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SubjectRe: [PATCH] riscv: lib: Optimize 'strlen' function
From
On 12/17/23 17:00, David Laight wrote:
> I'd also guess that pretty much all the calls in-kernel are short.
> You might try counting as: histogram[ilog2(strlen_result)]++
> and seeing what it shows for some workload.
> I bet you (a beer if I see you!) that you won't see many over 1k.

Hi David,

Here is the statistics for strlen result:

[ 223.169575] Calls count for 2^0: 6150
[ 223.173293] Calls count for 2^1: 184852
[ 223.177142] Calls count for 2^2: 313896
[ 223.180990] Calls count for 2^3: 185844
[ 223.184881] Calls count for 2^4: 87868
[ 223.188660] Calls count for 2^5: 9916
[ 223.192368] Calls count for 2^6: 1865
[ 223.196062] Calls count for 2^7: 0
[ 223.199483] Calls count for 2^8: 0
[ 223.202952] Calls count for 2^9: 0
...

Looks like I've just lost a beer :)

Considering this statistics, I'd say implementing the word-oriented
strlen is an overcomplication - we wouldn't get any performance gain and
it just doesn't worth it.

I simplified your code a little bit, it looks like the alignment there
is unnecessary: QEMU test shows the same performance independently from
alignment. Tests on the board gave the same result (perhaps because the
CPU on the board has 2 DDR channels?)

mv t0, a0
1:
lbu t1, 0(a0)
lbu t2, 1(a0)
addi a0, a0, 2
beqz t1, 2f
bnez t2, 1b
addi a0, a0, 1
2:
addi a0, a0, -2
sub a0, a0, t0
ret

If it looks good to you, would you mind if I send the patch with it?
Could I add you to suggested-by tag?

--
Kind regards,
Ivan Orlov


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