================ @@ -0,0 +1,300 @@ +=================== +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 ---------------- llvm-beanz wrote:
I don't think we explicitly define it, but we need the Clang DXIL generation to match otherwise we could expose subtle bugs. https://github.com/llvm/llvm-project/pull/75397 _______________________________________________ cfe-commits mailing list cfe-commits@lists.llvm.org https://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-commits
