[Zlib-devel] crc32 big/little endian

[Török Edwin]

On 04/22/2010 05:43 AM, Mark Adler wrote:
> John and Edwin,
>
> Thank you for all of the testing.  My conclusion for now is just to
leave crc32 as is.
>

Yeah, it is very compiler/CPU dependent, see below.

On 04/22/2010 02:57 AM, John Bowler wrote:
> From: Joakim Tjernlund 
>> gcc has always had a hard time optimizing crc32. I recently discovered that
>> -O1 was noticeable faster than -O2 with gcc 4.3.4 in some crc32 tests I was
>> doing a while back.
> 
> Wow, you are correct.  Silly me - I just blindly assumed that -O1 would be slightly worse than -O2 (and this is *true* on ARM gcc 3.4.4 where -O1 performs worse of all but still much better than -O0).  Here's an updated BYFOUR table:
> 
> buffer	-O3	-Os	-O2	-O1	-O0
> 64	18644	19035	18650	17279	40816
> 128	17060	17250	17080	17755	36057
> 256	16280	16366	16276	15802	34619
> 512	15874	15926	15890	14901	33596
> 1024	15902	15928	15903	14650	33742
> 2048	15722	15710	15699	14311	32548
> 4096	15586	15602	15586	14129	33543
> 8192	15624	15590	15587	14080	34835
> 16384	18162	18146	18149	17126	37775

Here are some comparisons on my x86-64 (Intel Core 2 Quad Q9550),
comparing only compilers, and optimization levels.
Buffer size is 16384 (8192 is slower for me (17120 vs 16700).
I used -march=native for all compilers.

The numbers are in percentages, 100% is slowest (18000), and 82.2% is
fastest (14800).

        (uint64) -O1 -O2 -O3 -Os (uint32) -O1 -O2 -O3 -Os
gcc-4.4          86.7  92.8  92.8  98.9  90.0  92.8  92.8 100.0
gcc-4.5          93.9  92.8  92.8  98.9  92.2  92.8  92.8  99.4
clang 2.6        82.2  88.9  89.4  88.9  85.6  86.7  86.7  86.7
clang 2.7pre2    92.2  90.6  92.2  90.6  86.1  85.6  87.2  84.4

The absolute best is clang 2.6 -O1, 64-bit crc32.
The second best is clang 2.7pre2 -Os, 32-bit crc32.

I can post the generated assembly, if anybody is interested in it.

So it looks like gcc generates better code with 64-bit crc32, and clang
with 32-bit crc32 (except clang 2.6 -O1, which is interesting).

Best regards,
--Edwin