OK, so just to summarize: The approach suggested by Roland is to have the outputs be one-dimensional and only representing the current invocation's per-vertex outputs. Each invocation would also get access to other invocations' per-vertex outputs via a 2d input array.
So a shader might look something like TESSC DECL IN[][0], POSITION (input patch's per-vertex position) DECL IN[][1], GENERIC (input patch's per-vertex generic attribute) DECL IN[][2], TCS_POSITION (output patch's per-vertex position) DECL IN[][3], TCS_OUTPUT (output patch's per-vertex generic attribute) DECL OUT[0], POSITION DECL OUT[1], GENERIC DECL OUT[2], PATCH And then anything written to OUT[0] would be aliased via IN[][2]. Roland, does that sound right? This seems kinda nasty that there are going to be 2 types of position/pointsize/clipdistance inputs -- do you have a better suggestion for handling that? I still sorta like my hacky idea of declaring the outputs 1-dimensionally, but then allowing them as sources and reading them as though they were 2d... Although that still doesn't deal with a foo[gl_InvocationID] += 1 type of situation very gracefully. On Mon, Sep 1, 2014 at 1:47 PM, Roland Scheidegger <srol...@vmware.com> wrote: > Am 01.09.2014 18:53, schrieb Ilia Mirkin: >> On Mon, Sep 1, 2014 at 12:47 PM, Roland Scheidegger <srol...@vmware.com> >> wrote: >>> Am 01.09.2014 18:19, schrieb Ilia Mirkin: >>>> On Mon, Sep 1, 2014 at 12:00 PM, Roland Scheidegger <srol...@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. >> > > I think you could do that easily either way (you need to recognize it's > really reading the current invocation data anyway, and it's not really > dependent on the tgsi representation). > btw there is some other reason why I think separate inputs/outputs has > some merit: outputs are usually uninitialized - but if you just declare > outputs as a 2d array then this is obviously not really the case, except > for the one with invocationID (and subject to the barrier stuff actually > for the other ones though that is true even for separate > inputs/outputs), which makes for a somewhat awkard register model I > guess. But still it should be workable. > > Roland > _______________________________________________ mesa-dev mailing list mesa-dev@lists.freedesktop.org http://lists.freedesktop.org/mailman/listinfo/mesa-dev
