[Mesa-dev] [PATCH 01/10] tgsi: add MEMBAR opcode to handle memoryBarrier* GLSL intrinsics

[Marek Olšák]

On Tue, Jan 19, 2016 at 3:25 AM, Ilia Mirkin <imirkin at alum.mit.edu> wrote:
> On Mon, Jan 18, 2016 at 6:06 AM, Marek Olšák <maraeo at gmail.com> wrote:
>> For 1-4,
>>
>> Reviewed-by: Marek Olšák <marek.olsak at amd.com>
>>
>> I'm not very familiar with the code in 2, but the changes seem reasonable.
>>
>> Also, and I know this is not your mistake, but still, mtypes.h has:
>>
>> struct gl_atomic_buffer_binding
>>       AtomicBufferBindings[MAX_COMBINED_ATOMIC_BUFFERS];
>>
>> But it should be:
>>
>> struct gl_atomic_buffer_binding
>>       AtomicBufferBindings[16 /* or use a proper definition here */];
>>
>> It's not possible to use more than MaxAtomicBufferBindings, because
>> the slots are shared among all shader stages.
>
> Besides the suboptimal name, I don't see what's so terribly wrong
> about this. They're saying there are 15*6 binding points (the value of
> MAX_COMBINED_ATOMIC_BUFFERS), and that's also the
> MaxCombinedAtomicBuffers thing. I guess MaxCombinedAtomicBuffers
> should be N * the max # of bindings? So this is a bit more restrictive
> than it could be, but... meh.

I don't understand. AtomicBufferBindings are binding points. There is
a fixed number of binding points shared by all shader stages, for
example 15. It's not possible to bind a buffer above that. From the
spec:

"BindBufferBase and BindBufferRange will generate an INVALID_VALUE
error if <index> is greater than or equal to the value of
MAX_ATOMIC_COUNTER_BUFFER_BINDINGS".

This is why AtomicBufferBindings[MAX_COMBINED_ATOMIC_BUFFERS] wastes a
lot of memory.

MAX_COMBINED_ATOMIC_BUFFERS is about shader references, not bindings.
You can have 15 global bindings (slots), but 5 shaders can reference
them, which leads to 15*5 references. This is a thing that only the
linker should care about. It's only possible to get the maximum if all
5 stages use the same slots (because there are only 15). The spec is
pretty clear about it:

"If an atomic counter buffer is used by multiple shaders, each such
use counts separately against this combined limit. The combined atomic
counter buffer use limit can be obtained by calling GetIntegerv with a
<pname> of MAX_COMBINED_ATOMIC_COUNTER_BUFFERS"

Marek