[Mesa-dev] TGSI and Tessellation Control Shader outputs

[Ilia Mirkin]

On Mon, Sep 1, 2014 at 12:47 PM, Roland Scheidegger <sroland at vmware.com> wrote:
> Am 01.09.2014 18:19, schrieb Ilia Mirkin:
>> On Mon, Sep 1, 2014 at 12:00 PM, Roland Scheidegger <sroland at vmware.com> wrote:
>>> Am 29.08.2014 22:44, schrieb Ilia Mirkin:
>>>> Hello,
>>>>
>>>> I've been thinking a bit about how to properly implement TCS outputs
>>>> in TGSI. As a quick reminder, there are per-vertex (i.e. invocation)
>>>> and per-patch outputs in TCS. And while you can only write to the
>>>> current invocation's per-vertex outputs, you can read from any of
>>>> them. (With barrier() used to synchronize invocations.)
>>>>
>>>> Per-patch outputs map quite nicely onto the existing infrastructure,
>>>> so the rest of the questions will be about per-vertex outputs.
>>>>
>>>> One can represent per-vertex outputs as 2D output arrays. That means
>>>> support for them needs to be added all over (which I've actually done,
>>>> so I'm not complaining about the extra work but rather asking if it's
>>>> a good idea). And then you might have
>>>>
>>>> DCL OUT[][0], GENERIC
>>>> MOV ADDR[1].x, SV[0] /* invocation id */
>>>> MOV OUT[ADDR[1].x][0], TEMP[0] /* store value */
>>>> BARRIER
>>>> MOV TEMP[0], OUT[3][0] /* read output from invocation == 3 */
>>>>
>>>> The advantage here is that it's all nice and consistent. However the
>>>> disadvantage is that we have to add a totally useless read of the
>>>> invocation id and use it as a relative index for the store. At least
>>>> the nvidia shaders don't even have a way of writing other invocations'
>>>> data even if they wanted to (without resorting to global memory
>>>> accesses). So it's complicating all sorts of logic for apparently no
>>>> real benefit.
>>>>
>>>> Another approach might be to bypass the invocation id on storing the
>>>> output, but using it on reads. For example code like
>>>>
>>>> DCL OUT[0], GENERIC
>>>> MOV OUT[0], TEMP[0]
>>>> BARRIER
>>>> MOV TEMP[0], OUT[3][0]
>>>>
>>>> This avoids having to teach tgsi about 2d outputs (esp reladdr ones).
>>>> This seems a lot simpler, but it ignores the gl_InvocationID indexing
>>>> that happens when writing the output. However I don't think that's so
>>>> bad. It also means that reads and writes are interpreted a little
>>>> differently for OUT's, but that doesn't seem so bad either.
>>>>
>>>> Thoughts?
>>>>
>>>
>>> I think in the second case though it should be required to declare the
>>> inputs separately. It sounds to me like at least on nv50 the access
>>> works different in any case (even if the actual data accessed is the
>>> same). Though I have no idea how other hw handles this, but in any case
>>
>> On nvc0 there are load and store instructions (nv50 is a little
>> different, but it also doesn't support tess). When storing, there's no
>> way to provide it the invocation offset. When loading, there is.
>>
>>> hull shader from d3d11 uses 2d addressed inputs but 1d addressed outputs
>>> too -
>>> https://urldefense.proofpoint.com/v1/url?u=http://msdn.microsoft.com/en-us/library/windows/desktop/hh447211%28v%3Dvs.85%29.aspx&k=oIvRg1%2BdGAgOoM1BIlLLqw%3D%3D%0A&r=F4msKE2WxRzA%2BwN%2B25muztFm5TSPwE8HKJfWfR2NgfY%3D%0A&m=nYcD1FcBz0UnqCOOj%2B2wurf%2F3rjQNi1sQmGxNT2xfPQ%3D%0A&s=f81f9c26e90f61f613539e68b7a0cfe070451d77be957c6dc28b2107b03fe497
>>> (though I don't know how that looks like at the ddi level). Probably GL
>>
>> Hmmm... well from a quick read of it, they've bypassed this problem by
>> creating substages with inputs consuming previous stages' outputs.
> Doesn't exactly look like this to me. They still have this both as input
> and output in multiple stages.
>
>>
>>> used 2d outputs because it indeed looks more consistent (or perhaps some
>>> extension could lift the restriction that only the current invocation be
>>> written, though I'm not sure if that would ever make sense).
>>> So I think if it doesn't actually make sense to try writing to other
>>> outputs, option 2) makes more sense. I think though in this case the
>>> outputs should probably be strictly write-only, I'd guess it would get
>>> messy otherwise if you try to read some other invocations data vs.
>>> reading back the current one.
>>
>> If they were write-only, how would you read another invocation's
>> outputs? Or are you suggesting that some new input type be used which
>> maps onto the invocations' outputs?
>
> Yes that's what d3d11 seems to do (as far as I can tell they just have
> input control points and output control points). That's why you'd
> declare it both as inputs and outputs, even though it is sort of the
> same. Can't really tell though if this makes more sense as the gl model,
> but this looks cleaner to me than accessing the same var differently (1d
> output, 2d input).

One thing that occurred to me, and it's a problem with any approach
that hides any aspect of what's going on, which is that you might have
like

out int foo[];
...
foo[gl_InvocationID] = ...
if (...) foo[gl_InvocationID] += 1;

Now, it would be nice if the += 1 step could be done without the
(presumably expensive) shader input load, instead reusing whatever
TEMP was used above. Not sure whether that's too important though.

  -ilia