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