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

[Damyan Pepper 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
----------------
damyanp wrote:

> The sample type is an enum value from the DXIL ABI

Do we want to tie this so closely to the DXIL ABI?  Would an extra level of indirection here allow LLVM and DXIL to evolve independently?

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