On Wed, Oct 24, 2012 at 10:41 PM, Marc Glisse <marc.gli...@inria.fr> wrote:
> On Wed, 24 Oct 2012, Mike Stump wrote:
>
>> Well, I suspect the OpenCL community had a ton of people sweat over the
>> details and take into consideration the realities and the needs of people.
>> I'd like to believe they had more people in on this and that this was a
>> compromise for someones vector unit.
>
>
> Intel's, apparently...
>
>
>> The problem is, what if your vector unit produces 0, 1, to be compatible
>> with C? Suddenly, spilling this onto ? is annoying, both because it doesn't
>> match hardware, nor the expected semantics of a person that just knows C.
>> Maybe we run a poll of vector units that prefer -1 or prefer 1 for true, and
>> then decide. SSE has CMPPS, which likes the -1. Altivec's vec_cmpgt says
>> true is all bits 1. Gosh, I guess we can stop there. Neon, for fun VCGE is
>> defined to set to all ones for true. OpenCL it is.
>
>
> We already decided that comparisons return -1, most processors agree on that
> except sparc, which doesn't return a vector at all. The question is about
> the selection instructions.
>
> {-2,-1,0,1} ? {x,x,x,x} : {y,y,y,y}
>
> OpenCL says this should be {x,x,y,y}. We are considering making it {x,x,y,x}
> instead. Hardware selection instructions vary a lot. OpenCL follows x86,
> what we are considering matches Power IIRC, and ARM only has a bitwise
> selection (I only quickly glanced at all of these, I may have read them
> wrong).
>
>
> I am fine with both alternatives, but the choice is important...
I would go with != 0, too. It is what people would expect. We can deviate
when someone introduces -fopencl.
Richard.
> --
> Marc Glisse
