Development support for future VK_KHR_portability_subset_metal extension.

[billhollings]

• fetchDependencies script pulls VK_KHR_portability_subset_metal content from private Khronos repo.
• Add VK_KHR_portability_subset_metal extension.
• Include VkPhysicalDevicePortabilitySubsetMetalFeaturesKHR in pNext of VkPhysicalDeviceFeatures2.

[billhollings]

GitHub CI is failing because it doesn't have access to the private Khronos repo. This is expected.

[alyssarosenzweig]

shaderImageGatherExtendedConstOffsets

This is straightforward to lower, as most native drivers do? https://gitlab.freedesktop.org/mesa/mesa/-/blob/main/src/compiler/nir/nir_lower_tex.c?ref_type=heads#L1253

[cdavis5e]

shaderImageGatherExtendedConstOffsets

This is straightforward to lower, as most native drivers do? https://gitlab.freedesktop.org/mesa/mesa/-/blob/main/src/compiler/nir/nir_lower_tex.c?ref_type=heads#L1253

Sadly, not as straightforward as one would like. It's simple enough to expand it to four texture::read() calls, but the problem is handling sampler coordinate wrapping. We can't introspect the sampler object for its wrap modes, so we'd have to pass this information alongside the sampler. This becomes particularly problematic when not using argument buffers, which we don't at all by default.

[alyssarosenzweig]

shaderImageGatherExtendedConstOffsets

This is straightforward to lower, as most native drivers do? https://gitlab.freedesktop.org/mesa/mesa/-/blob/main/src/compiler/nir/nir_lower_tex.c?ref_type=heads#L1253

Sadly, not as straightforward as one would like. It's simple enough to expand it to four texture::read() calls, but the problem is handling sampler coordinate wrapping. We can't introspect the sampler object for its wrap modes, so we'd have to pass this information alongside the sampler. This becomes particularly problematic when not using argument buffers, which we don't at all by default.

See the linked lowering. You turn it into 4 gathers

[billhollings]

See the linked lowering. You turn it into 4 gathers

Thanks for the suggestion, and example. I've added it to the full issue discussion.

[billhollings]

See the linked lowering. You turn it into 4 gathers

Thanks for the suggestion, and example. I've added it to the full issue discussion.

SPIRV-Cross PR #2325 adds support for image gather with ConstOffsets.