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

[Simon Pilgrim via cfe-commits]

================
@@ -11575,6 +11575,46 @@ static bool handleVectorElementCast(EvalInfo &Info, const FPOptions FPO,
   return false;
 }
 
+static bool evalPackBuiltin(const CallExpr *E, EvalInfo &Info, APValue &Result,
+                            llvm::function_ref<APInt(const APSInt &)> PackFn) {
+  APValue LHS, RHS;
+  if (!EvaluateAsRValue(Info, E->getArg(0), LHS) ||
+      !EvaluateAsRValue(Info, E->getArg(1), RHS))
+    return false;
+
+  unsigned LHSVecLen = LHS.getVectorLength();
+  unsigned RHSVecLen = RHS.getVectorLength();
+
+  assert(LHSVecLen != 0 && LHSVecLen == RHSVecLen &&
+         "pack builtin LHSVecLen must equal to RHSVecLen");
+
+  const VectorType *VT0 = E->getArg(0)->getType()->castAs<VectorType>();
+  const unsigned SrcBits = Info.Ctx.getIntWidth(VT0->getElementType());
+
+  const VectorType *DstVT = E->getType()->castAs<VectorType>();
+  QualType DstElemTy = DstVT->getElementType();
+  const bool DstIsUnsigned = DstElemTy->isUnsignedIntegerType();
+
+  const unsigned srcPerLane = 128 / SrcBits;
----------------
RKSimon wrote:

(style) Capitalize first letter of variables: SrcPerLane, Lanes, Lane, I etc.

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