Constant Sampler in shaders

[Karpov-Ruslan]

The Metal shader language allows the use of compile-time samplers. Thanks to them, it is possible not to rely on the state of the samplers from the client API, and the GPU can optimize texture sampling operations in advance. An extension for SPIR-V would allow to take advantage of the listed features when using various client graphics APIs. However, there are several ways to implement such an extension in my opinion:

1. Option №1: Using the OpConstantSampler instruction. This instruction is already provided in SPIR-V and is a direct analogue of compile-time samplers from the Metal shader language in terms of a set of parameters. The only drawback is the lack of the ability to fine-tune the sampler. For example, you cannot use different magnification and minification filters, and there is no border color setting.

2. Option №2: Create new instruction(-s) in which fine-tuning of the sampler will be available. This is feasible, since most of the sampler settings are the selection of one parameter from a set (for example, the filter is either the closest, linear, or cubic, but we only choose one thing), so some sampler parameters can be merged into a single instruction parameter.

3. Option №3: Using the decoration instructions. Each sampler parameter will have its own decoration, so it will also be possible to fine-tune the sampler parameters.

Is it possible to implement such an extension in SPIR-V? From my point of view, since OpenCL supports constant samplers, then there should be no problems for graphics APIs either, especially as part of the extension.

P.S. I am expanding this question to Vulkan and GLSL.
For GLSL, in my opinion, it is worth using layout qualifiers to specify the sampler parameters and instead of using the uniform keyword, use const to emphasize the constancy of the sampler.