Implement the `fwidth` HLSL Function

[farzonl]

• Implement fwidth clang builtin,
• Link fwidth clang builtin with hlsl_intrinsics.h
• Add sema checks for fwidth to CheckHLSLBuiltinFunctionCall in SemaChecking.cpp
• Add codegen for fwidth to EmitHLSLBuiltinExpr in CGBuiltin.cpp
• Add codegen tests to clang/test/CodeGenHLSL/builtins/fwidth.hlsl
• Add sema tests to clang/test/SemaHLSL/BuiltIns/fwidth-errors.hlsl
• Create the int_dx_fwidth intrinsic in IntrinsicsDirectX.td
• Create the DXILOpMapping of int_dx_fwidth to 83 in DXIL.td
• Create the fwidth.ll and fwidth_errors.ll tests in llvm/test/CodeGen/DirectX/
• Create the int_spv_fwidth intrinsic in IntrinsicsSPIRV.td
• In SPIRVInstructionSelector.cpp create the fwidth lowering and map it to int_spv_fwidth in SPIRVInstructionSelector::selectIntrinsic.
• Create SPIR-V backend test case in llvm/test/CodeGen/SPIRV/hlsl-intrinsics/fwidth.ll

DirectX

DXIL Opcode	DXIL OpName	Shader Model	Shader Stages
83	DerivCoarseX	6.0	('library', 'pixel', 'compute', 'amplification', 'mesh', 'node')

SPIR-V

OpFwidth:

Description:

Result is the same as computing the sum of the absolute values of
OpDPdx and OpDPdy on P.

Result Type must be a scalar or vector of floating-point
type using the IEEE 754 encoding. The component width must
be 32 bits.

The type of P must be the same as Result Type. P is the value to
take the derivative of.

This instruction is only valid in the Fragment Execution
Model.

Capability:
Shader

Word Count	Opcode	Results	Operands
4	209	<id> Result Type	Result <id>	<id> P

Test Case(s)

Example 1

//dxc fwidth_test.hlsl -T lib_6_8 -enable-16bit-types -O0

export float4 fn(float4 p1) {
    return fwidth(p1);
}

HLSL:

Returns the absolute value of the partial derivatives of the specified value.

ret fwidth(x)

This function computes the following: abs(ddx(x)) + abs(ddy(x)).

This function is only supported in pixel shaders.

Parameters

Item	Description
x	[in] The specified value.

Return Value

The absolute value of the partial derivatives of the x parameter.

Type Description

Name	Template Type	Component Type	Size
x	scalar, vector, or matrix	float	any
ret	same as input x	float	same dimension(s) as input x

Minimum Shader Model

This function is supported in the following shader models.

Shader Model	Supported
Shader Model 3 (DirectX HLSL) and higher shader models	yes
Shader Model 2 (DirectX HLSL)	yes (ps_2_x only)
Shader Model 1 (DirectX HLSL)	no

See also

Intrinsic Functions (DirectX HLSL)

[Alexander-Johnston]

I can pick this up. I don't have permissions to assign myself.