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