[Mesa-dev] [RFC PATCH 03/26] glsl: introduce new base types for bindless samplers/images

[Samuel Pitoiset]

On 04/18/2017 11:26 PM, Nicolai Hähnle wrote:
> On 18.04.2017 21:49, Dave Airlie wrote:
>> On 19 April 2017 at 05:30, Samuel Pitoiset <samuel.pitoiset at gmail.com> 
>> wrote:
>>>
>>>
>>> On 04/18/2017 08:14 PM, Nicolai Hähnle wrote:
>>>>
>>>> On 11.04.2017 18:48, Samuel Pitoiset wrote:
>>>>>
>>>>> Bindless sampler/image types are really different from the existing
>>>>> sampler/image types. They are considered 64-bit unsigned integers,
>>>>> they can be declared as temporary, shader inputs/outputs and are
>>>>> non-opaque types.
>>>>>
>>>>> For these reasons, it looks more convenient to introduce new
>>>>> internal base types to the GLSL compiler, called
>>>>> GLSL_TYPE_BINDLESS_SAMPLER and respectively GLSL_TYPE_BINDLESS_IMAGE.
>>>>
>>>>
>>>> Sorry for taking so long to get to this series, but could you 
>>>> explain the
>>>> rationale here a bit more?
>>>
>>>
>>> No worries, all feedbacks are always welcome, even late. :)
>>>
>>> So, the bindless sampler/image types introduced in 
>>> ARB_bindless_texture are
>>> really different from the "standard" ones.
>>>
>>> They are considered as 64-bit unsigned integers (not 32-bit) and they 
>>> are
>>> non-opaque types. The latter means they can be declared as temporary
>>> variables, as shader inputs/outputs, inside an interface block, etc.
> 
> I don't know that that's the best way to think about it at the GLSL 
> level. Mostly they're still opaque types, it's just that they can be 
> explicitly converted to/from 64-bit ints (or uvec2... not having an 
> interaction with GL_ARB_gpu_shader_int64 seems like a spec oversight).

Well, it's not *only* about the explicit conversions, 
ARB_bindless_texture allows more flexibility for sampler/image types.

An oversight? Unfortunately, it looks like to me it's not the only one...

> 
> 
>>> That said, the current sampler/image types are opaque (cf,
>>> glsl_type::is_opaque()) and it seemed quite impossible to change the
>>> glsl_type helpers to fit my needs.
> 
> I see no is_opaque, maybe you mean contains_opaque? I agree that it's 
> annoying that the restrictions expressed in ast_to_hir.cpp need to be 
> modified. What other helpers are an issue?

Yes, contains_opaque() not is_opaque(). Well, how do you plan to handle 
the fact that bindless sampler types are 64-bit? It seemed quite logical 
to make glsl_type::is_64bit() returns true for them.

I don't remember all the restrictions to be honest (but they are many 
others) because this solution has been adopted for weeks. But the way 
the compiler is implemented, doing the other solution *really* need a 
ton of changes.

> 
> 
>>> I tried many different solutions before figuring this one which seems 
>>> better
>>> for some reasons:
>>>
>>> - easy to make bindless sampler/image types 64-bit unsigned int
>>> - easy to make bindless sampler/image types non-opaque
>>> - should avoid breakage because the base type is different
>>> - reduce the amount of changes in most places in the compiler
> 
> I still don't see a *positive* justification for having the two 
> different type families. Where do you actually need to *distinguish* 
> between bindless and bound samplers in the compiler? The spec really 
> doesn't make that distinction at all, except in one single place:
> 
>     "These modifiers control whether default-block uniforms of
>      the corresponding types may have their values set via both
>      UniformHandle* and Uniform1i (bindless_sampler and
>      bindless_image) or only via Uniform1i (bound_sampler and
>      bound_image)."
> 
> It seems like the only place to distinguish between the two is actually 
> *outside* the compiler, in the uniforms API. And that's not even really 
> have about the *type* of the variable; it's about the variable's memory 
> layout. So it really behaves more similarly to the layout qualifiers 
> (offset, alignment) you have in SSBO and UBO blocks rather than a 
> property of a type.
> 
> So as I see it, this code is messing with the type system for no good 
> reason, and doing so in a fundamentally incoherent way that goes against 
> all that is good and sane in compiler design. And I think it shows. 
> Consider the following example shader:
> 
>     layout (bound_sampler) uniform sampler2D tex_bound;
>     layout (bindless_sampler) uniform sampler2D tex_bindless;
> 
>     vec4 f(sampler2D s, ...)
>     {
>        return texture2D(s, ...);
>     }
> 
>     void main()
>     {
>        vec4 a = f(tex_bound, ...);
>        vec4 b = f(tex_bindless, ...);
>     }
> 
> What's the type of the parameter `s' of function `f'? Will this compile, 
> and if so, how?

Yeah, this should compile. f() has to be duplicated, one with bound, the 
other one with bindless, that's it. If we do agree about the function 
explosion as Ilia said, it's all good.

> 
> I guess the answer is: as is, the type is sampler2DBindless, which makes 
> no sense whatsoever if the function f appears in a shader which 
> *doesn't* enable ARB_bindless_texture (which is allowed!). But having 
> the type of f depend on whether the extension is enabled makes even 
> *less* sense.

How? bindless_sampler/bound_sampler are *not* allowed if 
ARB_bindless_texture is not enabled. This can't happen.

> 
> And does it compile? Well, I don't recall seeing code that would 
> automatically do a type conversion -- you probably either end up 
> generating an incorrect compiler error, or you end up with IR that 
> assigns a value of bindless-type to a variable of non-bindless-type, or 
> vice versa.
> 
> None of this gives me confidence from a compiler design perspective. 
> What's left are some vague implementation concerns that I don't 
> understand yet, admittedly in part because I haven't tried to implement 
> the alternative :-)

Yeah. :)

I tried the alternative(s). They have different pros/cons, no one looks 
"perfect".

By the way, I have asked like 2 months ago about this concern on the 
list, no answer. Thus, I discussed with Dave, Timothy and Ilia over IRC 
and the solution happened.

https://lists.freedesktop.org/archives/mesa-dev/2017-February/145541.html

> 
> 
>>> At the Gallium level, the changes are really small. Basically, if the 
>>> type
>>> is a bindless sampler, the ir_dereference variable is visited and it 
>>> can be
>>> considered as PROGRAM_UNIFORM or PROGRAM_TEMPORARY (for shader
>>> inputs/outputs).
>>>
>>> Hopefully, you are convinced now. :)
>>
>> When I did my initial implementation I went with merged types, I don't 
>> think
>> it was a good idea either. Apart from making old non-bindless samples 
>> take up
>> twice as much space, there's a lot or corner cases.
> 
> Non-bindless samplers taking up twice as much space for the accounting 
> of the default uniform block is something the spec explicitly calls out 
> as being okay, though.
> 
> Even if we decide that it isn't, it's really something that belongs into 
> struct gl_uniform_storage during linking, because it's clearly a 
> property of... uniform storage layout.
> 
> 
>> Unless we hit some problem in the future where they need to be the same
>> type I'd say this is preferable.
> 
> I hope the code sample above is convincing evidence that using different 
> types for the same thing just makes no sense.
> 
> Cheers,
> Nicolai
> 
> 
>>
>> Dave.
>>
> 
>