[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

llvm-svn: 252036

1 files changed, 14 insertions, 0 deletions

diff --git a/clang/test/PCH/make-integer-seq.cpp b/clang/test/PCH/make-integer-seq.cpp
new file mode 100644
index 00000000000..3622c33449d
--- /dev/null
+++ b/clang/test/PCH/make-integer-seq.cpp
@@ -0,0 +1,14 @@
+// RUN: %clang_cc1 -std=c++14 -x c++-header %s -emit-pch -o %t.pch
+// RUN: %clang_cc1 -std=c++14 -x c++ /dev/null -include-pch %t.pch
+
+template <class T, T... I>
+struct Seq {
+    static constexpr T PackSize = sizeof...(I);
+};
+
+template <typename T, T N>
+using MakeSeq = __make_integer_seq<Seq, T, N>;
+
+void fn1() {
+  MakeSeq<int, 3> x;
+}