<table border="1" cellspacing="0" cellpadding="8">
    <tr>
        <th>Issue</th>
        <td>
            <a href=https://github.com/llvm/llvm-project/issues/155392>155392</a>
        </td>
    </tr>

    <tr>
        <th>Summary</th>
        <td>
            [Clang] VectorExprEvaluator::VisitCallExpr - add MMX/SSE/AVX/AVX512 PMADDWD/PMADDUBSW intrinsics to be used in constexpr
        </td>
    </tr>

    <tr>
      <th>Labels</th>
      <td>
            good first issue,
            backend:X86,
            clang:frontend,
            constexpr
      </td>
    </tr>

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

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

<pre>
    The PMADDWD/PMADDUBSW intrinsics can all be used in constexpr with suitable handling of the __builtin_ia32_pmadd* builtins inside VectorExprEvaluator::VisitCallExpr similar to https://github.com/llvm/llvm-project/pull/152540
```
_mm_madd_pi16
_mm_madd_epi16 _mm256_madd_epi16 _mm512_madd_epi16
_mm_mask_madd_epi16 _mm256_mask_madd_epi16 _mm512_mask_madd_epi16
_mm_maskz_madd_epi16 _mm256_maskz_madd_epi16 _mm512_maskz_madd_epi16

_mm_maddubs_pi16
_mm_maddubs_epi16 _mm256_maddubs_epi16 _mm512_maddubs_epi16
_mm_mask_maddubs_epi16 _mm256_mask_maddubs_epi16 _mm512_mask_maddubs_epi16
_mm_maskz_maddubs_epi16 _mm256_maskz_maddubs_epi16 _mm512_maskz_maddubs_epi16
```
The constant folding doesn't map to an APInt operation - you should handle it locally, plus its a horizontal pairwise addition so that will need handling as well:
```
  // Split Lo/Hi elements pairs, extend and add together.
  // PMADDWD(X,Y) =
  // add(mul(sext(lhs[0]),sext(rhs[0])),mul(sext(lhs[1]),sext(rhs[1])))
  // PMADDUBSW(X,Y) =
  // sadd_sat(mul(zext(lhs[0]),sext(rhs[0])),mul(zext(lhs[1]),sext(rhs[1])))
```
We might need to consider adjusting the PMADDUBSW builtins to take an unsigned lhs argument as well.
</pre>
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