[Bug 92760] Add FP64 support to the i965 shader backends

bugzilla-daemon at freedesktop.org bugzilla-daemon at freedesktop.org
Thu Apr 7 06:54:58 UTC 2016


--- Comment #76 from Iago Toral <itoral at igalia.com> ---
(In reply to Francisco Jerez from comment #73)
> (In reply to Jason Ekstrand from comment #72)
> > (In reply to Iago Toral from comment #66)
> > > Jason, Connor:
> > > 
> > > last week Curro spent some time looking at our fp64 branch and testing some
> > > things and we have been discussing some aspects of the hardware in fp64 that
> > > are not all that well documented (or not even documented at all :)) and that
> > > may have some important implications in the implementation, specifically for
> > > the vec4 backend.
> > >
> > > Opinions?
> > 
> > Short version:
> > 
> > a) The hardware is busted.
> > b) I think Curro knows what he's talking about.  :-)
> > 
> > Longer version:
> > 
> > I see a couple of options here:  One is to just scalarize all double stuff. 
> > On Ivy Bridge, I think you would also have to double up instructions, one
> > for each half.  It's not a great option from the perspective of performance
> > but is perhaps the easiest to implement.
> > 
> > The second option is what Curro's suggesting where you try and use the
> > hardware as much as possible and fall back to nasty things only when you
> > have to.  Unfortunately, this is going to cause a lot of pain in the
> > generator because suddenly lots of stuff may become align1 at least on IVB.
> > 
> Just a short comment on this point: I don't think IVB will be much worse.
> From the functional point of view it's not that much different from HSW+,
> its primary limitation is that Align16 FP64 instructions can't do more than
> one dvec4 at a time, but that's easily solvable by hooking up the SIMD width
> lowering pass (in addition to the swizzle and writemask lowering pass that
> could be used on later gens), because NibCtrl behaves as expected on FP64
> Align16 instructions even on IVB thankfully.

I think there is still a problem with this: the fact that NibCtrl only works
with DF instructions, but we would still need to deal with UD access to DF
data... wouldn't that be broken in this case?

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