[DirectX] Documenting Root Signature Binary representation

llvm/docs/DirectX/DXContainer.rst

@@ -111,7 +111,7 @@ FXC are marked with \*.
 #. `PSV0`_ - Stores Pipeline State Validation data.
 #. RDAT† - Stores Runtime Data.
 #. RDEF\* - Stores resource definitions.
-#. RTS0 - Stores compiled root signature.
+#. `RTS0`_ - Stores compiled root signature.
 #. `SFI0`_ - Stores shader feature flags.
 #. SHDR\* - Stores compiled DXBC bytecode.
 #. SHEX\* - Stores compiled DXBC bytecode.
@@ -393,6 +393,187 @@ bit in the mask vector identifies one column of a patch constant input and a
 column of an output. A value of 1 means the output is impacted by the primitive
 input.
 
+Root Signature (RTS0) Part
+--------------------------
+.. _RTS0:
+
+The Root Signature data defines the shader's resource interface with Direct3D 
+12, specifying what resources the shader needs to access and how they're 
+organized and bound to the pipeline. 
+
+The RTS0 part comprises three data structures: ``RootSignatureHeader``, 
+``RootParameters`` and ``StaticSamplers``. The details of each will be described 
+in the following sections. All ``RootParameters`` will be serialized following 
+the order they were defined in the metadata representation.
+
+The table below summarizes the data being serialized as well as it's size. The 
+details of it part will be discussed in further details on the next sections 
+of this document.
+
+======================== =========================================== =============================
+Part Name                Size In Bytes                                 Maximum number of Instances 
+======================== =========================================== =============================
+Root Signature Header    24                                          1               
+Root Parameter Headers   12                                          Many              
+Root Parameter           ================================ ===        Many
+                         Root Constants                   12                                      
+                         Root Descriptor Version 1.0      8                                      
+                         Root Descriptor Version 1.1      12                                      
+                         Descriptors Tables Version 1.0   20                                     
+                         Descriptors Tables Version 1.1   24                                      
+                         ================================ ===       
+
+Static Samplers          52                                          Many              
+======================== =========================================== =============================
+
+
+Root Signature Header
+~~~~~~~~~~~~~~~~~~~~~
+
+The root signature header is 24 bytes long, consisting of six 32 bit values 
+representing the version, number and offset of parameters, number and offset 
+of static samplers, and a flags field for global behaviours:
+
+.. code-block:: c
+
+   struct RootSignatureHeader {
+     uint32_t Version;
+     uint32_t NumParameters;
+     uint32_t ParametersOffset;
+     uint32_t NumStaticSamplers;
+     uint32_t StaticSamplerOffset;
+     uint32_t Flags;
+   }
+
+
+Root Parameters
+~~~~~~~~~~~~~~~
+
+Root parameters define how resources are bound to the shader pipeline, each 
+type having different size and fields. 
+
+The slot of root parameters is preceded by a variable size section containing 
+the header information for such parameters. Such structure is 12 bytes long, 
+composed of three 32 bit values, representing the parameter type, a flag 
+encoding the pipeline stages where the data is visible, and an offset 
+calculated from the start of RTS0 section.
+
+.. code-block:: c
+
+   struct RootParameterHeader {
+     uint32_t ParameterType;
+     uint32_t ShaderVisibility;
+     uint32_t ParameterOffset;
+   };
+
+After the header information has been serialized, the actual data for each of the
+root parameters is layout in a single continous blob. The parameters can be fetch 
+from such using the offset information, present in the header.
+
+The following sections will describe each of the root parameters types and their 
+encodings.
+
+Root Constants
+''''''''''''''
+
+The root constants are inline 32-bit values that show up in the shader 
+as a constant buffer. It is a 12 bytes long structure, two 32 bit values 
+encoding the register and space the constant is assigned to, and 
+the last 32 bits encode the number of constants being defined in the buffer.
+
+.. code-block:: c
+
+  struct RootConstants {
+    uint32_t Register;
+    uint32_t Space;
+    uint32_t NumOfConstants;
+  };
+
+Root Descriptor
+'''''''''''''''
+
+Root descriptors provide direct GPU memory addresses to resources.
+
+In version 1.0, the root descriptor is 8 bytes. It encodes the register and
+space as 2 32-bit values.
+
+In version 1.1, the root descriptor is 12 bytes. It matches the 1.0 descriptor
+but adds a 32-bit access flag.
+
+.. code-block:: c
+
+   struct RootDescriptor_V1_0 {
+      uint32_t ShaderRegister;
+      uint32_t RegisterSpace;
+   };
+
+   struct RootDescriptor_V1_1 {
+      uint32_t ShaderRegister;
+      uint32_t RegisterSpace;      
+      uint32_t Flags;
+   };
+
+Root Descriptor Table
+'''''''''''''''''''''
+
+Descriptor tables let shaders access multiple resources through a single pointer 
+to a descriptor heap. 
+
+The tables are made of a collection of descriptor ranges. In Version 1.0, the 
+descriptor range is 20 bytes, containing five 32 bit values. It encodes a range 
+of registers, including the register type, range length, register numbers and 
+space within range and the offset locating each range inside the table.
+
+In version 1.1, the descriptor range is 24 bytes. It matches the 1.0 descriptor
+but adds a 32-bit access flag.
+
+.. code-block:: c
+
+   struct DescriptorRange_V1_0 {
+      uint32_t RangeType;
+      uint32_t NumDescriptors;
+      uint32_t BaseShaderRegister;
+      uint32_t RegisterSpace;
+      uint32_t OffsetInDescriptorsFromTableStart;
+   };
+
+   struct DescriptorRange_V1_1 {
+      dxbc::DescriptorRangeType RangeType;
+      uint32_t NumDescriptors;
+      uint32_t BaseShaderRegister;
+      uint32_t RegisterSpace;
+      uint32_t OffsetInDescriptorsFromTableStart;      
+      uint32_t Flags;
+   };
+
+Static Samplers
+~~~~~~~~~~~~~~~
+
+Static samplers are predefined filtering settings built into the root signature, 
+avoiding descriptor heap lookups. 
+
+This section also has a variable size, since it can contain multiple static 
+samplers definitions. However, the definition is a fixed sized struct, 
+containing 13 32-byte fields of various enum, float, and integer values. 
+
+.. code-block:: c
+
+   struct StaticSamplerDesc {
+      FilterMode Filter; 
+      TextureAddressMode AddressU;
+      TextureAddressMode AddressV;
+      TextureAddressMode AddressW;
+      float MipLODBias;
+      uint32_t MaxAnisotropy;
+      ComparisonFunc ComparisonFunc; 
+      StaticBorderColor BorderColor;
+      float MinLOD;
+      float MaxLOD;
+      uint32_t ShaderRegister;
+      uint32_t RegisterSpace;
+      ShaderVisibility ShaderVisibility;
+   };
+
 SFI0 Part
 ---------
 .. _SFI0: