// Copyright (c) 2016-2017 Khronos Group. This work is licensed under a
// Creative Commons Attribution 4.0 International License; see
// http://creativecommons.org/licenses/by/4.0/

include::meta/VK_KHR_variable_pointers.txt[]

*Last Modified Date*::
    2017-03-14
*IP Status*::
    No known IP claims.
*Interactions and External Dependencies*::
  - Requires the
    https://www.khronos.org/registry/spir-v/extensions/KHR/SPV_KHR_variable_pointers.html[+SPV_KHR_variable_pointers+]
    SPIR-V extension.
*Contributors*::
  - John Kessenich, Google
  - Neil Henning, Codeplay
  - David Neto, Google
  - Daniel Koch, Nvidia
  - Graeme Leese, Broadcom
  - Weifeng Zhang, Qualcomm
  - Stephen Clarke, Imagination Technologies
  - Jason Ekstrand, Intel
  - Jesse Hall, Google

The +VK_KHR_variable_pointers+ extension allows implementations to indicate
their level of support for the +SPV_KHR_variable_pointers+ SPIR-V extension.
The SPIR-V extension allows shader modules to use invocation-private
pointers into uniform and/or storage buffers, where the pointer values can
be dynamic and non-uniform.

The +SPV_KHR_variable_pointers+ extension introduces two capabilities.
The first, code:VariablePointersStorageBuffer, must: be supported by all
implementations of this extension.
The second, code:VariablePointers, is optional.

=== New Enum Constants

  * Extending elink:VkStructureType:
  ** ename:VK_STRUCTURE_TYPE_PHYSICAL_DEVICE_VARIABLE_POINTER_FEATURES_KHR

=== New Structures

  * slink:VkPhysicalDeviceVariablePointerFeaturesKHR

=== New SPIR-V Capabilities

  * <<spirvenv-capabilities-table-variablepointers,VariablePointersStorageBuffer>>
  * <<spirvenv-capabilities-table-variablepointers,VariablePointers>>

=== Issues

1) Do we need an optional property for the SPIR-V
code:VariablePointersStorageBuffer capability or should it be mandatory when
this extension is advertised?

*RESOLVED*: Add it as a distinct feature, but make support mandatory.
Adding it as a feature makes the extension easier to include in a future
core API version.
In the extension, the feature is mandatory, so that presence of the
extension guarantees some functionality.
When included in a core API version, the feature would be optional.

2) Can support for these capabilities vary between shader stages?

*RESOLVED*: No, if the capability is supported in any stage it must be
supported in all stages.

3) Should the capabilities be features or limits?

*RESOLVED*: Features, primarily for consistency with other similar
extensions.

=== Version History

  * Revision 1, 2017-03-14 (Jesse Hall and John Kessenich)
    - Internal revisions