<table border="1" cellspacing="0" cellpadding="8">
    <tr>
        <th>Issue</th>
        <td>
            <a href=https://github.com/llvm/llvm-project/issues/99111>99111</a>
        </td>
    </tr>

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

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

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

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

<pre>
    - [ ] Implement `EvaluateAttributeSnapped` clang builtin,
- [ ] Link `EvaluateAttributeSnapped` clang builtin with `hlsl_intrinsics.h`
- [ ] Add sema checks for `EvaluateAttributeSnapped` to `CheckHLSLBuiltinFunctionCall` in `SemaChecking.cpp`
- [ ] Add codegen for `EvaluateAttributeSnapped` to `EmitHLSLBuiltinExpr` in `CGBuiltin.cpp`
- [ ] Add codegen tests to `clang/test/CodeGenHLSL/builtins/EvaluateAttributeSnapped.hlsl`
- [ ] Add sema tests to `clang/test/SemaHLSL/BuiltIns/EvaluateAttributeSnapped-errors.hlsl`
- [ ] Create the `int_dx_EvaluateAttributeSnapped` intrinsic in `IntrinsicsDirectX.td`
- [ ] Create the `DXILOpMapping` of `int_dx_EvaluateAttributeSnapped` to  `87` in `DXIL.td`
- [ ] Create the  `EvaluateAttributeSnapped.ll` and `EvaluateAttributeSnapped_errors.ll` tests in `llvm/test/CodeGen/DirectX/`

## DirectX

| DXIL Opcode | DXIL OpName | Shader Model | Shader Stages |
| ----------- | ----------- | ------------ | ------------- |
| 87 | EvalSnapped | 6.0 | ('pixel',) |

## SPIR-V

There is no support for `EvaluateAttributeSnapped` when targeting SPIR-V.

## Test Case(s)

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

[numthreads(1, 1, 1)]
[shader("pixel")]
float4 fn(float4 p1 : COLOR ) : SV_Target {
        int2 p2;
        return EvaluateAttributeSnapped(p1, p2);
}
```
## HLSL:

Evaluates at the pixel centroid with an offset.

## Syntax

``` syntax
numeric EvaluateAttributeSnapped(
  in attrib numeric value,
  in 
            int2 offset
);
```

## Parameters

<dl> <dt>

*value* \[in\]
</dt> <dd>

Type: **attrib numeric**

The input value.

</dd> <dt>

*offset* \[in\]
</dt> <dd>

Type: **int2**

A 2D offset from the pixel center using a 16x16 grid.

</dd> </dl>

## Remarks

The range for the *offset* parameter must be defined by the following byte code.

Only the least significant 4 bits of the first two components (U, V) of the pixel offset are used. The conversion from the 4-bit fixed point to float is as follows (MSB...LSB), where the MSB is both a part of the fraction and determines the sign:

-   1000 = -0.5f (-8 / 16)
-   1001 = -0.4375f (-7 / 16)
-   1010 = -0.375f (-6 / 16)
-   1011 = -0.3125f (-5 / 16)
-   1100 = -0.25f (-4 / 16)
-   1101 = -0.1875f (-3 / 16)
-   1110 = -0.125f (-2 / 16)
-   1111 = -0.0625f (-1 / 16)
-   0000 = 0.0f ( 0 / 16)
-   0001 = 0.0625f ( 1 / 16)
-   0010 = 0.125f ( 2 / 16)
-   0011 = 0.1875f ( 3 / 16)
-   0100 = 0.25f ( 4 / 16)
-   0101 = 0.3125f ( 5 / 16)
-   0110 = 0.375f ( 6 / 16)
-   0111 = 0.4375f ( 7 / 16)

> [!Note]  
> The left and top edges of a pixel are included in the offset; however, the bottom and right edges are not included. All other bits in the 32-bit integer U and V offset values are ignored.

 

An implementation can take the offset provided by the shader and obtain a full 32-bit fixed point value (28.4), which spans the valid range, by performing the following calculation:


```
iU = (iU<<28)>>28  // keep lowest 4 bits and sign extend, which yields [-8..7]
```



If an implementation must map the offset to a floating-point offset, it performs the following calculation:


```
fU = ((float)iU)/16
```



### Minimum Shader Model

This function is supported in the following shader models.



| Shader Model                                                                | Supported |
|-----------------------------------------------------------------------------|-----------|
| [Shader Model 5](https://github.com/MicrosoftDocs/win32/blob/docs/desktop-src//direct3dhlsl/d3d11-graphics-reference-sm5.md) and higher shader models | yes       |



 

This function is supported in the following types of shaders:



| Vertex | Hull | Domain | Geometry | Pixel | Compute |
|--------|------|--------|----------|-------|---------|
|        |      |        |          | x     |         |



 

## See also

<dl> <dt>

[Intrinsic Functions](https://github.com/MicrosoftDocs/win32/blob/docs/desktop-src//direct3dhlsl/dx-graphics-hlsl-intrinsic-functions.md)
</dt> <dt>

[Shader Model 5](https://github.com/MicrosoftDocs/win32/blob/docs/desktop-src//direct3dhlsl/d3d11-graphics-reference-sm5.md)
</dt> </dl>
</pre>
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