[DirectX][docs] Expand DXILArchitecture documentation (#84077)

This just seeks to give a high level overview of the current state of
the DirectX backend. It doesn't go in too deep, but should be a starting
point for further documentation.

---------

Co-authored-by: S. Bharadwaj Yadavalli <[email protected]>

Author: llvm-beanz

llvm/docs/DirectX/DXILArchitecture.rst

@@ -89,6 +89,52 @@ leverage LLVM's current bitcode libraries to do a lot of the work, but
 it's possible that at some point in the future it will need to be
 completely separate as modern LLVM bitcode evolves.
 
+DirectX Backend Flow
+--------------------
+
+The code generation flow for DXIL is broken into a series of passes. The passes
+are grouped into two flows:
+
+#. Generating DXIL IR.
+#. Generating DXIL Binary.
+
+The passes to generate DXIL IR follow the flow:
+
+  DXILOpLowering -> DXILPrepare -> DXILTranslateMetadata
+
+Each of these passes has a defined responsibility:
+
+#. DXILOpLowering translates LLVM intrinsic calls to dx.op calls.
+#. DXILPrepare transforms the DXIL IR to be compatible with LLVM 3.7, and
+   inserts bitcasts to allow typed pointers to be inserted.
+#. DXILTranslateMetadata emits the DXIL Metadata structures.
+
+The passes to encode DXIL to binary in the DX Container follow the flow:
+
+  DXILEmbedder -> DXContainerGlobals -> AsmPrinter
+
+Each of these passes have the following defined responsibilities:
+
+#. DXILEmbedder runs the DXIL bitcode writer to generate a bitcode stream and
+   embeds the binary data inside a global in the original module.
+#. DXContainerGlobals generates binary data globals for the other DX Container
+   parts based on computed analysis passes.
+#. AsmPrinter is the standard LLVM infrastructure for emitting object files.
+
+When emitting DXIL into a DX Container file the MC layer is used in a similar
+way to how the Clang ``-fembed-bitcode`` option operates. The DX Container
+object writer knows how to construct the headers and structural fields of the
+container, and reads global variables from the module to fill in the remaining
+part data.
+
+DirectX Container
+-----------------
+
+The DirectX container format is treated in LLVM as an object file format.
+Reading is implemented between the BinaryFormat and Object libraries, and
+writing is implemented in the MC layer. Additional testing and inspection
+support are implemented in the ObjectYAML library and tools.
+
 Testing
 =======