Re: What exactly is our CRC algorithm?

On 2015-04-02 20:57:24 +0530, Abhijit Menon-Sen wrote:
> At 2015-04-02 17:58:23 +0300, hlinnaka(at)iki(dot)fi wrote:
> >
> > We're only using inline assembly to force producing SSE 4.2 code, even
> > when -msse4.2 is not used. That feels wrong.
>
> Why? It feels OK to me (and to Andres, per earlier discussions about
> exactly this topic). Doing it this way allows the binary to run on a
> non-SSE4.2 platform (and not use the CRC instructions).

Right. And SSE4.2 isn't that widespread yet.

> > 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.

To me this seems like a somewhat pointless exercise. I actually think
from a performance POV it's better to have all the functions in one
source file, so the compiler can inline things into the trampoline if it
feels like it.

Greetings,

Andres Freund