[Mesa-dev] Implementation of VK_KHR_draw_indirect_count extension for anv

Danylo Piliaiev danylo.piliaiev at gmail.com
Mon Sep 17 13:34:25 UTC 2018

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 - 
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 

        /* 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
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 

- 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.piliaiev at gmail.com <mailto:danylo.piliaiev at 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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