[Sema] Implement __make_integer_seq

This new builtin template allows for incredibly fast instantiations of
templates like std::integer_sequence.

Performance numbers follow:
My work station has 64 GB of ram + 20 Xeon Cores at 2.8 GHz.

__make_integer_seq<std::integer_sequence, int, 90000> takes 0.25
seconds.

std::make_integer_sequence<int, 90000> takes unbound time, it is still
running.  Clang is consuming gigabytes of memory.

Differential Revision: http://reviews.llvm.org/D13786

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@252036 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 30 insertions, 0 deletions

diff --git a/include/clang/AST/DeclTemplate.h b/include/clang/AST/DeclTemplate.h
index bd38ac8823..a23fe4b5e5 100644
--- a/include/clang/AST/DeclTemplate.h
+++ b/include/clang/AST/DeclTemplate.h
@@ -25,6 +25,7 @@
 
 namespace clang {
 
+enum BuiltinTemplateKind : int;
 class TemplateParameterList;
 class TemplateDecl;
 class RedeclarableTemplateDecl;
@@ -1490,6 +1491,35 @@ public:
   friend TrailingObjects;
 };
 
+/// \brief Represents the builtin template declaration which is used to
+/// implement __make_integer_seq.  It serves no real purpose beyond existing as
+/// a place to hold template parameters.
+class BuiltinTemplateDecl : public TemplateDecl {
+  void anchor() override;
+
+  BuiltinTemplateDecl(const ASTContext &C, DeclContext *DC,
+                      DeclarationName Name, BuiltinTemplateKind BTK);
+
+  BuiltinTemplateKind BTK;
+
+public:
+  // Implement isa/cast/dyncast support
+  static bool classof(const Decl *D) { return classofKind(D->getKind()); }
+  static bool classofKind(Kind K) { return K == BuiltinTemplate; }
+
+  static BuiltinTemplateDecl *Create(const ASTContext &C, DeclContext *DC,
+                                     DeclarationName Name,
+                                     BuiltinTemplateKind BTK) {
+    return new (C, DC) BuiltinTemplateDecl(C, DC, Name, BTK);
+  }
+
+  SourceRange getSourceRange() const override LLVM_READONLY {
+    return SourceRange();
+  }
+
+  BuiltinTemplateKind getBuiltinTemplateKind() const { return BTK; }
+};
+
 /// \brief Represents a class template specialization, which refers to
 /// a class template with a given set of template arguments.
 ///