[clang] 66f6929 - [HLSL][Doc] Add doc about expected differences (#82395)
via cfe-commits
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Thu Feb 22 12:32:29 PST 2024
Author: Chris B
Date: 2024-02-22T14:32:24-06:00
New Revision: 66f6929fec3ae4770368b60aa1920623ab835f9d
URL: https://github.com/llvm/llvm-project/commit/66f6929fec3ae4770368b60aa1920623ab835f9d
DIFF: https://github.com/llvm/llvm-project/commit/66f6929fec3ae4770368b60aa1920623ab835f9d.diff
LOG: [HLSL][Doc] Add doc about expected differences (#82395)
This document covers expected differences between Clang and the HLSL
reference compiler implementations (FXC & DXC). The document is not
intended to be exhaustive, but it should be a best effort to cover known
cases.
This document should document both the behavioral difference and the
explanation of why Clang differs.
The initail document covers known overload resolution differences.
---------
Co-authored-by: S. Bharadwaj Yadavalli <Bharadwaj.Yadavalli at microsoft.com>
Added:
clang/docs/HLSL/ExpectedDifferences.rst
Modified:
clang/docs/HLSL/HLSLDocs.rst
Removed:
################################################################################
diff --git a/clang/docs/HLSL/ExpectedDifferences.rst b/clang/docs/HLSL/ExpectedDifferences.rst
new file mode 100644
index 00000000000000..60001b22dc7920
--- /dev/null
+++ b/clang/docs/HLSL/ExpectedDifferences.rst
@@ -0,0 +1,110 @@
+
+Expected Differences vs DXC and FXC
+===================================
+
+.. contents::
+ :local:
+
+Introduction
+============
+
+HLSL currently has two reference compilers, the `DirectX Shader Compiler (DXC)
+<https://github.com/microsoft/DirectXShaderCompiler/>`_ and the
+`Effect-Compiler (FXC) <https://learn.microsoft.com/en-us/windows/win32/direct3dtools/fxc>`_.
+The two reference compilers do not fully agree. Some known disagreements in the
+references are tracked on
+`DXC's GitHub
+<https://github.com/microsoft/DirectXShaderCompiler/issues?q=is%3Aopen+is%3Aissue+label%3Afxc-disagrees>`_,
+but many more are known to exist.
+
+HLSL as implemented by Clang will also not fully match either of the reference
+implementations, it is instead being written to match the `draft language
+specification <https://microsoft.github.io/hlsl-specs/specs/hlsl.pdf>`_.
+
+This document is a non-exhaustive collection the known
diff erences between
+Clang's implementation of HLSL and the existing reference compilers.
+
+General Principles
+------------------
+
+Most of the intended
diff erences between Clang and the earlier reference
+compilers are focused on increased consistency and correctness. Both reference
+compilers do not always apply language rules the same in all contexts.
+
+Clang also deviates from the reference compilers by providing
diff erent
+diagnostics, both in terms of the textual messages and the contexts in which
+diagnostics are produced. While striving for a high level of source
+compatibility with conforming HLSL code, Clang may produce earlier and more
+robust diagnostics for incorrect code or reject code that a reference compiler
+incorrectly accepted.
+
+Language Version
+================
+
+Clang targets language compatibility for HLSL 2021 as implemented by DXC.
+Language features that were removed in earlier versions of HLSL may be added on
+a case-by-case basis, but are not planned for the initial implementation.
+
+Overload Resolution
+===================
+
+Clang's HLSL implementation adopts C++ overload resolution rules as proposed for
+HLSL 202x based on proposal
+`0007 <https://github.com/microsoft/hlsl-specs/blob/main/proposals/0007-const-instance-methods.md>`_
+and
+`0008 <https://github.com/microsoft/hlsl-specs/blob/main/proposals/0008-non-member-operator-overloading.md>`_.
+
+Clang's implementation extends standard overload resolution rules to HLSL
+library functionality. This causes subtle changes in overload resolution
+behavior between Clang and DXC. Some examples include:
+
+.. code-block:: c++
+
+ void halfOrInt16(half H);
+ void halfOrInt16(uint16_t U);
+ void halfOrInt16(int16_t I);
+
+ void takesDoubles(double, double, double);
+
+ cbuffer CB {
+ uint U;
+ int I;
+ float X, Y, Z;
+ double3 A, B;
+ }
+
+ export void call() {
+ halfOrInt16(U); // DXC: Fails with call ambiguous between int16_t and uint16_t overloads
+ // Clang: Resolves to halfOrInt16(uint16_t).
+ halfOrInt16(I); // All: Resolves to halfOrInt16(int16_t).
+ half H;
+ #ifndef IGNORE_ERRORS
+ // asfloat16 is a builtin with overloads for half, int16_t, and uint16_t.
+ H = asfloat16(I); // DXC: Fails to resolve overload for int.
+ // Clang: Resolves to asfloat16(int16_t).
+ H = asfloat16(U); // DXC: Fails to resolve overload for int.
+ // Clang: Resolves to asfloat16(uint16_t).
+ #endif
+ H = asfloat16(0x01); // DXC: Resolves to asfloat16(half).
+ // Clang: Resolves to asfloat16(uint16_t).
+
+ takesDoubles(X, Y, Z); // Works on all compilers
+ #ifndef IGNORE_ERRORS
+ fma(X, Y, Z); // DXC: Fails to resolve no known conversion from float to double.
+ // Clang: Resolves to fma(double,double,double).
+ #endif
+
+ double D = dot(A, B); // DXC: Resolves to dot(double3, double3), fails DXIL Validation.
+ // FXC: Expands to compute double dot product with fmul/fadd
+ // Clang: Resolves to dot(float3, float3), emits conversion warnings.
+
+ }
+
+.. note::
+
+ In Clang, a conscious decision was made to exclude the ``dot(vector<double,N>, vector<double,N>)``
+ overload and allow overload resolution to resolve the
+ ``vector<float,N>`` overload. This approach provides ``-Wconversion``
+ diagnostic notifying the user of the conversion rather than silently altering
+ precision relative to the other overloads (as FXC does) or generating code
+ that will fail validation (as DXC does).
diff --git a/clang/docs/HLSL/HLSLDocs.rst b/clang/docs/HLSL/HLSLDocs.rst
index 1f232129548d0b..97b2425f013b34 100644
--- a/clang/docs/HLSL/HLSLDocs.rst
+++ b/clang/docs/HLSL/HLSLDocs.rst
@@ -11,6 +11,7 @@ HLSL Design and Implementation
.. toctree::
:maxdepth: 1
+ ExpectedDifferences
HLSLIRReference
ResourceTypes
EntryFunctions
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