Merge pull request swiftlang#36553 from zoecarver/cxx/fix-deep-template-spec

[cxx-interop] Improve performance of deep template instantiations.

Author: varungandhi-apple

lib/ClangImporter/ImportDecl.cpp

@@ -3725,6 +3725,14 @@ namespace {
 
     Decl *VisitClassTemplateSpecializationDecl(
                  const clang::ClassTemplateSpecializationDecl *decl) {
+      // Before we go any further, check if we've already got tens of thousands
+      // of specializations. If so, it means we're likely instantiating a very
+      // deep/complex template, or we've run into an infinite loop. In either
+      // case, its not worth the compile time, so bail.
+      // TODO: this could be configurable at some point.
+      if (llvm::size(decl->getSpecializedTemplate()->specializations()) > 10000)
+        return nullptr;
+
       // `Sema::isCompleteType` will try to instantiate the class template as a
       // side-effect and we rely on this here. `decl->getDefinition()` can
       // return nullptr before the call to sema and return its definition

test/Interop/Cxx/templates/Inputs/large-class-templates.h

@@ -4,3 +4,47 @@ struct HasTypeWithSelfAsParam {
 };
 
 using WillBeInfinite = HasTypeWithSelfAsParam<int>;
+
+namespace RegressionTest {
+
+// This is a regression test to check that we don't instantiate exponentially
+// large templates. The below template will instantiate hundreds of thousands
+// (or more) of specializations before hitting the 8-template-param-deep limit.
+template<class E, int>
+struct SliceExpr {
+  using type = typename E::type;
+  E expr;
+  
+  typename E::type* test() { return nullptr; }
+};
+
+template<class T>
+struct Array {
+  using type = T;
+};
+
+template<class E>
+struct ValExpr {
+  using type = typename E::type;
+  E expr;
+  
+  ValExpr<SliceExpr<E, 1>> test1() { return {SliceExpr<E, 1>{expr}}; }
+  ValExpr<SliceExpr<E, 2>> test2() { return {SliceExpr<E, 2>{expr}}; }
+  ValExpr<SliceExpr<E, 3>> test3() { return {SliceExpr<E, 3>{expr}}; }
+  ValExpr<SliceExpr<E, 4>> test4() { return {SliceExpr<E, 4>{expr}}; }
+  ValExpr<SliceExpr<E, 5>> test5() { return {SliceExpr<E, 5>{expr}}; }
+  ValExpr<SliceExpr<E, 6>> test6() { return {SliceExpr<E, 6>{expr}}; }
+  ValExpr<SliceExpr<E, 7>> test7() { return {SliceExpr<E, 7>{expr}}; }
+  ValExpr<SliceExpr<E, 8>> test8() { return {SliceExpr<E, 8>{expr}}; }
+  ValExpr<SliceExpr<E, 9>> test9() { return {SliceExpr<E, 8>{expr}}; }
+  ValExpr<SliceExpr<E, 11>> test11() { return {SliceExpr<E, 11>{expr}}; }
+  ValExpr<SliceExpr<E, 12>> test12() { return {SliceExpr<E, 12>{expr}}; }
+};
+
+// This class template is exponentially slow to *fully* instantiate (and the
+// exponent is the number of testX methods). Make sure that we can still
+// partially import it without importing all of its child template
+// instantiations.
+void user(ValExpr<Array<int>> *) { }
+
+} // end namespace 'RegressionTest'

test/Interop/Cxx/templates/large-class-templates-module-interface.swift

@@ -9,3 +9,9 @@
 // CHECK: }
 
 // CHECK: typealias WillBeInfinite = __CxxTemplateInst22HasTypeWithSelfAsParamIiE
+
+// Make sure we fail to import super slow templates.
+// CHECK-NOT: __CxxTemplateInstN14RegressionTest7ValExprINS_9SliceExprINS_5ArrayIiEELi1EEEEE
+// TODO: we should not be importing functions that use this type in their
+// signature (such as the function below).
+// CHECK: mutating func test1() -> RegressionTest.__CxxTemplateInstN14RegressionTest7ValExprINS_9SliceExprINS_5ArrayIiEELi1EEEEE