Re: What exactly is our CRC algorithm?

On 04/02/2015 12:39 PM, Abhijit Menon-Sen wrote:
> At 2015-03-25 19:18:51 +0200, hlinnaka@iki.fi wrote:
>>
>> I think we'll need a version check there. […]
>>
>> You want to write that or should I?
>
> I'm not familiar with MSVC at all, so it would be nice if you did it.

Thinking more about the configure checks, I think the current approach 
of using inline assembly on gcc is not quite right. We're only using 
inline assembly to force producing SSE 4.2 code, even when -msse4.2 is 
not used. That feels wrong.

And who's to say that the assembler supports the SSE instructions 
anyway? At least we'd need a configure check for that too. We have a 
buildfarm animal that still uses gcc 2.95.3, which was released in 2001. 
I don't have a compiler of that vintage to test with, but I assume an 
old enough assembler would not know about the crc32q instruction and 
fail to compile.

I believe the GCC way to do this would be to put the SSE4.2-specific 
code into a separate source file, and compile that file with "-msse4.2". 
And when you compile with -msse4.2, gcc actually also supports the 
_mm_crc32_u8/u64 intrinsics.

- Heikki