From e533381b755faadf8f1a7f7329f32a7102a0c950 Mon Sep 17 00:00:00 2001
From: Nathan Ridge <zeratul976@hotmail.com>
Date: Wed, 2 May 2018 01:36:46 -0400
Subject: [PATCH] Bug 534126 - Cache instantiations of alias template instances

This avoid runtime that's exponential in the nesting depth of
alias template instances.

Change-Id: Ibde6a6d98753df54e8e495a8b4547a90e8313191
---
 .../parser/tests/ast2/AST2TemplateTests.java  | 108 ++++++++++++++++++
 .../dom/parser/cpp/CPPASTTranslationUnit.java |  11 ++
 .../parser/cpp/semantics/CPPTemplates.java    |  49 +++++++-
 3 files changed, 163 insertions(+), 5 deletions(-)

diff --git a/core/org.eclipse.cdt.core.tests/parser/org/eclipse/cdt/core/parser/tests/ast2/AST2TemplateTests.java b/core/org.eclipse.cdt.core.tests/parser/org/eclipse/cdt/core/parser/tests/ast2/AST2TemplateTests.java
index 496f8958f62..48a9cd6539a 100644
--- a/core/org.eclipse.cdt.core.tests/parser/org/eclipse/cdt/core/parser/tests/ast2/AST2TemplateTests.java
+++ b/core/org.eclipse.cdt.core.tests/parser/org/eclipse/cdt/core/parser/tests/ast2/AST2TemplateTests.java
@@ -10826,4 +10826,112 @@ public class AST2TemplateTests extends AST2CPPTestBase {
 	public void testOverloadResolutionWithInitializerList_531322() throws Exception {
 		parseAndCheckBindings();
 	}
+	
+	//	using size_t = decltype(sizeof(int));
+	//
+	//	template <class Fn, class... Ts>
+	//	using MetaApply = typename Fn::template apply<Ts...>;
+	//
+	//	template <class... Ts>
+	//	struct TypeList {
+	//		using type = TypeList;
+	//
+	//		template <class Fn>
+	//		using apply = MetaApply<Fn, Ts...>;
+	//	};
+	//
+	//	struct Empty {};
+	//
+	//	namespace impl {
+	//		template <bool B>
+	//		struct If_ {
+	//			template <class T, class U>
+	//			using apply = T;
+	//		};
+	//		template <>
+	//		struct If_<false> {
+	//			template <class T, class U>
+	//			using apply = U;
+	//		};
+	//	}
+	//
+	//	template <bool If_, class Then, class Else>
+	//	using If = MetaApply<impl::If_<If_>, Then, Else>;
+	//
+	//	template <template <class...> class C, class... Ts>
+	//	class MetaDefer {
+	//		template <template <class...> class D = C, class = D<Ts...>>
+	//		static char(&try_(int))[1];
+	//		static char(&try_(long))[2];
+	//		struct Result {
+	//			using type = C<Ts...>;
+	//		};
+	//
+	//	public:
+	//		template <class... Us>
+	//		using apply = typename If<sizeof(try_(0)) - 1 || sizeof...(Us), Empty, Result>::type;
+	//	};
+	//
+	//	struct MetaIdentity {
+	//		template <class T>
+	//		using apply = T;
+	//	};
+	//
+	//	template <template <class...> class C>
+	//	struct MetaQuote {
+	//		template <class... Ts>
+	//		using apply = MetaApply<MetaDefer<C, Ts...>>;
+	//	};
+	//
+	//	template <>
+	//	struct MetaQuote<TypeList> {
+	//		template <class... Ts>
+	//		using apply = TypeList<Ts...>;
+	//	};
+	//
+	//	template <class Fn>
+	//	struct MetaFlip {
+	//		template <class A, class B>
+	//		using apply = MetaApply<Fn, B, A>;
+	//	};
+	//
+	//	namespace impl {
+	//		template <class Fn>
+	//		struct FoldL_ {
+	//			template <class... Ts>
+	//			struct Lambda : MetaIdentity {};
+	//			template <class A, class... Ts>
+	//			struct Lambda<A, Ts...> {
+	//				template <class State>
+	//				using apply = MetaApply<Lambda<Ts...>, MetaApply<Fn, State, A>>;
+	//			};
+	//			template <class... Ts>
+	//			using apply = Lambda<Ts...>;
+	//		};
+	//	}
+	//
+	//	template <class List, class State, class Fn>
+	//	using TypeReverseFold = MetaApply<MetaApply<List, impl::FoldL_<Fn>>, State>;
+	//
+	//	template <class Car, class Cdr = Empty>
+	//	struct Cons {};
+	//	using Fn = MetaQuote<Cons>;
+	//	using T4 = TypeReverseFold<
+	//      // Make it long enough to be sure that if the runtime is exponential
+	//      // in the length of the list, the test suite times out.
+	//		TypeList<int, short, void, float, double, long, char
+    //               int*, short*, void*, float*, double*, long*, char*>,
+	//		Empty,
+	//		MetaFlip<Fn>>;
+	//
+	//	template <class T>
+	//	struct Dummy {
+	//		static const bool value = true;
+	//	};
+	//	int main() {
+	//		static_assert(Dummy<T4>::value, "");
+	//	}
+	public void testMetaprogrammingWithAliasTemplates_534126() throws Exception {
+		parseAndCheckBindings();
+	}
 }
diff --git a/core/org.eclipse.cdt.core/parser/org/eclipse/cdt/internal/core/dom/parser/cpp/CPPASTTranslationUnit.java b/core/org.eclipse.cdt.core/parser/org/eclipse/cdt/internal/core/dom/parser/cpp/CPPASTTranslationUnit.java
index 9ff0ec7ae5c..1d0410f50b2 100644
--- a/core/org.eclipse.cdt.core/parser/org/eclipse/cdt/internal/core/dom/parser/cpp/CPPASTTranslationUnit.java
+++ b/core/org.eclipse.cdt.core/parser/org/eclipse/cdt/internal/core/dom/parser/cpp/CPPASTTranslationUnit.java
@@ -41,6 +41,7 @@ import org.eclipse.cdt.internal.core.dom.parser.ASTTranslationUnit;
 import org.eclipse.cdt.internal.core.dom.parser.IASTAmbiguityParent;
 import org.eclipse.cdt.internal.core.dom.parser.cpp.semantics.CPPInheritance.FinalOverriderMap;
 import org.eclipse.cdt.internal.core.dom.parser.cpp.semantics.CPPVisitor;
+import org.eclipse.cdt.internal.core.dom.parser.cpp.semantics.TypeInstantiationRequest;
 import org.eclipse.cdt.internal.core.index.IIndexScope;
 import org.eclipse.cdt.internal.core.parser.scanner.InternalFileContent;
 
@@ -55,6 +56,12 @@ public class CPPASTTranslationUnit extends ASTTranslationUnit implements ICPPAST
 
 	// Caches.
 	private final Map<ICPPClassType, FinalOverriderMap> fFinalOverriderMapCache = new HashMap<>();
+	// Cache for type instantiations. This is currently only used for instantiations of
+	// alias template instances, but its use could potentially be expanded to cover other
+	// instantiations. Note that class template instances are already cached by the 
+	// template definition, so we wouldn't want to double-cache those. (But we could e.g. 
+	// cache instantiations of function types if we found it worthwhile.)
+	private final Map<TypeInstantiationRequest, IType> fInstantiationCache = new HashMap<>();
 
 	public CPPASTTranslationUnit() {
 		fScopeMapper= new CPPScopeMapper(this);
@@ -249,6 +256,10 @@ public class CPPASTTranslationUnit extends ASTTranslationUnit implements ICPPAST
 	public Map<ICPPClassType, FinalOverriderMap> getFinalOverriderMapCache() {
 		return fFinalOverriderMapCache;
 	}
+	
+	public Map<TypeInstantiationRequest, IType> getInstantiationCache() {
+		return fInstantiationCache;
+	}
 
 	public void recordPartialSpecialization(ICPPClassTemplatePartialSpecialization indexSpec,
 			ICPPClassTemplatePartialSpecialization astSpec) {
diff --git a/core/org.eclipse.cdt.core/parser/org/eclipse/cdt/internal/core/dom/parser/cpp/semantics/CPPTemplates.java b/core/org.eclipse.cdt.core/parser/org/eclipse/cdt/internal/core/dom/parser/cpp/semantics/CPPTemplates.java
index 379abee791f..ed26f0a81d4 100644
--- a/core/org.eclipse.cdt.core/parser/org/eclipse/cdt/internal/core/dom/parser/cpp/semantics/CPPTemplates.java
+++ b/core/org.eclipse.cdt.core/parser/org/eclipse/cdt/internal/core/dom/parser/cpp/semantics/CPPTemplates.java
@@ -26,6 +26,7 @@ import static org.eclipse.cdt.internal.core.dom.parser.cpp.semantics.SemanticUti
 import java.util.ArrayList;
 import java.util.HashSet;
 import java.util.List;
+import java.util.Map;
 import java.util.Set;
 
 import org.eclipse.cdt.core.CCorePlugin;
@@ -45,6 +46,7 @@ import org.eclipse.cdt.core.dom.ast.IASTName;
 import org.eclipse.cdt.core.dom.ast.IASTNameOwner;
 import org.eclipse.cdt.core.dom.ast.IASTNode;
 import org.eclipse.cdt.core.dom.ast.IASTSimpleDeclaration;
+import org.eclipse.cdt.core.dom.ast.IASTTranslationUnit;
 import org.eclipse.cdt.core.dom.ast.IASTTypeId;
 import org.eclipse.cdt.core.dom.ast.IArrayType;
 import org.eclipse.cdt.core.dom.ast.IBinding;
@@ -134,6 +136,7 @@ import org.eclipse.cdt.internal.core.dom.parser.ProblemBinding;
 import org.eclipse.cdt.internal.core.dom.parser.ProblemFunctionType;
 import org.eclipse.cdt.internal.core.dom.parser.ProblemType;
 import org.eclipse.cdt.internal.core.dom.parser.cpp.CPPASTName;
+import org.eclipse.cdt.internal.core.dom.parser.cpp.CPPASTTranslationUnit;
 import org.eclipse.cdt.internal.core.dom.parser.cpp.CPPAliasTemplateInstance;
 import org.eclipse.cdt.internal.core.dom.parser.cpp.CPPAliasTemplateSpecialization;
 import org.eclipse.cdt.internal.core.dom.parser.cpp.CPPArrayType;
@@ -1607,21 +1610,34 @@ public class CPPTemplates {
 			// to the target type but can SFINAE out during instantiation, so it's not
 			// sufficient to handle it in the ITypeContainer case.
 			if (type instanceof ICPPAliasTemplateInstance) {
+				// Cache instantiations of alias templates. This is necessary because below we can
+				// potentially instantiate types that appear in the arguments of an alias template
+				// instance up to three times (once in instantiateArguments(), once in
+				// instantiateArgumentMap(), and if the argument appears in the aliased type, then
+				// a third time in instantiateType()), leading to exponential runtime in cases of
+				// nested alias template instances (which can be common in metaprogramming code
+				// implemented using alias templates). 
+				IType result = getCachedInstantiation(instantiationRequest);
+				if (result != null) {
+					return result;
+				}
 				ICPPAliasTemplateInstance instance = (ICPPAliasTemplateInstance) type;
 				ICPPAliasTemplate template = instance.getTemplateDefinition();
 				ICPPTemplateArgument[] args = instance.getTemplateArguments();
 				ICPPTemplateArgument[] newArgs = instantiateArguments(args, context, true);
 				if (newArgs == null) {
-					return (IType) createProblem(template, 
+					result = (IType) createProblem(template, 
 							IProblemBinding.SEMANTIC_INVALID_TEMPLATE_ARGUMENTS);
-				}
-				if (args != newArgs) {
+				} else if (args != newArgs) {
 					IType target = instantiateType(instance.getType(), context);
 					CPPTemplateParameterMap map = 
 							instantiateArgumentMap(instance.getTemplateParameterMap(), context);
-					return new CPPAliasTemplateInstance(template, target, instance.getOwner(), map, newArgs);
+					result = new CPPAliasTemplateInstance(template, target, instance.getOwner(), map, newArgs);
+				} else {
+					result = type;
 				}
-				return type;
+				putCachedInstantiation(instantiationRequest, result);
+				return result;
 			}
 
 			if (type instanceof ITypeContainer) {
@@ -3342,4 +3358,27 @@ public class CPPTemplates {
 		}
 		return true;
 	}
+
+	private static Map<TypeInstantiationRequest, IType> getInstantiationCache() {
+		IASTNode lookupPoint = CPPSemantics.getCurrentLookupPoint();
+		if (lookupPoint != null) {
+			IASTTranslationUnit tu = lookupPoint.getTranslationUnit();
+			if (tu instanceof CPPASTTranslationUnit) {
+				return ((CPPASTTranslationUnit) tu).getInstantiationCache();
+			}
+		}
+		return null;
+	}
+	
+	private static IType getCachedInstantiation(TypeInstantiationRequest instantiationRequest) {
+		Map<TypeInstantiationRequest, IType> cache = getInstantiationCache();
+		return cache != null ? cache.get(instantiationRequest) : null;
+	}
+	
+	private static void putCachedInstantiation(TypeInstantiationRequest instantiationRequest, IType result) {
+		Map<TypeInstantiationRequest, IType> cache = getInstantiationCache();
+		if (cache != null) {
+			cache.put(instantiationRequest, result);
+		}
+	}
 }