Re: What exactly is our CRC algorithm?

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.

Andres

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Andres Freund http://www.2ndQuadrant.com/
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