Add proposal for scalar layout for constant buffers

proposals/0023-scalar-constant-buffer-layout.md

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+# Scalar constant buffer layout
+
+* Proposal: [0023](0023-scalar-constant-buffer-layout.md)
+* Author(s): [Tobias Hector](https://github.com/tobski)
+* Sponsor: Chris Bieneman
+* Status: **Under Consideration**
+
+
+## Introduction
+
+This proposal adds the ability to define constant buffers with alignment
+defined only by the scalar components of its structure members.
+This allows layout compatibility with native C/C++ structure layouts, where
+there is no native vector type.
+This functionality already exists in Vulkan via the
+[VK_EXT_scalar_block_layout](https://registry.khronos.org/vulkan/specs/1.3-extensions/man/html/VK_EXT_scalar_block_layout.html)
+extension, but not yet in DirectX.
+
+
+## Motivation
+
+When generating data to be passed to a shader from the CPU programming
+language, applications need to be very deliberate with how that data is packed
+if they use vector and array types in HLSL.
+The packing rules for constant buffers are described here:
+https://learn.microsoft.com/en-us/windows/win32/direct3dhlsl/dx-graphics-hlsl-packing-rules.
+The gist is that most things are aligned to either 4 or 16 bytes, but members
+are packed down to a smaller alignment where possible, such that information
+about all elements is necessary to determine the final layout.
+
+
+## Proposed solution
+
+Given that scalar block layout is now widely supported across all recent
+hardware in Vulkan, this document proposes adding a new `bufferlayout`
+attribute that takes one of two values; either `legacy`, matching the current
+layout constraints, or the new `scalar` layout.
+The new `scalar` layout aligns structure members to the size of the scalar
+element of each type, with no additional alignment for arrays, vectors, or
+matrices; effectively matching standard C/C++ memory layouts.
+
+The new attribute would have a default value of `legacy` if not specified,
+but this could be changed with a command line option.
+A future version of HLSL may choose to make `scalar` the default, or even
+remove the `legacy` path altogether.
+
+
+## Detailed design
+
+*The detailed design is not required until the feature is under review.*
+
+This section should grow into a feature specification that will live in the
+specifications directory once complete. Each feature will need different levels
+of detail here, but some common things to think through are:
+
+### HLSL Additions
+
+* How is this feature represented in the grammar?
+* How does it interact with different shader stages?
+* How does it work interact other HLSL features (semantics, buffers, etc)?
+* How does this interact with C++ features that aren't already in HLSL?
+* Does this have implications for existing HLSL source code compatibility?
+
+### Interchange Format Additions
+
+* What DXIL changes does this change require?
+* What Metadata changes does this require?
+* How will SPIRV be supported?
+
+### Diagnostic Changes
+
+* What additional errors or warnings does this introduce?
+* What existing errors or warnings does this remove?
+
+#### Validation Changes
+
+* What additional validation failures does this introduce?
+* What existing validation failures does this remove?
+
+### Runtime Additions
+
+#### Runtime information
+
+* What information does the compiler need to provide for the runtime and how?
+
+#### Device Capability
+
+* How does it interact with other Shader Model options?
+* What shader model and/or optional feature is prerequisite for the bulk of
+  this feature?
+* What portions are only available if an existing or new optional feature
+  is present?
+* Can this feature be supported through emulation or some other means
+  in older shader models?
+
+## Testing
+
+* How will correct codegen for DXIL/SPIRV be tested?
+* How will the diagnostics be tested?
+* How will validation errors be tested?
+* How will validation of new DXIL elements be tested?
+* How will the execution results be tested?
+
+## Transition Strategy for Breaking Changes (Optional)
+
+As the default layout of buffers is not changing without user intervention,
+there should be no breakages introduced by this change.
+What may break things is if the default layout is ever changed by a new HLSL
+version; but while that seems likely, whether and how that happens is outside
+of the scope of this proposal.
+
+If the new attribute is specified without targeting the new shader model, the
+compiler should raise an error directing the user to use a later shader model.
+
+
+## Alternatives considered (Optional)
+
+One alternative method to solve this issue would be to add something similar
+to the `#pragma pack` directive in C/C++, but this would need to be set on all
+members, and requires a more complex implementation to be fully supported,
+given the push/pop variants supported by compilers.
+It may still be worthwhile to support something like this for more control over
+ layouts, but that can be considered as a separate feature.
+
+
+## Acknowledgments (Optional)
+
+ - Chris Bieneman
+ - Jesse Natalie
+ - Contributors to
+   [VK_EXT_scalar_block_layout](https://registry.khronos.org/vulkan/specs/1.3-extensions/man/html/VK_EXT_scalar_block_layout.html)
+
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