[clang] [Doc][HLSL] Add documentation for root signature (PR #88781)

[Damyan Pepper via cfe-commits]

================
@@ -0,0 +1,291 @@
+====================
+HLSL Root Signatures
+====================
+
+.. contents::
+   :local:
+
+Usage
+=====
+
+In HLSL, the `root signature
+<https://learn.microsoft.com/en-us/windows/win32/direct3d12/root-signatures>`_
+defines what types of resources are bound to the graphics pipeline.
+
+A root signature can be specified in HLSL as a `string
+<https://learn.microsoft.com/en-us/windows/win32/direct3d12/specifying-root-signatures-in-hlsl#an-example-hlsl-root-signature>`_.
+The string contains a collection of comma-separated clauses that describe root
+signature constituent components.
+
+There are two mechanisms to compile an HLSL root signature. First, it is
+possible to attach a root signature string to a particular shader via the
+RootSignature attribute (in the following example, using the main entry
+point):
+
+.. code-block::
+
+    #define RS "RootFlags( ALLOW_INPUT_ASSEMBLER_INPUT_LAYOUT | " \
+              "DENY_VERTEX_SHADER_ROOT_ACCESS), " \
+              "CBV(b0, space = 1, flags = DATA_STATIC), " \
+              "SRV(t0), " \
+              "UAV(u0), " \
+              "DescriptorTable( CBV(b1), " \
+              "                 SRV(t1, numDescriptors = 8, " \
+              "                     flags = DESCRIPTORS_VOLATILE), " \
+              "                 UAV(u1, numDescriptors = unbounded, " \
+              "                     flags = DESCRIPTORS_VOLATILE)), " \
+              "DescriptorTable(Sampler(s0, space=1, numDescriptors = 4)), " \
+              "RootConstants(num32BitConstants=3, b10), " \
+              "StaticSampler(s1)," \
+              "StaticSampler(s2, " \
+              "              addressU = TEXTURE_ADDRESS_CLAMP, " \
+              "              filter = FILTER_MIN_MAG_MIP_LINEAR )"
+
+    [RootSignature(MyRS)]
+    float4 main(float4 coord : COORD) : SV_Target
+    {
+    …
+    }
+
+The compiler will create and verify the root signature blob for the shader and
----------------
damyanp wrote:

I wonder if it'd be helpful to show the runtime side of this?  Just by reading this, I think it's hard to understand what a root signature is, why I'd need to specify one, and what I'd do with one once I'd specified it.

https://github.com/llvm/llvm-project/pull/88781