[ObjC] Use empty Objective-C collection literal constants when

available.

Original patch by Douglas Gregor with minor modifications.

This recommits r300389, which broke bots because there have been API
changes since the original patch was written.

rdar://problem/20689633

llvm-svn: 300396

1 files changed, 14 insertions, 2 deletions

diff --git a/clang/lib/CodeGen/CGObjC.cpp b/clang/lib/CodeGen/CGObjC.cpp
index c011ab4dfc9..3a09a15dbc1 100644
--- a/clang/lib/CodeGen/CGObjC.cpp
+++ b/clang/lib/CodeGen/CGObjC.cpp
@@ -117,10 +117,22 @@ llvm::Value *CodeGenFunction::EmitObjCCollectionLiteral(const Expr *E,
   const ObjCArrayLiteral *ALE = dyn_cast<ObjCArrayLiteral>(E);
   if (!ALE)
     DLE = cast<ObjCDictionaryLiteral>(E);
-  
-  // Compute the type of the array we're initializing.
+
+  // Optimize empty collections by referencing constants, when available.
   uint64_t NumElements = 
     ALE ? ALE->getNumElements() : DLE->getNumElements();
+  if (NumElements == 0 && CGM.getLangOpts().ObjCRuntime.hasEmptyCollections()) {
+    StringRef ConstantName = ALE ? "__NSArray0__" : "__NSDictionary0__";
+    QualType IdTy(CGM.getContext().getObjCIdType());
+    llvm::Constant *Constant =
+        CGM.CreateRuntimeVariable(ConvertType(IdTy), ConstantName);
+    Address Addr(Constant, Context.getTypeAlignInChars(IdTy));
+    LValue LV = MakeAddrLValue(Addr, IdTy);
+    return Builder.CreateBitCast(EmitLoadOfScalar(LV, E->getLocStart()),
+                                 ConvertType(E->getType()));
+  }
+
+  // Compute the type of the array we're initializing.
   llvm::APInt APNumElements(Context.getTypeSize(Context.getSizeType()),
                             NumElements);
   QualType ElementType = Context.getObjCIdType().withConst();