[Mesa-dev] [PATCH 12/13] nir: add pack_double_2x32_split_y opcode

Wed Apr 13 14:57:13 UTC 2016

On Wed, Apr 13, 2016 at 3:24 AM, Iago Toral <itoral at igalia.com> wrote:
> On Tue, 2016-04-12 at 13:16 -0400, Connor Abbott wrote:
>> I'm not sure I'm so comfortable with this. For one, this is the sort
>> of thing the backend can (and should) do. Why should be be adding
>> stuff at the NIR level to help the backend's copy propagation pass?
>> And perhaps more importantly, why should we be adding stuff at the NIR
>> level to workaround the lack of spilling support for doubles? I know
>> it's more difficult to implement than normal spilling, but it just
>> feels dirty to workaround it because some tests happen to use too many
>> registers.
>
> My thoughts for this were that NIR is producing suboptimal output for
> this case: it is unpacking and repacking an entire 64-bit float when it
> only really needs to operate on the high 32-bit, so fixing that in NIR,
> where the issue is introduced, instead of having each backend fix it,
> seemed to make more sense to me.
>
> I am not sure that backends can easily fix this either: what they are
> going to see is a nir_op_pack_double_2x32_split operation, they don't
> have any context to tell if the low 32-bit argument is actually
> different from what is already in the register, other than emitting code
> to check for that which kind of defeats the purpose of the optimization.
> I suppose they could still see if the instruction that generated the low
> 32-bit value is actually an unpack of the same register and do the right
> thing in that case. Would you like that solution better?
>
> In any case, I don't think this is something for copy-propagation to
> handle this is really not about copy-propagating a value, it is about
> not needing that value to begin with. Register pressure comes from
> needing to have the low 32-bit value alive until we recombine it in the
> 64-bit value. This patch makes it so that the low 32-bit is not alive at
> any moment, we don't need to care about it at any point, and that is why
> it helps reduce register pressure.

Think of what the backend is going to turn this into, though. Say you
have a sequence like:

hi = unpack_double_2x32_split_y foo

new_hi = ...

lo = unpack_double_2x32_split_x foo
bar = pack_double_2x32_split lo, new_hi

then this gets transformed to, in the backend:

hi = foo (stride 2, offset 1)

new_hi  = ..

lo = foo (stride 2, offset 0)
bar (stride 2, offset 0) = lo (stride 2, offset 0) # <- notice this
bar (stride 2, offset 1) hi (stride 2, offset 1)

Now the move from lo to bar has the same stride, and the same offset,
and the same offset, and bar obviously doesn't conflict with lo, so
register coalescing can combine lo and bar. This will result in:

hi = foo (stride 2, offset 1)

new_hi  = ..

bar (stride 2, offset 0) = foo (stride 2, offset 0)
bar (stride 2, offset 1) hi (stride 2, offset 1)

This is the same code (with the same register pressure) as if you had
used pack_double_2x32_split_y. In fact, in the bugzilla bug you gave
me an example of a shader where this happens, and I pointed it out to
you. I think all you need to do to fix your problem is move down the
unpack_2x32_split_x to right before the pack, since the backend isn't
smart enough to reorder it.

In any case, I think that we should get spilling to work on doubles regardless.

>
> Jason also mentioned that what we should do is fix the spilling code. I
> agree that we should do that, but I think the underlying issue here is
> not that, but that NIR's output is leading to inefficient code, which is
> then leading to unnecessary spilling in some cases. If we fix the
> spilling then we can live with the problem, but the problem is still
> there: we are still having live values that we don't need taking some
> register space and leading to unnecessary spilling.
>
> In any case, I understand why you don't like the patch, so I am fine
> with holding it back and focus on fixing spilling code first. FWIW, I
> already had a stab at that, but I run into something that I didn't quite
> figure out at that time:
>
> https://lists.freedesktop.org/archives/mesa-dev/2016-March/109370.html

I didn't notice this fly by, but you're not doing the spilling right.
For things where the stride * type_size is 64, you can't decompose it
into two scratch reads/writes, since then the exec mask will be wrong.
You only want to read/write a channel if the exec mask is enabled for
it. You have to use a different message to spill doubles, since the
normal one (the scattered read/write) assumes the exec mask channels
correspond to 32-bit chunks. Jason would know more about this.

>
> I still have plans to get back to that though.
>
> Iago
>
>> Jason, what do you think?
>>
>> On Tue, Apr 12, 2016 at 4:05 AM, Samuel Iglesias Gonsálvez
>> <siglesias at igalia.com> wrote:
>> > From: Iago Toral Quiroga <itoral at igalia.com>
>> >
>> > This is useful when we only need to modify the high 32-bit chunk of a double.
>> > This is a common case, because this is the part that encodes the exponent
>> > which we manipulate in some double lowering passes. Although we can accomplish
>> > the same by using pack_double_2x32, this new opcode is better for register
>> > pressure, since we don't have to unpack both parts of the double and keep
>> > the low 32-bits around until we can recombine the new exponent.
>> >
>> > We will use this opcode in the set_exponent() function of the double lowering
>> > pass and with that we will fix spilling issues in some dmat4 divide
>> > piglit tests on Intel.
>> > ---
>> >  src/compiler/nir/nir_opcodes.py | 14 ++++++++++++++
>> >  1 file changed, 14 insertions(+)
>> >
>> > diff --git a/src/compiler/nir/nir_opcodes.py b/src/compiler/nir/nir_opcodes.py
>> > index 9f62e08..f92dd8d 100644
>> > --- a/src/compiler/nir/nir_opcodes.py
>> > +++ b/src/compiler/nir/nir_opcodes.py
>> > @@ -276,6 +276,20 @@ di.i2 = src0.y;
>> >  dst.x = di.u64;
>> >  """)
>> >
>> > +opcode("pack_double_2x32_split_y", 0, tuint64, [0, 0], [tuint64, tuint32], "", """
>> > +union {
>> > +    uint64_t u64;
>> > +    struct {
>> > +        uint32_t i1;
>> > +        uint32_t i2;
>> > +    };
>> > +} di;
>> > +
>> > +di.u64 = src0;
>> > +di.i1 = src1;
>> > +dst = di.u64;
>> > +""")
>> > +
>> >  unop_horiz("unpack_double_2x32", 2, tuint32, 1, tuint64, """
>> >  union {
>> >      uint64_t u64;
>> > --
>> > 2.5.0
>> >
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