[Piglit] [PATCH 5/8] arb_shader_image_load_store: add additional coherency test

Nicolai Hähnle nhaehnle at gmail.com
Sat Apr 16 15:19:33 UTC 2016

On 15.04.2016 17:12, Francisco Jerez wrote:
>>>>> For a test doing almost the same thing but not relying on unspecified
>>>>> invocation ordering, see
>>>>> "tests/spec/arb_shader_image_load_store/shader-mem-barrier.c" -- It
>>>>> would be interesting to see whether you can get it to reproduce the GCN
>>>>> coherency bug using different framebuffer size and modulus parameters.
>>>> I tried that, but couldn't reproduce. Whether I just wasn't thorough
>>>> enough/"unlucky" or whether the in-order nature of the hardware and L1
>>>> cache behavior just makes it impossible to fail the shader-mem-barrier
>>>> test, I'm not sure.
>>> Now I'm curious about the exact nature of the bug ;), some sort of
>>> missing L1 cache-flushing which could potentially affect dependent
>>> invocations?
>> I'm not sure I remember everything, to be honest.
>> One issue that I do remember is that load/store by default go through
>> L1, but atomics _never_ go through L1, no matter how you compile them.
>> This means that if you're working on two different images, one with
>> atomics and the other without, then the atomic one will always behave
>> coherently but the other one won't unless you explicitly tell it to.
>> Now that I think about this again, there should probably be a
>> shader-mem-barrier-style way to test for that particular issue in a way
>> that doesn't depend on the specifics of the parallelization. Something
>> like, in a loop:
>> Thread 1: increasing imageStore into image 1 at location 1, imageLoad
>> from image 1 location 2
>> Thread 2: same, but exchange locations 1 and 2
>> Both threads: imageAtomicAdd on the same location in image 2
>> Then each thread can check that _if_ the imageAtomicAdd detects the
>> buddy thread operating in parallel, _then_ they must also observe
>> incrementing values in the location that the buddy thread stores to.
>> Does that sound reasonable?
> Yeah, that sounds reasonable, but keep in mind that even if both image
> variables are marked coherent you cannot make assumptions about the
> ordering of the image stores performed on image 1 relative to the
> atomics performed on image 2 unless there is an explicit barrier in
> between, which means that some level of L1 caching is legitimate even in
> that scenario (and might have some performance benefit over skipping L1
> caching of coherent images altogether) -- That's in fact the way that
> the i965 driver implements coherent image stores: We just write to L1
> and flush later on to the globally coherent L3 on the next
> memoryBarrier().

Okay, adding the barrier makes sense.

> What about a test along the lines of the current coherency test?  Any
> idea what's the reason you couldn't get it to reproduce the issue?  Is
> it because threads with dependent inputs are guaranteed to be spawned in
> the same L1 cache domain as the threads that generated their inputs or
> something like that?

 From what I understand (though admittedly the documentation I have on 
this is not the clearest...), the hardware flushes the L1 cache 
automatically at the end of each shader invocation, so that dependent 
invocations are guaranteed to pick it up.


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