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

[Shafik Yaghmour via cfe-commits]

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@@ -11599,6 +11599,89 @@ static bool handleVectorElementCast(EvalInfo &Info, const FPOptions FPO,
   return false;
 }
 
+enum class PackKind {
+  SSWB,
+  USWB,
+  SSDW,
+  USDW
+}; // 16→8 signed/unsigned; 32→16 signed/unsigned
+
+static bool evalPackBuiltin(const CallExpr *E, EvalInfo &Info, APValue &Result,
+                            PackKind K) {
+  APValue L, R;
+  if (!EvaluateAsRValue(Info, E->getArg(0), L) ||
+      !EvaluateAsRValue(Info, E->getArg(1), R))
+    return false;
+
+  unsigned SrcBits = (K == PackKind::SSWB || K == PackKind::USWB) ? 16 : 32;
+  unsigned DstBits = SrcBits / 2;
+
+  unsigned NL = L.getVectorLength();
+  unsigned NR = R.getVectorLength();
+  if (NL == 0 || NR == 0 || NL != NR)
+    return false;
+
+  // Bounds for saturation (extended to SrcBits for compares).
+  APInt Lo = (K == PackKind::USWB || K == PackKind::USDW)
+                 ? APInt(SrcBits, 0)
+                 : APInt::getSignedMinValue(DstBits).sext(SrcBits);
+  APInt Hi = (K == PackKind::USWB || K == PackKind::USDW)
+                 ? APInt::getMaxValue(DstBits).zext(SrcBits)
+                 : APInt::getSignedMaxValue(DstBits).sext(SrcBits);
+
+  // Result element signedness follows the builtin's return vector element type.
+  QualType DestEltTy = E->getType()->castAs<VectorType>()->getElementType();
+  bool DestIsUnsigned = DestEltTy->isUnsignedIntegerType();
+
+  // Clamp one source element to the target range and narrow to DstBits.
+  auto clampOne = [&](const APSInt &X) -> APSInt {
+    APInt V = X;
+    if (V.getBitWidth() != SrcBits)
+      V = V.sextOrTrunc(SrcBits);
+
+    if (K == PackKind::USWB || K == PackKind::USDW) {
----------------
shafik wrote:

Please make sure to verify that you cover each branch, I see a lot of combination but I am sure we cover them all in the tests.

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