[DirectX][DXIL] Design document for TableGen Spec of DXIL Operations (llvm#85170)

Add an initial design document for TableGen specification of DXIL Operations.

Author: bharadwajy

llvm/docs/DirectX/DXILOpTableGenDesign.rst

@@ -0,0 +1,160 @@
+==============================================================
+Specification of DXIL Operations using TableGen Representation
+==============================================================
+.. contents::
+   :local:
+
+.. toctree
+   :hidden
+
+Introduction
+============
+
+`DirectXShaderCompiler <https://github.com/microsoft/DirectXShaderCompiler>`_
+encapsulates, among other information, various DXIL Operations in
+`hctdb.py <https://github.com/microsoft/DirectXShaderCompiler/blob/main/utils/hct/hctdb.py>`_.
+DXIL Operations are represented in one of the following `two ways
+<https://github.com/microsoft/DirectXShaderCompiler/blob/130877392c263888ef06bab768856d3dab1f1c9a/docs/DXIL.rst#L1978>`_:
+
+#. Using LLVM instructions.
+#. Using LLVM External functions. These are represented in LLVM IR as follows:
+   * "Standard" LLVM intrinsics (e.g., ``llvm.sin.*``) and
+   * HLSL intrinsics (defined as LLVM intrinsics in ``llvm/include/llvm/IR/IntrinsicsDirectX.td``, e.g., ``llvm.dx.*``)
+
+   These are  collectively referred to as `LLVM Intrinsics` in this note.
+
+Following is the complete list of properties of DXIL Ops with the corresponding field name
+as used in ``hctdb.py``. A DXIL Op is represented by a set of associated properties. These
+are categorized into two groups - viz., those that are (1) consumed in DXIL backend passes
+and (2) consumed in other usage scenarios such as validation, DXIL reader, etc.
+
+A. Properties consumed in DXIL backend passes
+
+   1. Name of operation (``dxil_op``)
+   2. The generic or HLSL-specific intrinsic that maps to the operation (``llvm_name``).
+   3. Unique Integer ID (``dxil_opid``)
+   4. Operation Class signifying the name and function signature of the operation (``dxil_class``).
+      This string is an integral part of the DXIL Op function name and is constructed in
+      the format ``dx.op.<class-name>.<overload-type>``. The DXIL validator checks for any
+      deviation from this for each of the DXIL Op call.
+   5. List of valid overload types for the operation (``oload_types``).
+   6. Required DXIL Version with support for the operation.
+   7. A string that documents the operation (``doc``) - This is not strictly necessary but is included
+      for readability and documentation of the operation.
+
+B. Properties consumed in other usage scenarios
+
+   1. Required minimum Shader Model (``shader_model``).
+   2. Minimum shader model required with translation by linker (``shader_model_translated``)
+   3.  List of shader stages applicable to (``shader_stages``), empty for all.
+   4.  Memory access attributes of the operation (``fn_attr``).
+   5.  Boolean attributes of operation to indicate if it
+
+       * is some kind of a derivative (``is_derivative``)
+       * requires gradient calculation (``is_gradient``)
+       * is a sampler feedback (``is_feedback``)
+       * requires in-wave, cross-lane functionality (``is_wave``)
+       * requires that all of its inputs are uniform across the wave (``requires_uniform_inputs``).
+       * is a barrier operation (``is_barrier``).
+
+Motivation
+==========
+
+DXIL backend passes depend on various properties of DXIL Operations. For example, ``DXILLowering``
+pass will need information such as the DXIL operation an LLVM intrinsic is to be lowered to,
+along with valid overload and parameter types etc. The TableGen file -
+``llvm/lib/Target/DirectX/DXIL.td`` - is used to represent DXIL Operations
+by specifying their properties listed above. ``DXIL.td`` is designed to be the single source
+of reference of DXIL Operations for DXIL backend implementation in ``llvm-project`` repo -
+analogous to ``hctdb.py`` for ``DirectXShadeCompiler`` repo. It needs to have a rich
+representation capabilities that TableGen backends (such as ``DXILEmitter``) can rely on.
+Additionally, the DXIL Op specification should be easy to read and comprehend.
+
+This note focuses on specification of the set of properties consumed by DXIL backend
+passes identified above in category A. Any of the properties from category B are expected to be
+included as deemed necessary during implementation.
+
+Design
+======
+
+1. Each DXIL Operation is represented as a TableGen record. The name of each of the records
+   signifies operation name.
+2. The LLVM Intrinsic that maps to the operation is represented using ``Intrinsic::*``.
+3. The unique operation id is represented by an integer.
+4. DXIL Operation Class is represented as follows
+
+   .. code-block::
+
+        // Abstraction of DXIL Operation class.
+        // It encapsulates an associated function signature viz.,
+        // returnTy(param1Ty, param2Ty, ...) represented as a list of LLVMTypes.
+        // DXIL Ops that belong to a DXILOpClass record the signature of that DXILOpClass
+
+        class DXILOpClass<list<LLVMType> OpSig> {
+          list<LLVMType> OpSignature = OpSig;
+        }
+
+   Concrete operation classes, such as ``unary`` are defined inheriting from ``DXILOpClass``.
+5. Valid overload types are represented as a list of ``LLVMType``.
+6. Concrete records of DXIL versions and are defined by inheriting from the class
+
+   .. code-block::
+
+        // Abstract class to represent major and minor version values
+        class Version<int major, int minor> {
+          int Major = major;
+          int Minor = minor;
+        }
+
+7. A documentation string for the operation.
+
+
+A DXIL Operation is represented by the following TableGen class by encapsulating the various
+TableGen representations of its properties described above.
+
+.. code-block::
+
+  // Abstraction DXIL Operation
+  class DXILOpPropertiesBase {
+    int OpCode = 0;                           // Opcode of DXIL Operation
+    DXILOpClass OpClass = UnknownOpClass;     // Class of DXIL Operation.
+    Intrinsic LLVMIntrinsic = ?;              // LLVM Intrinsic DXIL Operation maps to
+    list<LLVMType> OpOverloadTypes = ?; // Valid overload type
+                                              // of DXIL Operation
+    Version DXILVer = ?;                      // Min DXIL version
+    string Doc = "";                          // A short description of the operation
+  }
+
+
+The following convenience class, definitions of ``unary`` and ``DXVer1_0`` are used to
+illustrate the definitions of ``Sin`` and ``Cos`` operations:
+
+  .. code-block::
+
+      class DXILOpProperties<int opCode,
+                    Intrinsic intrinsic,
+                    list<LLVMType> overloadTypes,
+                    string doc> : DXILOpPropertiesBase {
+        int OpCode = opCode;
+        Intrinsic LLVMIntrinsic = intrinsic;
+        list<LLVMType> OpOverloadTypes = overloadTypes;
+        string Doc = doc;
+      }
+
+      def unary : DXILOpClass<[llvm_any_ty, LLVMMatchType<0>]>;
+      def DXVer1_0 : Version<1, 0>;
+
+      let OpClass = unary, DXILVer = DXVer1_0 in {
+        def Cos  : DXILOpProperties<12, int_cos, [llvm_half_ty, llvm_float_ty],
+                                   "Returns cosine(theta) for theta in radians.">;
+        def Sin  : DXILOpProperties<13, int_sin, [llvm_half_ty, llvm_float_ty],
+                                   "Returns sine(theta) for theta in radians.">;
+      }
+
+Summary
+=======
+
+This note sketches the design of a readable and maintainable TableGen specification of
+DXIL Ops in ``DXIL.td`` intended to serve as a single source of reference for TableGen
+backends (such as ``DXILEmitter``) that generates C++ representations used in DXIL
+backend passes.