<table border="1" cellspacing="0" cellpadding="8">
    <tr>
        <th>Issue</th>
        <td>
            <a href=https://github.com/llvm/llvm-project/issues/99158>99158</a>
        </td>
    </tr>

    <tr>
        <th>Summary</th>
        <td>
            Implement the `WaveIsFirstLane` HLSL Function
        </td>
    </tr>

    <tr>
      <th>Labels</th>
      <td>
            metabug,
            backend:DirectX,
            HLSL,
            backend:SPIR-V,
            bot:HLSL
      </td>
    </tr>

    <tr>
      <th>Assignees</th>
      <td>
      </td>
    </tr>

    <tr>
      <th>Reporter</th>
      <td>
          farzonl
      </td>
    </tr>
</table>

<pre>
    - [ ] Implement `WaveIsFirstLane` clang builtin,
- [ ] Link `WaveIsFirstLane` clang builtin with `hlsl_intrinsics.h`
- [ ] Add sema checks for `WaveIsFirstLane` to `CheckHLSLBuiltinFunctionCall` in `SemaChecking.cpp`
- [ ] Add codegen for `WaveIsFirstLane` to `EmitHLSLBuiltinExpr` in `CGBuiltin.cpp`
- [ ] Add codegen tests to `clang/test/CodeGenHLSL/builtins/WaveIsFirstLane.hlsl`
- [ ] Add sema tests to `clang/test/SemaHLSL/BuiltIns/WaveIsFirstLane-errors.hlsl`
- [ ] Create the `int_dx_WaveIsFirstLane` intrinsic in `IntrinsicsDirectX.td`
- [ ] Create the `DXILOpMapping` of `int_dx_WaveIsFirstLane` to  `110` in `DXIL.td`
- [ ] Create the  `WaveIsFirstLane.ll` and `WaveIsFirstLane_errors.ll` tests in `llvm/test/CodeGen/DirectX/`
- [ ] Create the `int_spv_WaveIsFirstLane` intrinsic in `IntrinsicsSPIRV.td`
- [ ] In SPIRVInstructionSelector.cpp create the `WaveIsFirstLane` lowering and map  it to `int_spv_WaveIsFirstLane` in `SPIRVInstructionSelector::selectIntrinsic`.
- [ ] Create SPIR-V backend test case in `llvm/test/CodeGen/SPIRV/hlsl-intrinsics/WaveIsFirstLane.ll`

## DirectX

| DXIL Opcode | DXIL OpName | Shader Model | Shader Stages |
| ----------- | ----------- | ------------ | ------------- |
| 110 | WaveIsFirstLane | 6.0 | ('library', 'compute', 'amplification', 'mesh', 'pixel', 'vertex', 'hull', 'domain', 'geometry', 'raygeneration', 'intersection', 'anyhit', 'closesthit', 'miss', 'callable', 'node') |

## SPIR-V

# [OpGroupNonUniformElect](https://registry.khronos.org/SPIR-V/specs/unified1/SPIRV.html#OpGroupNonUniformElect):

## Description:
  
Result is **true** only in the active [invocation](https://registry.khronos.org/SPIR-V/specs/unified1/SPIRV.html#Invocation) with the
lowest id in the [group](https://registry.khronos.org/SPIR-V/specs/unified1/SPIRV.html#Group), otherwise result is false.  
  
*Result Type* must be a [*Boolean type*](https://registry.khronos.org/SPIR-V/specs/unified1/SPIRV.html#Boolean).  
  
*Execution* is a [*Scope*](https://registry.khronos.org/SPIR-V/specs/unified1/SPIRV.html#Scope_-id-) that identifies the group of
invocations affected by this command. It must be **Subgroup**.

[Capability](https://registry.khronos.org/SPIR-V/specs/unified1/SPIRV.html#Capability):  
**GroupNonUniform**  
  
[Missing before](https://registry.khronos.org/SPIR-V/specs/unified1/SPIRV.html#Unified) **version 1.3**.

<table>
<colgroup>
<col style="width: 20%" />
<col style="width: 20%" />
<col style="width: 20%" />
<col style="width: 20%" />
<col style="width: 20%" />
</colgroup>
<thead>
<tr>
<th>Word Count</th>
<th>Opcode</th>
<th>Results</th>
<th>Operands</th>
<th></th>
</tr>
</thead>
<tbody>
<tr>
<td class="tableblock halign-left valign-top"><p>4</p></td>
<td class="tableblock halign-left valign-top"><p>333</p></td>
<td
class="tableblock halign-left valign-top"><p><em>&lt;id&gt;</em><br />
<em>Result Type</em></p></td>
<td class="tableblock halign-left valign-top"><p><a
href="#ResultId"><em>Result &lt;id&gt;</em></a></p></td>
<td class="tableblock halign-left valign-top"><p><a
href="#Scope_-id-"><em>Scope &lt;id&gt;</em></a><br />
<em>Execution</em></p></td>
</tr>
</tbody>
</table>



## Test Case(s)

 
 ### Example 1
```hlsl
//dxc WaveIsFirstLane_test.hlsl -T lib_6_8 -enable-16bit-types -O0

export bool fn() {
    return WaveIsFirstLane();
}
```
## HLSL:

Returns true only for the active lane in the current wave with the smallest index.

## Syntax


``` syntax
bool WaveIsFirstLane(void);
```



## Parameters

This function has no parameters.

## Return value

True only for the active lane in the current wave with the smallest index.

## Remarks

This function can be used to identify operations that are to be executed only once per wave.

This function is supported from shader model 6.0 in all shader stages. 



 

## Examples

``` syntax
 if ( WaveIsFirstLane() )
{
    . . . // once per-wave code
}
```

## See also

<dl> <dt>

[Overview of Shader Model 6](https://github.com/MicrosoftDocs/win32/blob/docs/desktop-src//direct3dhlsl/hlsl-shader-model-6-0-features-for-direct3d-12.md)
</dt> <dt>

[Shader Model 6](https://github.com/MicrosoftDocs/win32/blob/docs/desktop-src//direct3dhlsl/shader-model-6-0.md)
</dt> </dl>
</pre>
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