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(at)momjian(dot)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.

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Bruce Momjian <bruce(at)momjian(dot)us> http://momjian.us
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