Alexander Kuleshov <[email protected]> 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 <[email protected]>
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