of instructions or really what is going on, only the organisation
and flow of the code is more inline - but that shouldn't make much
of a difference. Yet, there is a definite repeatable measurable
difference between the two:
random-filled:
arm : find_first_bit: 17778911 ns, 16448 iterations
generic: find_first_bit: 18596622 ns, 16401 iterations
sparse:
arm : find_first_bit: 7301363 ns, 656 iterations
generic: find_first_bit: 7589120 ns, 655 iterations
The bigger difference is in the find_next_bit operations, and this
likely comes from the arm32 code not having the hassles of the "_and"
and other conditionals that the generic code has:
random-filled:
arm : find_next_bit: 2242618 ns, 163949 iterations
generic: find_next_bit: 2632859 ns, 163743 iterations
sparse:
arm : find_next_bit: 40078 ns, 656 iterations
generic: find_next_bit: 69943 ns, 655 iterations
find_next_zero_bit show a greater difference:
random-filled:
arm : find_next_zero_bit: 2049129 ns, 163732 iterations
generic: find_next_zero_bit: 2844221 ns, 163938 iterations
sparse:
arm : find_next_zero_bit: 3939309 ns, 327025 iterations
generic: find_next_zero_bit: 5529553 ns, 327026 iterations
Here's the disassemblies for comparison. Note that the arm versions
share code paths between the functions which makes the code even more
compact - so the loop in the find_first gets re-used for find_next
after we check the current word.
generic:
00000000 <_find_first_bit>:
0: e1a02000 mov r2, r0
4: e2510000 subs r0, r1, #0
8: 012fff1e bxeq lr
c: e2422004 sub r2, r2, #4
10: e3a03000 mov r3, #0
14: ea000002 b 24 <_find_first_bit+0x24>
18: e2833020 add r3, r3, #32
1c: e1500003 cmp r0, r3
20: 912fff1e bxls lr
24: e5b2c004 ldr ip, [r2, #4]!
28: e35c0000 cmp ip, #0
2c: 0afffff9 beq 18 <_find_first_bit+0x18>
30: e6ffcf3c rbit ip, ip
34: e16fcf1c clz ip, ip
38: e08c3003 add r3, ip, r3
3c: e1500003 cmp r0, r3
40: 21a00003 movcs r0, r3
44: e12fff1e bx lr
000000e8 <_find_next_bit>:
e8: e92d4070 push {r4, r5, r6, lr}
ec: e1530002 cmp r3, r2
f0: e59d4010 ldr r4, [sp, #16]
f4: e59d5014 ldr r5, [sp, #20]
f8: 2a000024 bcs 190 <_find_next_bit+0xa8>
fc: e1a0e2a3 lsr lr, r3, #5
100: e3510000 cmp r1, #0
104: e203601f and r6, r3, #31
108: e3c3301f bic r3, r3, #31
10c: e790c10e ldr ip, [r0, lr, lsl #2]
110: 1791e10e ldrne lr, [r1, lr, lsl #2]
114: 100cc00e andne ip, ip, lr
118: e3e0e000 mvn lr, #0
11c: e02cc004 eor ip, ip, r4
120: e3550000 cmp r5, #0
124: e1a0e61e lsl lr, lr, r6
128: 1a00001a bne 198 <_find_next_bit+0xb0>
12c: e01cc00e ands ip, ip, lr
130: 1a000011 bne 17c <_find_next_bit+0x94>
134: e2833020 add r3, r3, #32
138: e1530002 cmp r3, r2
13c: 3a000003 bcc 150 <_find_next_bit+0x68>
140: ea000012 b 190 <_find_next_bit+0xa8>
144: e2833020 add r3, r3, #32
148: e1520003 cmp r2, r3
14c: 9a00000f bls 190 <_find_next_bit+0xa8>
150: e1a0e2a3 lsr lr, r3, #5
154: e3510000 cmp r1, #0
158: e790c10e ldr ip, [r0, lr, lsl #2]
15c: 1791e10e ldrne lr, [r1, lr, lsl #2]
160: 100cc00e andne ip, ip, lr
164: e15c0004 cmp ip, r4
168: 0afffff5 beq 144 <_find_next_bit+0x5c>
16c: e02cc004 eor ip, ip, r4
170: e3550000 cmp r5, #0
174: 0a000000 beq 17c <_find_next_bit+0x94>
178: e6bfcf3c rev ip, ip
17c: e6ffcf3c rbit ip, ip
180: e16fcf1c clz ip, ip
184: e08c3003 add r3, ip, r3
188: e1520003 cmp r2, r3
18c: 21a02003 movcs r2, r3
190: e1a00002 mov r0, r2
194: e8bd8070 pop {r4, r5, r6, pc}
198: e6bfef3e rev lr, lr
19c: e01cc00e ands ip, ip, lr
1a0: 0affffe3 beq 134 <_find_next_bit+0x4c>
1a4: eafffff3 b 178 <_find_next_bit+0x90>
==========
arm optimised:
00000060 <_find_first_bit_le>:
60: e3310000 teq r1, #0
64: 0a000006 beq 84 <_find_first_bit_le+0x24>
68: e3a02000 mov r2, #0
6c: e4903004 ldr r3, [r0], #4
70: e1b03003 movs r3, r3
74: 1a000011 bne c0 <_find_next_bit_le+0x34>
78: e2822020 add r2, r2, #32
7c: e1520001 cmp r2, r1
80: 3afffff9 bcc 6c <_find_first_bit_le+0xc>
84: e1a00001 mov r0, r1
88: e12fff1e bx lr
0000008c <_find_next_bit_le>:
8c: e1520001 cmp r2, r1
90: 2afffffb bcs 84 <_find_first_bit_le+0x24>
94: e1a0c2a2 lsr ip, r2, #5
98: e080010c add r0, r0, ip, lsl #2
9c: e212c01f ands ip, r2, #31
a0: 0afffff1 beq 6c <_find_first_bit_le+0xc>
a4: e4903004 ldr r3, [r0], #4
a8: e1b03c33 lsrs r3, r3, ip
ac: 1a000003 bne c0 <_find_next_bit_le+0x34>
b0: e382201f orr r2, r2, #31
b4: e2822001 add r2, r2, #1
b8: eaffffef b 7c <_find_first_bit_le+0x1c>
bc: e6bf3f33 rev r3, r3
c0: e6ff3f33 rbit r3, r3
c4: e16f3f13 clz r3, r3
c8: e0820003 add r0, r2, r3
cc: e1510000 cmp r1, r0
d0: 31a00001 movcc r0, r1
d4: e12fff1e bx lr
--
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