[llvm] [HLSLSemanticSignatures] Implement the stacked packing of elements (PR #218060)

[Finn Plummer via llvm-commits]

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@@ -125,3 +125,51 @@ The following container fields are derived from the operands above:
 
 A metadata node of one or more semantic indices. Its length must equal the
 `Rows` field of the containing signature element.
+
+## Signature Packing
+
+Before a semantic signature is serialized, each element that participates in
+packing is assigned a location in a fixed register space of 32 rows and 4
+columns. An element occupies a rectangle of `Rows` consecutive registers and
+`Cols` consecutive components. Its allocated location is recorded in
+`StartRow` and `StartCol`.
+
+The packing helper classifies each element from its semantic kind, shader stage,
+and I/O type. Elements with the `NotAllocated` interpretation are accessed by
+other means and retain the unallocated row and column sentinels. The remaining
+interpretations accepted by a packing algorithm are assigned locations
+according to that algorithm's rules. If an eligible element cannot be placed,
+packing returns a `SignaturePackingError` identifying the element that failed.
+
+The packing APIs and their in-memory element representation are declared in
+[SemanticSignaturePacking.h].
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inbelic wrote:

Updated to clarify

https://github.com/llvm/llvm-project/pull/218060