[clang] [llvm] [DirectX] Start documenting DXIL Resource handling (PR #90553)

[Justin Bogner via llvm-commits]

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+======================
+DXIL Resource Handling
+======================
+
+.. contents::
+   :local:
+
+.. toctree::
+   :hidden:
+
+Introduction
+============
+
+Resources in DXIL are represented via ``TargetExtType`` in LLVM IR and
+eventually lowered by the DirectX backend into metadata in DXIL.
+
+In DXC and DXIL, static resources are represented as lists of SRVs (Shader
+Resource Views), UAVs (Uniform Access Views), CBVs (Constant Bffer Views), and
+Samplers. This metadata consists of a "resource record ID" which uniquely
+identifies a resource and type information. As of shader model 6.6, there are
+also dynamic resources, which forgo the metadata and are described via
+``annotateHandle`` operations in the instruction stream instead.
+
+In LLVM we attempt to unify some of the alternative representations that are
+present in DXC, with the aim of making handling of resources in the middle end
+of the compiler simpler and more consistent.
+
+Resource Type Information and Properties
+========================================
+
+There are a number of properties associated with a resource in DXIL.
+
+`Resource ID`
+   An arbitrary ID that must be unique per resource type (SRV, UAV, etc).
+
+   In LLVM we don't bother representing this, instead opting to generate it at
+   DXIL lowering time.
+
+`Binding information`
+   Information about where the resource comes from. This is either (a) a
+   binding space, lower bound in that space, and size of the binding, or (b) an
+   index into a dynamic resource heap.
+
+   In LLVM we represent binding information in the arguments of the
+   :ref:`handle creation intrinsics <dxil-resources-handles>`. When generating
+   DXIL we transform these calls to metadata, ``dx.op.createHandle``,
+   ``dx.op.createHandleFromBinding``, ``dx.op.createHandleFromHeap``, and
+   ``dx.op.createHandleForLib`` as needed.
+
+`Type information`
+   The type of data that's accessible via the resource. For buffers and
+   textures this can be a simple type like ``float`` or ``float4``, a struct,
+   or raw bytes. For constant buffers this is just a size. For samplers this is
+   the kind of sampler.
+
+   In LLVM we embed this information as a parameter on the ``target()`` type of
+   the resource. See :ref:`dxil-resources-types-of-resource`.
+
+`Resource kind information`
+   The kind of resource. In HLSL we have things like ``ByteAddressBuffer``,
+   ``RWTexture2D``, and ``RasterizerOrderedStructuredBuffer``. These map to a
+   set of DXIL kinds like ``RawBuffer`` and ``Texture2D`` with fields for
+   certain properties such as ``IsUAV`` and ``IsROV``.
+
+   In LLVM we represent this in the ``target()`` type. We omit information
+   that's deriveable from the type information, but we do have fields to encode
+   ``IsWriteable``, ``IsROV``, and ``SampleCount`` when needed.
+
+.. note:: TODO: There are two fields in the DXIL metadata that are not
+   represented as part of the target type: ``IsGloballyCoherent`` and
+   ``HasCounter``.
+
+   Since these are derived from analysis, storing them on the type would mean
+   we need to change the type during the compiler pipeline. That just isn't
+   practical. It isn't entirely clear to me that we need to serialize this info
+   into the IR during the compiler pipeline anyway - we can probably get away
+   with an analysis pass that can calculate the information when we need it.
+
+   If analysis is insufficient we'll need something akin to ``annotateHandle``
+   (but limited to these two properties) or to encode these in the handle
+   creation.
+
+.. _dxil-resources-types-of-resource:
+
+Types of Resource
+=================
+
+We define a set of ``TargetExtTypes`` that is similar to the HLSL
+representations for the various resources, albeit with a few things
+parameterized. This is different than DXIL, as simplifying the types to
+something like "dx.srv" and "dx.uav" types would mean the operations on these
+types would have to be overly generic.
+
+Samplers
+--------
+
+.. code-block:: llvm
+
+   target("dx.Sampler", SamplerType)
+
+The "dx.Sampler" type is used to represent sampler state. The sampler type is
+an enum value from the DXIL ABI, and these appear in sampling operations as
----------------
bogner wrote:

Other than a different but very similar enum I'm not sure how we would abstract this. FWIW, the set of sampler types is pretty small:

```c++
enum class SamplerKind : unsigned {
  Default = 0,
  Comparison,
  Mono,
  Invalid,
};
```

https://github.com/llvm/llvm-project/pull/90553