Hi Jason,
I have implemented the extension and it works, however before sending
the patch I decided to see how it can interact with other extension -
VK_EXT_conditional_render
and got confused:
From the spec it is not disallowed to call functions of
VK_KHR_draw_indirect_count in conditional rendering block. So let's say
that predicate of conditional rendering
will result in FALSE, we call vkCmdDrawIndirectCountKHR which sees that
there is already a predicate emitted and it should be taken into
account, since it will be FALSE
all next predicates should result in FALSE. The issue is that I don't
see an easy way to do this.
My current implementation uses the next predicate (it is same as in GL
implementation):
/* While draw_index < maxDrawCount the predicate's result will be
* (draw_index == maxDrawCount) ^ TRUE = TRUE
* When draw_index == maxDrawCount the result is
* (TRUE) ^ TRUE = FALSE
* After this all results will be:
* (FALSE) ^ FALSE = FALSE
*/
anv_batch_emit(&cmd_buffer->batch, GENX(MI_PREDICATE), mip) {
mip.LoadOperation = LOAD_LOAD;
mip.CombineOperation = COMBINE_XOR;
mip.CompareOperation = COMPARE_SRCS_EQUAL;
}
But if the initial predicate state is FALSE then when draw_index equals
maxDrawCount the result will be
(FALSE) ^ TRUE = TRUE
Which isn't something we want. But without "not equal" operation or
without MI_MATH I don't see how to fix this.
I don't see anything related in Vulkan or GL specs neither I see
anything in Piglit and CTS tests.
Maybe I'm missing something obvious, could you help me here?
You can find current implementation in
https://gitlab.freedesktop.org/GL/mesa/commit/9d1c7ae0db618c6f7281d5f667c96612ff0bb2c2
- Danil
On 9/12/18 6:30 PM, Danylo Piliaiev wrote:
Hi,
Thank you for the directions!
On 9/12/18 6:13 PM, Jason Ekstrand wrote:
Danylo,
You're free to implement anything not already implemented. Here are
some other (probably simpler) extensions that I think can be
reasonably implemented on Intel HW:
- VK_EXT_conservative_rasterization
- VK_EXT_conditional_render
Didn't see them, will take closer look later.
As far as VK_KHR_draw_indirect_count go, I haven't implemented it yet
because the "proper" implementation is actually kind-of painful
though not impossible. In general, there are two ways it can be done:
## 1. The cheap and easy way
The spec explicitly allows for the cheap and easy way by requiring
the caller to pass in a maxDrawCount. The idea here would be to emit
maxDrawCount draw calls only have each one of them predicated on
draw_id < draw_count_from_buffer. This one probably wouldn't take
much to wire up but it does mean doing maxDrawCount 3DPRIMITIVE
commands no matter how many of them are actually needed.
I saw such implementation for i965, looked straightforward and I
thought it will easily translate into Vulkan implementation. Didn't
know that it's possible to do it other way on Intel.
## 2. The hard but maybe more correct way
The Intel command streamer does have the ability, if used carefully,
to loop. The difficulty here isn't in looping; that can be done
fairly easily on gen8+ by emitting a predicated MI_BATCH_BUFFER_START
that's predicated off of the looping condition which jumps to the top
of the loop. The real difficult bit is taking your loop counter and
using it to indirectly access the array of draw information. In
order to do this, you have to have a self-modifying batch buffer. In
short, you would emit MI commands which read the draw information
into registers and also emit MI commands (which would probably come
before the first set) which write the actual address into the
location in the batch where the first set of MI commands has their
address to read from. This would be a painful to debug mess of GPU
hangs but could actually be kind-of fun to implement.
The correct way looks interesting, I'll need some time to understand
details.
I hope I haven't scarred you away from working on anv; I just wanted
to make it clear what you're getting yourself into. Both ways are
totally implementable and I think you'd pretty much have to do the
first method on gen7 if we really care about supporting it there.
The second is totally doable, it'll just involve some headaches when
it's broken. If you want to continue with this project after reading
my scarry e-mail, I recommend starting with method 1 to get your feet
wet and then we can look into method 2 once you have that working.
I'll follow your recommendation and will start from the first method.
- Danil
--Jason
On Wed, Sep 12, 2018 at 6:36 AM Danylo Piliaiev
<danylo.pilia...@gmail.com <mailto:danylo.pilia...@gmail.com>> wrote:
Hello everyone,
I would like to try to implement one of the Vulkan extensions -
VK_KHR_draw_indirect_count for anv,
unless someone is already working on it.
It's a relatively minor extension and I saw that the same
functionality
is already implemented
for ARB_indirect_parameters in i965.
Also I would appreciate any tips if there are any known possible
tricky
parts.
- Danil
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