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

[Justin Bogner via llvm-commits]

================
@@ -0,0 +1,799 @@
+======================
+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
----------------
bogner wrote:

It's the "space" in `register(t3,  space0)`. From https://learn.microsoft.com/en-us/windows/win32/direct3d12/resource-binding-in-hlsl:

> The space keyword specifies to which logical register space the declared variable is bound. If the space keyword is omitted, then the default space index of 0 is implicitly assigned to the range (so the tex2 range above resides in space0). register(t3, space0) will never conflict with register(t3, space1), nor with any array in another space that might include t3.

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