Assign APValues by swapping from a temporary. Removes a bunch of unnecessary

copy-construction, which Daniel Dunbar reports as giving a 0.75% speedup on
403.gcc/combine.c. The performance differences on my constexpr torture tests
are below the noise floor.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@152455 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 8 insertions, 2 deletions

diff --git a/lib/AST/ExprConstant.cpp b/lib/AST/ExprConstant.cpp
index 5f9b0501b2..97c906c407 100644
--- a/lib/AST/ExprConstant.cpp
+++ b/lib/AST/ExprConstant.cpp
@@ -1518,7 +1518,7 @@ static bool ExtractSubobject(EvalInfo &Info, const Expr *E,
     // This object might be initialized later.
     return false;
 
-  const APValue *O = &Obj;
+  APValue *O = &Obj;
   // Walk the designator's path to find the subobject.
   for (unsigned I = 0, N = Sub.Entries.size(); I != N; ++I) {
     if (ObjType->isArrayType()) {
@@ -1616,7 +1616,13 @@ static bool ExtractSubobject(EvalInfo &Info, const Expr *E,
     }
   }
 
-  Obj = APValue(*O);
+  // This may look super-stupid, but it serves an important purpose: if we just
+  // swapped Obj and *O, we'd create an object which had itself as a subobject.
+  // To avoid the leak, we ensure that Tmp ends up owning the original complete
+  // object, which is destroyed by Tmp's destructor.
+  APValue Tmp;
+  O->swap(Tmp);
+  Obj.swap(Tmp);
   return true;
 }