<table border="1" cellspacing="0" cellpadding="8">
<tr>
<th>Issue</th>
<td>
<a href=https://github.com/llvm/llvm-project/issues/99114>99114</a>
</td>
</tr>
<tr>
<th>Summary</th>
<td>
Implement the `faceforward` 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 `faceforward` clang builtin,
- [ ] Link `faceforward` clang builtin with `hlsl_intrinsics.h`
- [ ] Add sema checks for `faceforward` to `CheckHLSLBuiltinFunctionCall` in `SemaChecking.cpp`
- [ ] Add codegen for `faceforward` to `EmitHLSLBuiltinExpr` in `CGBuiltin.cpp`
- [ ] Add codegen tests to `clang/test/CodeGenHLSL/builtins/faceforward.hlsl`
- [ ] Add sema tests to `clang/test/SemaHLSL/BuiltIns/faceforward-errors.hlsl`
- [ ] Create the `int_dx_faceforward` intrinsic in `IntrinsicsDirectX.td`
- [ ] Create the `DXILOpMapping` of `int_dx_faceforward` to `56` in `DXIL.td`
- [ ] Create the `faceforward.ll` and `faceforward_errors.ll` tests in `llvm/test/CodeGen/DirectX/`
- [ ] Create the `int_spv_faceforward` intrinsic in `IntrinsicsSPIRV.td`
- [ ] In SPIRVInstructionSelector.cpp create the `faceforward` lowering and map it to `int_spv_faceforward` in `SPIRVInstructionSelector::selectIntrinsic`.
- [ ] Create SPIR-V backend test case in `llvm/test/CodeGen/SPIRV/hlsl-intrinsics/faceforward.ll`
## DirectX
| DXIL Opcode | DXIL OpName | Shader Model | Shader Stages |
| ----------- | ----------- | ------------ | ------------- |
| 56 | Dot4 | 6.0 | () |
## SPIR-V
# FaceForward:
## Description:
**FaceForward**
If the dot product of *Nref* and *I* is negative, the result is *N*,
otherwise it is *-N*.
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.
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<thead>
<tr>
<th>Number</th>
<th>Operand 1</th>
<th>Operand 2</th>
<th>Operand 3</th>
<th>Operand 4</th>
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</thead>
<tbody>
<tr>
<td class="tableblock halign-left valign-top"><p>70</p></td>
<td
class="tableblock halign-left valign-top"><p><em><id></em><br />
<em>N</em></p></td>
<td
class="tableblock halign-left valign-top"><p><em><id></em><br />
<em>I</em></p></td>
<td
class="tableblock halign-left valign-top"><p><em><id></em><br />
<em>Nref</em></p></td>
<td></td>
</tr>
</tbody>
</table>
## Test Case(s)
### Example 1
```hlsl
//dxc faceforward_test.hlsl -T lib_6_8 -enable-16bit-types -O0
export float4 fn(float4 p1, float4 p2, float4 p3) {
return faceforward(p1, p2, p3);
}
```
## HLSL:
Flips the surface-normal (if needed) to face in a direction opposite to i; returns the result in n.
| *ret* faceforward(*n*, *i*, *ng*) |
|-----------------------------------|
This function uses the following formula: -*n* sign(dot(*i*, *ng*)).
## Parameters
| Item | Description |
|-----------------------------------------------------------|-----------------------------------------------------------------------------------------------------------------|
| <span id="n"></span><span id="N"></span>*n*<br/> | \[in\] The resulting floating-point surface-normal vector.<br/> |
| <span id="i"></span><span id="I"></span>*i*<br/> | \[in\] A floating-point, incident vector that points from the view position to the shading position.<br/> |
| <span id="ng"></span><span id="NG"></span>*ng*<br/> | \[in\] A floating-point surface-normal vector.<br/> |
## Return Value
A floating-point, surface normal vector that is facing the view direction.
## Type Description
| Name | [**Template Type**](https://github.com/MicrosoftDocs/win32/blob/docs/desktop-src//direct3dhlsl/dx-graphics-hlsl-intrinsic-functions.md) | [**Component Type**](https://github.com/MicrosoftDocs/win32/blob/docs/desktop-src//direct3dhlsl/dx-graphics-hlsl-intrinsic-functions.md) | Size |
|-------|-------------------------------------------------------------------------------------|----------------------------------------------------------------|--------------------------------|
| *n* | [**vector**](https://github.com/MicrosoftDocs/win32/blob/docs/desktop-src//direct3dhlsl/dx-graphics-hlsl-intrinsic-functions.md) | [**float**](/windows/desktop/WinProg/windows-data-types) | any |
| *i* | [**vector**](https://github.com/MicrosoftDocs/win32/blob/docs/desktop-src//direct3dhlsl/dx-graphics-hlsl-intrinsic-functions.md) | [**float**](/windows/desktop/WinProg/windows-data-types) | same dimension(s) as input *n* |
| *ng* | [**vector**](https://github.com/MicrosoftDocs/win32/blob/docs/desktop-src//direct3dhlsl/dx-graphics-hlsl-intrinsic-functions.md) | [**float**](/windows/desktop/WinProg/windows-data-types) | same dimensions as input *n* |
| *ret* | [**vector**](https://github.com/MicrosoftDocs/win32/blob/docs/desktop-src//direct3dhlsl/dx-graphics-hlsl-intrinsic-functions.md) | [**float**](/windows/desktop/WinProg/windows-data-types) | same dimensions as input *n* |
## Minimum Shader Model
This function is supported in the following shader models.
| Shader Model | Supported |
|------------------------------------------------------------------------------------|-----------------------|
| [Shader Model 2 (DirectX HLSL)](https://github.com/MicrosoftDocs/win32/blob/docs/desktop-src//direct3dhlsl/dx-graphics-hlsl-sm2.md) and higher shader models | yes |
| [Shader Model 1 (DirectX HLSL)](https://github.com/MicrosoftDocs/win32/blob/docs/desktop-src//direct3dhlsl/dx-graphics-hlsl-sm1.md) | vs\_1\_1 and ps\_1\_4 |
## See also
<dl> <dt>
[**Intrinsic Functions (DirectX HLSL)**](https://github.com/MicrosoftDocs/win32/blob/docs/desktop-src//direct3dhlsl/dx-graphics-hlsl-intrinsic-functions.md)
</dt> </dl>
</pre>
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