[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, 5 insertions, 0 deletions

diff --git a/clang/lib/Sema/SemaLookup.cpp b/clang/lib/Sema/SemaLookup.cpp
index 0ba34ea8ef2..66cfada9f43 100644
--- a/clang/lib/Sema/SemaLookup.cpp
+++ b/clang/lib/Sema/SemaLookup.cpp
@@ -627,6 +627,11 @@ static bool LookupBuiltin(Sema &S, LookupResult &R) {
         R.addDecl(S.getASTContext().getFloat128StubType());
         return true;
       }
+      if (S.getLangOpts().CPlusPlus && NameKind == Sema::LookupOrdinaryName &&
+          II == S.getASTContext().getMakeIntegerSeqName()) {
+        R.addDecl(S.getASTContext().getMakeIntegerSeqDecl());
+        return true;
+      }
 
       // If this is a builtin on this (or all) targets, create the decl.
       if (unsigned BuiltinID = II->getBuiltinID()) {