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

bugzilla-daemon at freedesktop.org bugzilla-daemon at freedesktop.org
Fri Dec 4 17:04:58 PST 2015


--- Comment #24 from Connor Abbott <cwabbott0 at gmail.com> ---
(In reply to Iago Toral from comment #23)
> We spent some time working on fixing code that needs the execsize hack. For
> now we decided to only fix the cases that are absolutely required, which
> involve instructions with a dst.width of 4 (we don't currently generate
> instructions on doubles with a width < 4). This makes the number of changes
> required for gen7/8 pretty small (fixing this for widths < 4 would require a
> huge amount of changes everywhere...). This keeps non-fp64 piglit tests on
> gen7/8 and fp64 tests on gen8 happy with a small number of changes.
> Basically with this we need to keep the exec_size hack as is for gen < 7 and
> tweak it for gen7/8 so it only acts on instructions with a dst.width < 4.
> I would not try to remove the execsize hack entirely (that is cover the
> cases where dst.witdh < 4), because that would mean a really huge number of
> things throughout the driver and across all hw gens.
> We could try to avoid special-casing gen7/8 by fixing instructions with a
> dst.width of 4 in previous gens too, but this will require a lot more
> patches, so I am not sure that it is worth it. Specifically:
> * In gen6 there are instances of instructions injected at the vec4_visitor
> level that need the exec_size hack to set the execution size to 4. The
> problem here is that vec4_instruction does not provide means to alter the
> execution size of the instruction, so that support would have to be added
> (or, alternatively, write generator opcodes that do that as needed). We
> think fixing gen6 would involve at least 5 patches, so it is not too bad.

In this case, I think the right thing to do would be to push the decision of
whether to do exec_size 4 or 8 based on dst.width into the vec4 generator --
essentially, move the hack up one level.

> * In gen5 the problem is that the strip-and-fans and clipping modules are
> full with code that mixes width4 and width8 instructions, so it requires to
> tweak execsizes in a fair amount of places. I have patches to do this that
> in fact change the default execution size for the strip and fans to 4 and
> fix the cases that need 8, since that seemed more reasonable to limit the
> number of necessary changes. This involves something like ~10 patches, but
> the changes look a bit scarier to me (piglit seems to be happy though).
> * We don't have gen4 or gen9 hardware available, so we would need help
> fixing these paths. I would expect a few more patches required for gen4 at
> least.
> So the summary is that if we don't want to special-case the execsize for
> gen7/8, we will need a significant number of changes, help from Intel to
> track down issues in gen4 and gen9 and cross fingers that piglit actually
> exercises all the paths we need to tweak, so all things considered I wonder
> if we should just special case the execsize hack for gen7/8 for now and
> forget about other gens for now, since we are not planning fp64 for them yet
> anyway.
> What do you think?

Ugh, I didn't realize we were using this so much... nice job looking into it!
Indeed, it seems like special-casing this for gen7/gen8 is the easiest thing
that at least gets us moving in the right direction. I would like to get
others' opinions on this, though. I think you should send out an RFC that does
the minimum necessary to solve the problem (i.e. only fix the width-4 case on
gen7/8), explaining the problem and what you came across when trying to solve
it (basically this comment).

I was thinking about this, and there is another way to fix the problem. Since
we don't actually use vstride or width in the destination, we could avoid
dividing the width and vstride by 2 for destination registers. However, I don't
think we want to go in this direction, since this is a step toward conflating
dst.width and exec_size when, at least in FS, there is already a
clearly-defined notion of what the exec_size should be that's part of the
instruction. Jason has already done a lot of work towards not inferring
exec_size from dst.width (although for somewhat different reasons, and at a
higher level), and we should try to get rid of the assumption at a lower level

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