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[DirectX] Documenting Root Signature Binary representation (#131011)
Closes: #131009, #129796, #129749, #129561 --------- Co-authored-by: joaosaffran <[email protected]>
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‎llvm/docs/DirectX/DXContainer.rst‎

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

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