[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
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damyanp wrote:

I've always called those "register spaces" (and it seems that's what the linked doc calls them as well).

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