Re: What exactly is our CRC algorithm?

Hi.

Here's a proposed patch to use CPUID at startup to determine if the
SSE4.2 CRC instructions are available, to use them instead of the
slice-by-8 implementation (posted earlier).

A few notes:

1. GCC has included cpuid.h since 4.3.0, so I figured it was safe to
   use. It can be replaced with some inline assembly otherwise.

2. I've also used the crc32b/crc32q instructions directly rather than
   using ".bytes" to encode the instructions; bintuils versions since
   2007 or so have supported them.

3. I've included the MSVC implementation mostly as an example of how to
   extend this to different compilers/platforms. It's written according
   to the documentation for MSVC intrinsics, but I have not tested it.
   Suggestions/improvements are welcome.

-- Abhijit