[clang] [Headers][X86] VectorExprEvaluator::VisitCallExpr - allow SSE/AVX2/AVX512 pack intrinsics to be used in constexpr (PR #156003)

[Simon Pilgrim via cfe-commits]

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@@ -2604,6 +2605,65 @@ static bool interp__builtin_elementwise_int_binop(
   return true;
 }
 
+static bool interp__builtin_x86_pack(
+    InterpState &S, CodePtr, const CallExpr *E, bool IsUnsat,
+    llvm::function_ref<APSInt(const APSInt &X, unsigned, unsigned, bool)>
+        narrowElement) {
+  const auto *VT0 = E->getArg(0)->getType()->castAs<VectorType>();
+  const auto *VT1 = E->getArg(1)->getType()->castAs<VectorType>();
+  assert(VT0 && VT1 && "pack builtin VT0 and VT1 must be VectorType");
+  assert(VT0->getElementType() == VT1->getElementType() &&
+         VT0->getNumElements() == VT1->getNumElements() &&
+         "pack builtin VT0 and VT1 ElementType must be same");
+
+  const Pointer &RHS = S.Stk.pop<Pointer>();
+  const Pointer &LHS = S.Stk.pop<Pointer>();
+  const Pointer &Dst = S.Stk.peek<Pointer>();
+
+  ASTContext &Ctx = S.getASTContext();
+  const unsigned SrcBits = Ctx.getIntWidth(VT0->getElementType());
+  const unsigned DstBits = SrcBits / 2;
+  const unsigned LHSVecLen = VT0->getNumElements();
+  const unsigned VectorBits = LHSVecLen * SrcBits;
+  const unsigned srcPerLane = VectorBits >= 128 ? (128 / SrcBits) : LHSVecLen;
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RKSimon wrote:

(style) Capitalise variable first letter

https://github.com/llvm/llvm-project/pull/156003