On 18.02.2015 21:32, OGAWA Hirofumi wrote:
> Alexander Kuleshov <kuleshovm...@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)
You have to put
__attribute__((optimize("unroll-loops")))
here to unroll the loop inside the function:
static __attribute__ ((noinline))
__attribute__((optimize("unroll-loops")))
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)
>
$ time ./c.inline aaaaaaaaaaa
14
real 0m0.743s
user 0m0.740s
sys 0m0.000s
Without __attribute__((optimize("unroll-loops"))) :
$ time ./c.loop aaaaaaaaaaa
14
real 0m1.482s
user 0m1.472s
sys 0m0.004s
With __attribute__((optimize("unroll-loops"))) :
$ time ./c.loop aaaaaaaaaaa
14
real 0m0.742s
user 0m0.740s
sys 0m0.000s
Best regards
Heinrich Schuchardt
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