Expected Differences vs DXC and FXC


HLSL currently has two reference compilers, the DirectX Shader Compiler (DXC) and the Effect-Compiler (FXC). The two reference compilers do not fully agree. Some known disagreements in the references are tracked on DXC’s GitHub, 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.

This document is a non-exhaustive collection the known differences between Clang’s implementation of HLSL and the existing reference compilers.

General Principles

Most of the intended differences 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 different 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 and 0008.

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:

void halfOrInt16(half H);
void halfOrInt16(uint16_t U);
void halfOrInt16(int16_t I);

void takesDoubles(double, double, double);

cbuffer CB {
  bool B;
  uint U;
  int I;
  float X, Y, Z;
  double3 A, B;

void twoParams(int, int);
void twoParams(float, float);

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;
  // 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).
  H = asfloat16(0x01); // DXC: Resolves to asfloat16(half).
                       // Clang: Resolves to asfloat16(uint16_t).

  takesDoubles(X, Y, Z); // Works on all compilers
  fma(X, Y, Z); // DXC: Fails to resolve no known conversion from float to double.
                // Clang: Resolves to fma(double,double,double).

  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.

  tan(B); // DXC: resolves to tan(float).
          // Clang: Fails to resolve, ambiguous between integer types.

  twoParams(I, X); // DXC: resolves twoParams(int, int).
                   // Clang: Fails to resolve ambiguous conversions.


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).