linux-kernel - Re: [PATCH] fs/fat: calculate checksum in a loop instead of directly calculating

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Date:	Thu, 19 Feb 2015 01:46:07 +0600
From:	Alexander Kuleshov <kuleshovmail@...il.com>
To:	OGAWA Hirofumi <hirofumi@...l.parknet.co.jp>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] fs/fat: calculate checksum in a loop instead of directly calculating

Hello,

I tested two simple program with this (gcc-4.9.1)

Results are following:

time ./test_with_loop

real    0m0.001s
user    0m0.000s
sys    0m0.001s

And

time ./test_direct_calculation:

real    0m0.002s
user    0m0.000s
sys    0m0.001s

As you can see result almsot the same. Also i compared assembly output
from two these programras and there are two notes:

1. Assembly output of the program with loop is half as much than
program with directly rotation, but ofcourse we can't take it for rule
here. Current version prepares stack and than just does:

    addq    $1, %rax
    movzbl    (%rax), %eax
    addl    %edx, %eax
    movb    %al, -1(%rbp)
    movzbl    -1(%rbp), %eax
    rorb    %al
    movl    %eax, %edx
    movq    -24(%rbp), %rax

11 times, but version with loop does the same but with cmp/jump.

Thank you.


2015-02-19 1:25 GMT+06:00 OGAWA Hirofumi <hirofumi@...l.parknet.co.jp>:
> Alexander Kuleshov <kuleshovmail@...il.com> writes:
>
>>  static inline unsigned char fat_checksum(const __u8 *name)
>>  {
>> +     u8 i;
>>       unsigned char s = name[0];
>> -     s = (s<<7) + (s>>1) + name[1];  s = (s<<7) + (s>>1) + name[2];
>> -     s = (s<<7) + (s>>1) + name[3];  s = (s<<7) + (s>>1) + name[4];
>> -     s = (s<<7) + (s>>1) + name[5];  s = (s<<7) + (s>>1) + name[6];
>> -     s = (s<<7) + (s>>1) + name[7];  s = (s<<7) + (s>>1) + name[8];
>> -     s = (s<<7) + (s>>1) + name[9];  s = (s<<7) + (s>>1) + name[10];
>> +
>> +     for (i = 1; i < 11; i++)
>> +             s = (s << 7) + (s >> 1) + name[i];
>> +
>>       return s;
>>  }
>
> When I wrote this, IIRC, there was measurable performance
> difference. All major gcc versions are enough smart now to optimize this?
> --
> OGAWA Hirofumi <hirofumi@...l.parknet.co.jp>
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