Re: MMX built-ins performance oddities

[Prakash Punnoor <prakashp at arcor dot de>]

Paolo Bonzini schrieb:
> > - vector version is about 3% faster than above instead of 10% slower -
> > wow!
> > So why is gcc 4.0 producing worse code when using intel style
> > intrinsics and why isn't the union version using builtins as fast as
> > using the vector version?
>
>
> I can answer why unions are slower: that's because they are spilled to
> memory on every assignment -- GCC 4.0 knows how to replace structs with
> different scalar variables (one per item), but not unions.  GCC 3.4 knew
> about none of these possibilities.

Ok, thanks for the explanation, though I am not sure, whether I understood
everythuing.



> About why vectors are faster, well, a lot of the vector support has been
> rewritten in GCC 4.0 so that may be the case.
>
> I do not know exactly why builtins are still slower, but you may want to
> create a PR and add me on the CC list (bonzini@gcc.gnu.org).


I have some good news: My tests yesterday was flawed. Somehow the compiler
wasn't completely switched. It seems gcc 3.4.3 was still used, but I don't
understand why it behaved differently after gcc 4 was installed. Perhaps
something in the script for switching compilers is wrong....

Nevertheless, today I did it correctly and gcc 4 just blew away. I didn't test
the unions version, only built-ins vs Intel style and this time they were
pretty identical and what is more, hell faster than gcc 3.4.3 generated code.
The devs did really good work.

Some numbers
Fastest runs with gcc3.4.3: about 2.9 sec (slowest when using Intel style: 3,1sec

gcc 4.0: <2.4 sec

Just astonishing!

So I guess considering gcc 4.0 this report can be forgotten, but gcc 3.4.3 has
serious issues. It even miscompiles SSE code. Are plans to fix this or won't
time be spent on it anymore? I'd rather see gcc 4.0 getting into a wonderful
stable state. :-)

Sorry for my mistake.

--
Prakash Punnoor

formerly known as Prakash K. Cheemplavam

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