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

[OGAWA Hirofumi <hirofumi@mail.parknet.co.jp>]

Alexander Kuleshov <kuleshovmail@gmail.com> writes:

> 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

Hm,

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>

typedef unsigned char	__u8;
typedef unsigned char	u8;

#if 1
static inline unsigned char fat_checksum(const __u8 *name)
{
	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];
	return s;
}
#else
static inline unsigned char fat_checksum(const __u8 *name)
{
	unsigned char s = name[0];
	u8 i;
	for (i = 1; i < 11; i++)
		s = (s << 7) + (s >> 1) + name[i];
	return s;
}
#endif

static __attribute__ ((noinline)) int test(unsigned char *name)
{
	long i;
	for (i = 0; i < 100000000L; i++)
		name[i % 11] = fat_checksum(name);
	return name[0];
}

int main(int argc, char *argv[])
{
	printf("%u\n", test((unsigned char *)argv[1]));
	return 0;
}


  $ gcc --version
  gcc (Debian 4.9.1-19) 4.9.1
  Copyright (C) 2014 Free Software Foundation, Inc.
  This is free software; see the source for copying conditions.  There is NO
  warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

  $ gcc -O2 -o c.inline c.c
  # change #if 1 => #if 0
  $ gcc -O2 -o c.loop c.c

  $ time ./c.inline aaaaaaaaaaa
  14

  real	0m0.550s
  user	0m0.548s
  sys	0m0.000s
  $ time ./c.loop aaaaaaaaaaa
  14

  real	0m0.901s
  user	0m0.896s
  sys	0m0.004s

This is my environment only? (gcc (Debian 4.9.1-19) 4.9.1)
-- 
OGAWA Hirofumi <hirofumi@mail.parknet.co.jp>