[HLSL][Docs] Add documentation for HLSL functions

This adds a new document that covers the HLSL approach to function
calls and parameter semantics. At time of writing this document is a
proposal for the implementation.

Author: llvm-beanz

clang/docs/HLSL/FunctionCalls.rst

@@ -0,0 +1,316 @@
+===================
+HLSL Function Calls
+===================
+
+.. contents::
+   :local:
+
+Introduction
+============
+
+This document descries the design and implementation of HLSL's function call
+semantics in Clang. This includes details related to argument conversion and
+parameter lifetimes.
+
+This document does not seek to serve as official documentation for HLSL's
+call semantics, but does provide an overview to assist a reader. The
+authoritative documentation for HLSL's language semantics is the `draft language
+specification <https://microsoft.github.io/hlsl-specs/specs/hlsl.pdf>`_.
+
+Argument Semantics
+==================
+
+In HLSL, all function arguments are passed by value in and out of functions.
+HLSL has 3 keywords which denote the parameter semantics (``in``, ``out`` and
+``inout``). In a function declaration a parameter may be annotated any of the
+following ways:
+
+#. <no parameter annotation> - denotes input
+#. ``in`` - denotes input
+#. ``out`` - denotes output
+#. ``in out`` - denotes input and output
+#. ``out in`` - denotes input and output
+#. ``inout`` - denotes input and output
+
+Parameters that are exclusively input behave like C/C++ parameters that are
+passed by value.
+
+For parameters that are output (or input and output), a temporary value is
+created in the caller. The temporary value is then passed by-address. For
+output-only parameters, the temporary is uninitialized when passed (it is
+undefined behavior to not explicitly initialize an ``out`` parameter inside a
+function). For input and output parameters, the temporary is initialized from
+the lvalue argument expression through implicit or explicit casting from the
+lvalue argument type to the parameter type.
+
+On return of the function, the values of any parameter temporaries are written
+back to the argument expression through an inverted conversion sequence (if an
+``out`` parameter was not initialized in the function, the uninitialized value
+may be written back).
+
+Parameters of constant-sized array type, are also passed with value semantics.
+This requires input parameters of arrays to construct temporaries and the
+temporaries go through array-to-pointer decay when initializing parameters.
+
+Implementations are allowed to avoid unnecessary temporaries, and HLSL's strict
+no-alias rules can enable some trivial optimizations.
+
+Array Temporaries
+-----------------
+
+Given the following example:
+
+.. code-block:: c++
+
+  void fn(float a[4]) {
+    a[0] = a[1] + a[2] + a[3];
+  }
+
+  float4 main() : SV_Target {
+    float arr[4] = {1, 1, 1, 1};
+    fn(arr);
+    return float4(a[0], a[1], a[2], a[3]);
+  }
+
+In C or C++, the array parameter decays to a pointer, so after the call to
+``fn``, the value of ``a[0]`` is ``3``. In HLSL, the array is passed by value,
+so modifications inside ``fn`` do not propagate out.
+
+.. note::
+
+  DXC supports unsized arrays passed directly as decayed pointers, which is an
+  unfortunate behavior divergence.
+
+Out Parameter Temporaries
+-------------------------
+
+.. code-block:: c++
+
+  void Init(inout int X, inout int Y) {
+    Y = 2;
+    X = 1;
+  }
+
+  void main() {
+    int V;
+    Init(V, V); // MSVC ABI V == 2, Itanium V == 1
+  }
+
+In the above example the ``Init`` function's behavior depends on the C++ ABI
+implementation. In the MSVC C++ ABI (used for the HLSL DXIL target), call
+arguments are emitted right-to-left and destroyed left-to-right. This means that
+the parameter initialization and destruction occurs in the order: {``Y``,
+``X``, ``~X``, ``~Y``}. This causes the write-back of the value of ``Y`` to occur
+last, so the resulting value of ``V`` is ``2``. In the Itanium C++ ABI, the
+parameter ordering is reversed, so the initialization and destruction occurs in
+the order: {``X``, ``Y``, ``~Y``, ``X``}. This causes the write-back of the
+value ``X`` to occur last, resulting in the value of ``V`` being set to ``1``.
+
+.. code-block:: c++
+
+  void Trunc(inout int3 V) { }
+
+
+  void main() {
+    float3 F = {1.5, 2.6, 3.3};
+    Trunc(F); // F == {1.0, 2.0, 3.0}
+  }
+
+In the above example, the argument expression ``F`` undergoes element-wise
+conversion from a float vector to an integer vector to create a temporary
+``int3``. On expiration the temporary undergoes elementwise conversion back to
+the floating point vector type ``float3``. This results in an implicit
+truncation of the vector even if the value is unused in the function.
+
+
+.. code-block:: c++
+
+  void UB(out int X) {}
+
+  void main() {
+    int X = 7;
+    UB(X); // X is undefined!
+  }
+
+In this example an initialized value is passed to an ``out`` parameter.
+Parameters marked ``out`` are not initialized by the argument expression or
+implicitly by the function. They must be explicitly initialized. In this case
+the argument is not initialized in the function so the temporary is still
+uninitialized when it is copied back to the argument expression. This is
+undefined behavior in HLSL, and may be illegal in generated programs.
+
+Clang Implementation 
+====================
+
+.. note::
+
+  The implementation described here is a proposal. It has not yet been fully
+  implemented, so the current state of Clang's sources may not reflect this
+  design. A prototype implementation was built on DXC which is Clang-3.7 based.
+  The prototype can be found
+  `here <https://github.com/microsoft/DirectXShaderCompiler/pull/5249>`_. A lot
+  of the changes in the prototype implementation are restoring Clang-3.7 code
+  that was previously modified to its original state.
+
+The implementation in clang depends on two new AST nodes and minor extensions to
+Clang's existing support for Objective-C write-back arguments. The goal of this
+design is to capture the semantic details of HLSL function calls in the AST, and
+minimize the amount of magic that needs to occur during IR generation.
+
+The two new AST nodes are ``HLSLArrayTemporaryExpr`` and ``HLSLOutParamExpr``,
+which respectively represent the temporaries used for passing arrays by value
+and the temporaries created for function outputs.
+
+Array Temporaries
+-----------------
+
+The ``HLSLArrayTemporaryExpr`` represents temporary values for input
+constant-sized array arguments. This applies for all constant-sized array
+arguments regardless of whether or not the parameter is constant-sized or
+unsized.
+
+.. code-block:: c++
+
+  void SizedArray(float a[4]);
+  void UnsizedArray(float a[]);
+
+  void main() {
+    float arr[4] = {1, 1, 1, 1};
+    SizedArray(arr);
+    UnsizedArray(arr);
+  }
+
+In the example above, the following AST is generated for the call to
+``SizedArray``:
+
+.. code-block:: text
+
+  CallExpr 'void'
+  |-ImplicitCastExpr 'void (*)(float [4])' <FunctionToPointerDecay>
+  | `-DeclRefExpr 'void (float [4])' lvalue Function 'SizedArray' 'void (float [4])'
+  `-HLSLArrayTemporaryExpr 'float [4]'
+    `-DeclRefExpr 'float [4]' lvalue Var 'arr' 'float [4]'
+
+In the example above, the following AST is generated for the call to
+``UnsizedArray``:
+
+.. code-block:: text
+
+  CallExpr 'void'
+  |-ImplicitCastExpr 'void (*)(float [])' <FunctionToPointerDecay>
+  | `-DeclRefExpr 'void (float [])' lvalue Function 'UnsizedArray' 'void (float [])'
+  `-HLSLArrayTemporaryExpr 'float [4]'
+    `-DeclRefExpr 'float [4]' lvalue Var 'arr' 'float [4]'
+
+In both of these cases the argument expression is of known array size so we can
+initialize an appropriately sized temporary.
+
+It is illegal in HLSL to convert an unsized array to a sized array:
+
+.. code-block:: c++
+
+  void SizedArray(float a[4]);
+  void UnsizedArray(float a[]) {
+    SizedArray(a); // Cannot convert float[] to float[4]
+  }
+
+When converting a sized array to an unsized array, an array temporary can also
+be inserted. Given the following code:
+
+.. code-block:: c++
+
+  void UnsizedArray(float a[]);
+  void SizedArray(float a[4]) {
+    UnsizedArray(a);
+  }
+
+An expected AST should be something like:
+
+.. code-block:: text
+
+  CallExpr 'void'
+  |-ImplicitCastExpr 'void (*)(float [])' <FunctionToPointerDecay>
+  | `-DeclRefExpr 'void (float [])' lvalue Function 'UnsizedArray' 'void (float [])'
+  `-HLSLArrayTemporaryExpr 'float [4]'
+    `-DeclRefExpr 'float [4]' lvalue Var 'arr' 'float [4]'
+
+Out Parameter Temporaries
+-------------------------
+
+Output parameters are defined in HLSL as *casting expiring values* (cx-values),
+which is a term made up for HLSL. A cx-value is a temporary value which may be
+the result of a cast, and stores its value back to an lvalue when the value
+expires.
+
+To represent this concept in Clang we introduce a new ``HLSLOutParamExpr``. An
+``HLSLOutParamExpr`` has two forms, one with a single sub-expression and one
+with two sub-expressions.
+
+The single sub-expression form is used when the argument expression and the
+function parameter are the same type, so no cast is required. As in this
+example:
+
+.. code-block:: c++
+
+  void Init(inout int X) {
+    X = 1;
+  }
+
+  void main() {
+    int V;
+    Init(V);
+  }
+
+The expected AST formulation for this code would be something like:
+
+.. code-block:: text
+
+  CallExpr 'void'
+  |-ImplicitCastExpr 'void (*)(int &)' <FunctionToPointerDecay>
+  | `-DeclRefExpr 'void (int &)' lvalue Function  'Init' 'void (int &)'
+  |-HLSLOutParamExpr 'int' lvalue inout
+    `-DeclRefExpr 'int' lvalue Var 'V' 'int'
+
+The ``HLSLOutParamExpr`` captures that the value is ``inout`` vs ``out`` to
+denote whether or not the temporary is initialized from the sub-expression. If
+no casting is required the sub-expression denotes the lvalue expression that the
+cx-value will be copied to when the value expires.
+
+The two sub-expression form of the AST node is required when the argument type
+is not the same as the parameter type. Given this example:
+
+.. code-block:: c++
+
+  void Trunc(inout int3 V) { }
+
+
+  void main() {
+    float3 F = {1.5, 2.6, 3.3};
+    Trunc(F);
+  }
+
+For this case the ``HLSLOutParamExpr`` will have sub-expressions to record both
+casting expression sequences for the initialization and write back:
+
+.. code-block:: text
+
+  -CallExpr 'void'
+    |-ImplicitCastExpr 'void (*)(int3 &)' <FunctionToPointerDecay>
+    | `-DeclRefExpr 'void (int3 &)' lvalue Function 'inc_i32' 'void (int3 &)'
+    `-HLSLOutParamExpr 'int3' lvalue inout
+      |-ImplicitCastExpr 'float3' <IntegralToFloating>
+      | `-ImplicitCastExpr 'int3' <LValueToRValue>
+      |   `-OpaqueValueExpr 'int3' lvalue
+      `-ImplicitCastExpr 'int3' <FloatingToIntegral>
+        `-ImplicitCastExpr 'float3' <LValueToRValue>
+          `-DeclRefExpr 'float3' lvalue 'F' 'float3'
+
+In this formation the write-back casts are captured as the first sub-expression
+and they cast from an ``OpaqueValueExpr``. In IR generation we can use the
+``OpaqueValueExpr`` as a placeholder for the ``HLSLOutParamExpr``'s temporary
+value on function return.
+
+In code generation this can be implemented with some targeted extensions to the
+Objective-C write-back support. Specifically extending CGCall.cpp's
+``EmitWriteback`` function to support casting expressions and emission of
+aggregate lvalues.

clang/docs/HLSL/HLSLDocs.rst

@@ -14,3 +14,4 @@ HLSL Design and Implementation
    HLSLIRReference
    ResourceTypes
    EntryFunctions
+   FunctionCalls