[clang] [Headers][X86] Allow AVX512 masked arithmetic intrinsics to be used in constexpr (PR #162816)

[Simon Pilgrim via cfe-commits]

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@@ -11705,6 +11705,23 @@ bool VectorExprEvaluator::VisitCallExpr(const CallExpr *E) {
         return Success(APValue(ResultElements.data(), SourceLen), E);
       };
 
+  auto EvalSelectScalar = [&](unsigned Len) -> std::optional<APValue> {
+    APSInt Mask;
+    APValue AVal, WVal;
+    if (!EvaluateInteger(E->getArg(0), Mask, Info) ||
+        !EvaluateAsRValue(Info, E->getArg(1), AVal) ||
+        !EvaluateAsRValue(Info, E->getArg(2), WVal))
+      return std::nullopt;
+
+    const bool TakeA0 = (Mask.getZExtValue() & 1u) != 0;
+    SmallVector<APValue, 4> Res;
+    Res.reserve(Len);
+    Res.push_back(TakeA0 ? AVal.getVectorElt(0) : WVal.getVectorElt(0));
+    for (unsigned i = 1; i < Len; ++i)
+      Res.push_back(WVal.getVectorElt(i));
+    return APValue(Res.data(), Res.size());
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RKSimon wrote:

why not just return Success / false?

https://github.com/llvm/llvm-project/pull/162816