Re: What exactly is our CRC algorithm?

On Fri, Dec 26, 2014 at 04:52:58PM +0100, Andres Freund wrote:
> On December 26, 2014 4:50:33 PM CET, Bruce Momjian <bruce@momjian.us> wrote:
> >On Thu, Dec 25, 2014 at 11:57:29AM +0530, Abhijit Menon-Sen wrote:
> >> 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.
> >
> >Uh, what happens if the system is compiled on a different CPU that it
> >is
> >run on?  Seems we would need a run-time CPU test.
> 
> That's the cpuid thing mentioned above.

Oh, so cpuid is not a macro exported by the compiler but a C API.  Good.

--  Bruce Momjian  <bruce@momjian.us>        http://momjian.us EnterpriseDB
http://enterprisedb.com
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