[Mesa-dev] Implementation of VK_KHR_draw_indirect_count extension for anv

Danylo Piliaiev danylo.piliaiev at gmail.com
Tue Sep 18 10:46:58 UTC 2018

On 9/18/18 1:08 PM, Jason Ekstrand wrote:
> On Tue, Sep 18, 2018 at 4:08 AM Danylo Piliaiev 
> <danylo.piliaiev at gmail.com <mailto:danylo.piliaiev at gmail.com>> wrote:
>     On 9/17/18 7:03 PM, Jason Ekstrand wrote:
>>     On Mon, Sep 17, 2018 at 10:08 AM Danylo Piliaiev
>>     <danylo.piliaiev at gmail.com <mailto:danylo.piliaiev at gmail.com>> wrote:
>>         On 9/17/18 5:34 PM, Jason Ekstrand wrote:
>>>         On Mon, Sep 17, 2018 at 8:34 AM Danylo Piliaiev
>>>         <danylo.piliaiev at gmail.com
>>>         <mailto:danylo.piliaiev at gmail.com>> wrote:
>>>             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.
>>>         First off, thanks for looking into the combination of these
>>>         two features. Getting them to work together nicely is half
>>>         of the difficulty of these two extensions.
>>>         On platforms which support MI_MATH, I think we're probably
>>>         better off just using it.  For Ivy Bridge, the only thing I
>>>         could think to do when both are in use would be to do two
>>>         MI_PREDICATEs for every draw call.  The first would be what
>>>         you describe above and the second would be the MI_PREDICATE
>>>         for the conditional render with COMBINE_AND. When the
>>>         condition is true, the AND would have no effect and you
>>>         would get the behavior above.  If the condition is false,
>>>         the above logic for implementing draw_indirect_count
>>>         wouldn't matter because it would get ANDed with false. On
>>>         Haswell and later, it's likely more efficient to just use
>>>         MI_MATH and avoid re-loading the draw count and condition on
>>>         every draw call.  (We could just leave the draw count in
>>>         CS_GPR0, for instance.)  Does that work?
>>         Looks like a plan. I'll try to go this path.
>>         Also there is another interaction which wasn't thought of before:
>>         Several vkCmdDrawIndirectCountKHR in conditional render block
>>         but using MI_MATH should solve it.
>>     In that case, we'll have to basically re-do the conditional bit
>>     for every draw call.  There may be some interesting interactions
>>     here with secondary command buffers as well.  I don't remember
>>     what we decided about inheriting conditions in secondaries. 
>>     Again, if we decide we need MI_MATH, then we'll just drop support
>>     for one or both extensions on Ivy Bridge.
>     About the secondary command buffers:
>     If inherited conditional rendering
>     <https://www.khronos.org/registry/vulkan/specs/1.1-extensions/html/vkspec.html#features-features-inheritedConditionalRendering>
>     is supported it means that secondary buffers can be executed
>     inside conditional rendering block and commands which can be
>     affected by conditional rendering are affected by it in secondary
>     buffer and also in primary, is it right?
>     However at this point the secondary buffer is already composed
>     without commands for conditions and since our implementation
>     depends on commands emitted to the buffer making its commands to
>     depend on condition either highly tricky to do (secondary buffer
>     needs to have certain points where to inject conditions?) or just
>     impossible.
>     And this secondary buffer may have been formed inside conditional
>     render block so they could be affected by two conditions if I
>     understand correctly.
>     Is is doable to implement?
> I think it is.  The obvious way to implement it would be to have a 
> boolean in the command buffer somewhere that tells you whether or not 
> conditional rendering is enabled and use that to set the 
> PredicateEnable bit on 3DPRIMITIVE commands.  For secondary command 
> buffers, however, we would have to assume that predication is enabled 
> and set the predicate to true in vkCmdExecuteCommands if conditional 
> rendering is disabled.
For primary buffers I'm already doing it this way.
So if we want to support inherited conditional rendering all relevant 
commands in command buffers should be constructed with dependency on 
predicate. Just to be sure: is the dependency on predicate cheap?
> The second issue is in communicating the predicate value.  If we don't 
> have secondaries, we can just hang on to the condition buffer and 
> re-read it whenever needed.  If we're going to use secondaries, we 
> won't have it available in the secondary.  One obvious option would be 
> to simply reserve one of the CS_GPR registers, say CS_GPR15, for 
> storing the predicate value.  On Ivy Bridge, the CS_GPRs don't exist 
> and we'd have to pick some other RW register. Digging through the 
> docs, I found the MI_PREDICATE_DATA which seems to exist for just this 
> sort of thing. :-) Exactly what register we use will have to depend on 
> how we want to compute predicate values; I could see CS_GPR15 being 
> more convenient if we use MI_MATH, for instance.
Thanks for suggestion!
> --Jason

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