Implement the `smoothstep` HLSL Function

[farzonl]

• Implement smoothstep using HLSL source in hlsl_intrinsics.h
• Implement the smoothstep SPIR-V target built-in in clang/include/clang/Basic/BuiltinsSPIRV.td
• Add sema checks for smoothstep to CheckSPIRVBuiltinFunctionCall in clang/lib/Sema/SemaSPIRV.cpp
• Add codegen for spv smoothstep to EmitSPIRVBuiltinExpr in CGBuiltin.cpp
• Add codegen tests to clang/test/CodeGenHLSL/builtins/smoothstep.hlsl
• Add spv codegen test to clang/test/CodeGenSPIRV/Builtins/smoothstep.c
• Add sema tests to clang/test/SemaHLSL/BuiltIns/smoothstep-errors.hlsl
• Add spv sema tests to clang/test/SemaSPIRV/BuiltIns/smoothstep-errors.c
• Create the int_spv_smoothstep intrinsic in IntrinsicsSPIRV.td
• In SPIRVInstructionSelector.cpp create the smoothstep lowering and map it to int_spv_smoothstep in SPIRVInstructionSelector::selectIntrinsic.
• Create SPIR-V backend test case in llvm/test/CodeGen/SPIRV/hlsl-intrinsics/smoothstep.ll
• Check for what OpenCL support is needed.

DirectX

DXIL Opcode	DXIL OpName	Shader Model	Shader Stages
7	Saturate	6.0	()

SPIR-V

SmoothStep:

Description:

SmoothStep

Result is 0.0 if x ≤ edge0 and 1.0 if x ≥ edge1 and performs
smooth Hermite interpolation between 0 and 1 when edge0 < x <
edge1. This is equivalent to:

t * t * (3 - 2 * t), where t = clamp ((x - edge0) /
(edge1 - edge0), 0, 1)

Result is undefined if edge0 ≥ edge1.

The operands must all be a scalar or vector whose component type is
floating-point.

Result Type and the type of all operands must be the same type.
Results are computed per component.

Number	Operand 1	Operand 2	Operand 3	Operand 4
49	<id> edge0	<id> edge1	<id> x

Test Case(s)

Example 1

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

export float4 fn(float4 p1, float4 p2, float4 p3) {
    return smoothstep(p1, p2, p3);
}

HLSL:

Returns a smooth Hermite interpolation between 0 and 1, if x is in the range [min, max].

ret smoothstep(min, max, x)

Parameters

Item	Description
min	[in] The minimum range of the x parameter.
max	[in] The maximum range of the x parameter.
x	[in] The specified value to be interpolated.

Return Value

Returns 0 if x is less than min; 1 if x is greater than max; otherwise, a value between 0 and 1 if x is in the range [min, max].

Remarks

Use the smoothstep HLSL intrinsic function to create a smooth transition between two values. For example, you can use this function to blend two colors smoothly.

Type Description

Name	Template Type	Component Type	Size
x	scalar, vector, or matrix	float	any
min	same as input x	float	same dimension(s) as input x
max	same as input x	float	same dimension(s) as input x
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 2 (DirectX HLSL) and higher shader models	yes
Shader Model 1 (DirectX HLSL)	yes (vs_1_1 only)

See also

Intrinsic Functions (DirectX HLSL)

[pow2clk]

Expanded in DXIL, opcode in SPIRV

[kmpeng]

I will be taking on this task.