[clang] [Clang] Avoid quadratic pack-indexing instantiation (store only the selected element) (PR #213790)

[Younan Zhang via cfe-commits]

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@@ -7121,6 +7127,56 @@ TreeTransform<Derived>::TransformPackIndexingType(TypeLocBuilder &TLB,
   if (Types.empty() && !PIT->expandsToEmptyPack())
     Types = llvm::ArrayRef<QualType>(&Pattern, 1);
 
+  // Fast path: substitute only the selected element instead of all N. A
+  // pack-indexing type inside a pack expansion (`T...[Is]...`) is transformed
+  // once per outer element, so substituting the whole pack each time is
+  // O(N^2) in time and memory.
+  if (Types.size() == 1 && Types[0]->containsUnexpandedParameterPack() &&
+      IndexExpr.isUsable() && !IndexExpr.get()->isInstantiationDependent()) {
+    QualType T = Types[0];
+    SmallVector<UnexpandedParameterPack, 2> Unexpanded;
+    getSema().collectUnexpandedParameterPacks(T, Unexpanded);
+    bool ShouldExpand = true, RetainExpansion = false;
+    UnsignedOrNone NumExpansions = std::nullopt;
+    if (getDerived().TryExpandParameterPacks(
+            TL.getEllipsisLoc(), SourceRange(), Unexpanded,
+            /*FailOnPackProducingTemplates=*/true, ShouldExpand, RetainExpansion,
+            NumExpansions))
+      return QualType();
+    if (ShouldExpand && !RetainExpansion) {
+      llvm::APSInt Value;
+      ExprResult CCE = SemaRef.CheckConvertedConstantExpression(
+          IndexExpr.get(), SemaRef.Context.getSizeType(), Value,
+          CCEKind::PackIndex);
+      if (!CCE.isUsable() || !Value.isRepresentableByInt64())
+        return QualType();
+      uint64_t V = Value.getZExtValue();
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zyn0217 wrote:

This is funny: if you look at PackIndexingType::getSelectedIndex(), it suggests that when IndexExpr is not instantiation dependent, the IndexExpr must be a ConstantExpr where you can fetch the value directly, without doing the constant evaluation dance.

Again I wonder if this is written by AI. I wish I'm not talking this PR with any of your agent.

https://github.com/llvm/llvm-project/pull/213790