Re: crc_fast: inefficiency

From: Vladimir Prus (ghost_at_[hidden])
Date: 2001-03-13 09:51:03

Daryle Walker wrote:

>I just posted a new version (5) of "crc.zip" that reverses the computation
>parameters instead of the byte. 
>....
>I wonder what this version speed will be.

For your system, you can find the speed by compiling/running attached 
program. For bcc, new code is 2.5-3 times slower than version I use for 
comparison. I think there's only one optimization to be made. You call 
crc_lookup inside checksumming loop, and that is slow. It is easy to change by
- making a routine get_crc_table which instread of table entry will return 
table itself
- adding member "table" to crc_fast
- assigning that member from call to get_crc_table in ctor.

After that change is made, code works within 25% of reference implementation' 
speed and much closer most of the time.

-- 
Regards,
Vladimir