Implement the `firstbitlow` HLSL Function

[farzonl]

• Implement firstbitlow clang builtin,
• Link firstbitlow clang builtin with hlsl_intrinsics.h
• Add sema checks for firstbitlow to CheckHLSLBuiltinFunctionCall in SemaChecking.cpp
• Add codegen for firstbitlow to EmitHLSLBuiltinExpr in CGBuiltin.cpp
• Add codegen tests to clang/test/CodeGenHLSL/builtins/firstbitlow.hlsl
• Add sema tests to clang/test/SemaHLSL/BuiltIns/firstbitlow-errors.hlsl
• Create the int_dx_firstbitlow intrinsic in IntrinsicsDirectX.td
• Create the DXILOpMapping of int_dx_firstbitlow to 32 in DXIL.td
• Create the firstbitlow.ll and firstbitlow_errors.ll tests in llvm/test/CodeGen/DirectX/
• Create the int_spv_firstbitlow intrinsic in IntrinsicsSPIRV.td
• In SPIRVInstructionSelector.cpp create the firstbitlow lowering and map it to int_spv_firstbitlow in SPIRVInstructionSelector::selectIntrinsic.
• Create SPIR-V backend test case in llvm/test/CodeGen/SPIRV/hlsl-intrinsics/firstbitlow.ll

DirectX

DXIL Opcode	DXIL OpName	Shader Model	Shader Stages
32	FirstbitLo	6.0	()

SPIR-V

FindILsb:

Description:

FindILsb

Integer least-significant bit.

Results in the bit number of the least-significant 1-bit in the binary
representation of Value. If Value is 0, the result is -1.

Result Type and the type of Value must both be integer scalar or
integer vector types. Result Type and operand types must have the same
number of components with the same component width. Results are computed
per component.

Number	Operand 1	Operand 2	Operand 3	Operand 4
73	<id> Value

Test Case(s)

Example 1

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

export uint4 fn(int4 p1) {
    return firstbitlow(p1);
}

HLSL:

Returns the location of the first set bit starting from the lowest order bit and working upward, per component.

Syntax

int firstbitlow(
  in int value
);

Parameters

value [in]

Type: int

The input value.

Return value

Type: int

The location of the first set bit.

Remarks

The following overloaded versions are also available:

uint2 firstbitlow(uint2 value);
uint3 firstbitlow(uint3 value);
uint4 firstbitlow(uint4 value);

Minimum Shader Model

This function is supported in the following shader models.

Shader Model	Supported
Shader Model 5 and higher shader models	yes

 

This function is supported in the following types of shaders:

Vertex	Hull	Domain	Geometry	Pixel	Compute
x	x	x	x	x	x

 

See also

Intrinsic Functions

Shader Model 5

[spall]

looking at this

[damyanp]

As discussed with @spall - keeping this around as a good starter issue.