Implement the `firstbithigh` HLSL Function

[farzonl]

• Implement firstbithigh clang builtin,
• Link firstbithigh clang builtin with hlsl_intrinsics.h
• Add sema checks for firstbithigh to CheckHLSLBuiltinFunctionCall in SemaChecking.cpp
• Add codegen for firstbithigh to EmitHLSLBuiltinExpr in CGBuiltin.cpp
• Add codegen tests to clang/test/CodeGenHLSL/builtins/firstbithigh.hlsl
• Add sema tests to clang/test/SemaHLSL/BuiltIns/firstbithigh-errors.hlsl
• Create the int_dx_firstbithigh intrinsic in IntrinsicsDirectX.td
• Create the DXILOpMapping of int_dx_firstbithigh to 34 in DXIL.td
• Create the firstbithigh.ll and firstbithigh_errors.ll tests in llvm/test/CodeGen/DirectX/
• Create the int_spv_firstbithigh intrinsic in IntrinsicsSPIRV.td
• In SPIRVInstructionSelector.cpp create the firstbithigh lowering and map it to int_spv_firstbithigh in SPIRVInstructionSelector::selectIntrinsic.
• Create SPIR-V backend test case in llvm/test/CodeGen/SPIRV/hlsl-intrinsics/firstbithigh.ll

DirectX

DXIL Opcode	DXIL OpName	Shader Model	Shader Stages
34	FirstbitSHi	6.0	()

SPIR-V

FindSMsb:

Description:

FindSMsb

Signed-integer most-significant bit, with Value interpreted as a
signed integer.

For positive numbers, the result will be the bit number of the most
significant 1-bit. For negative numbers, the result will be the bit
number of the most significant 0-bit. For a Value of 0 or -1, 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.

This instruction is currently limited to 32-bit width components.

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

Test Case(s)

Example 1

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

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

HLSL:

Gets the location of the first set bit starting from the highest order bit and working downward, per component.

Syntax

int firstbithigh(
  in int value
);

Parameters

value [in]

Type: int

The input value.

Return value

Type: int

The location of the first set bit.

Remarks

For a signed integer, the first significant bit is zero for a negative number.

The following overloaded versions are also available:

int2 firstbithigh(int2 value);
int3 firstbithigh(int3 value);
int4 firstbithigh(int4 value);
uint firstbithigh(uint value);
uint2 firstbithigh(uint2 value);
uint3 firstbithigh(uint3 value);
uint4 firstbithigh(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]

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