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

bugzilla-daemon at freedesktop.org bugzilla-daemon at freedesktop.org
Wed Apr 6 23:47:36 UTC 2016


--- Comment #73 from Francisco Jerez <currojerez at riseup.net> ---
(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.

> If we want to do option 2, I think we want some sort of lowering pass that
> runs after all the optimizations and before generation that applies all the
> nasty work-arounds.  Proior to the lowering pass we would treat swizzles and
> writemasks as something sensible such as actually working on dvec4
> components.  The lowering pass would turn those into something the hardware
> can handle and maybe add extra code to resolve swizzles and writemasks that
> the hardware can't do natively.

Sounds reasonable.

> On Ivy Bridge, the hardware can do almost
> nothing natively and the pass would be aware of that.  It would need to run
> after all the optimizations because we want optimizations to be able to
> assume swizzles are sensible so we don't have to implement crazy hardware
> restrictions at every level.  If it's practical, this seems like the best
> plan.
> As far as the NIR splitting pass goes: If it's not useful, throw it away! 
> No one will get their feelings hurt.  Connor and I were working under the
> assumption that the hardware was at least mildly sane and, as it turns out,
> we were wrong.

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