Rework our handling of copy construction of temporaries, which was a

poor (and wrong) approximation of the actual rules governing when to
build a copy and when it can be elided.

The correct implementation is actually simpler than the
approximation. When we only enumerate constructors as part of
initialization (e.g., for direct initialization or when we're copying
from a class type or one of its derived classes), we don't create a
copy. When we enumerate all conversion functions, we do create a
copy. Before, we created some extra copies and missed some
others. The new test copy-initialization.cpp shows a case where we
missed creating a (required, non-elidable) copy as part of a
user-defined conversion, which resulted in a miscompile. This commit
also fixes PR6757, where the missing copy made us reject well-formed
code in the ternary operator.

This commit also cleans up our handling of copy elision in the case
where we create an extra copy of a temporary object, which became
necessary now that we produce the right copies. The code that seeks to
find the temporary object being copied has moved into
Expr::getTemporaryObject(); it used to have two different
not-quite-the-same implementations, one in Sema and one in CodeGen.

Note that we still do not attempt to perform the named return value
optimization, so we miss copy elisions for return values and throw
expressions.

llvm-svn: 100196

1 files changed, 65 insertions, 0 deletions

diff --git a/clang/lib/AST/Expr.cpp b/clang/lib/AST/Expr.cpp
index 6764612c80b..ae4bc8c8012 100644
--- a/clang/lib/AST/Expr.cpp
+++ b/clang/lib/AST/Expr.cpp
@@ -1476,6 +1476,71 @@ bool Expr::isDefaultArgument() const {
   return isa<CXXDefaultArgExpr>(E);
 }
 
+/// \brief Skip over any no-op casts and any temporary-binding
+/// expressions.
+static const Expr *skipTemporaryBindingsAndNoOpCasts(const Expr *E) {
+  while (const ImplicitCastExpr *ICE = dyn_cast<ImplicitCastExpr>(E)) {
+    if (ICE->getCastKind() == CastExpr::CK_NoOp)
+      E = ICE->getSubExpr();
+    else
+      break;
+  }
+
+  while (const CXXBindTemporaryExpr *BE = dyn_cast<CXXBindTemporaryExpr>(E))
+    E = BE->getSubExpr();
+
+  while (const ImplicitCastExpr *ICE = dyn_cast<ImplicitCastExpr>(E)) {
+    if (ICE->getCastKind() == CastExpr::CK_NoOp)
+      E = ICE->getSubExpr();
+    else
+      break;
+  }
+  
+  return E;
+}
+
+const Expr *Expr::getTemporaryObject() const {
+  const Expr *E = skipTemporaryBindingsAndNoOpCasts(this);
+
+  // A cast can produce a temporary object. The object's construction
+  // is represented as a CXXConstructExpr.
+  if (const CastExpr *Cast = dyn_cast<CastExpr>(E)) {
+    // Only user-defined and constructor conversions can produce
+    // temporary objects.
+    if (Cast->getCastKind() != CastExpr::CK_ConstructorConversion &&
+        Cast->getCastKind() != CastExpr::CK_UserDefinedConversion)
+      return 0;
+
+    // Strip off temporary bindings and no-op casts.
+    const Expr *Sub = skipTemporaryBindingsAndNoOpCasts(Cast->getSubExpr());
+
+    // If this is a constructor conversion, see if we have an object
+    // construction.
+    if (Cast->getCastKind() == CastExpr::CK_ConstructorConversion)
+      return dyn_cast<CXXConstructExpr>(Sub);
+
+    // If this is a user-defined conversion, see if we have a call to
+    // a function that itself returns a temporary object.
+    if (Cast->getCastKind() == CastExpr::CK_UserDefinedConversion)
+      if (const CallExpr *CE = dyn_cast<CallExpr>(Sub))
+        if (CE->getCallReturnType()->isRecordType())
+          return CE;
+
+    return 0;
+  }
+
+  // A call returning a class type returns a temporary.
+  if (const CallExpr *CE = dyn_cast<CallExpr>(E)) {
+    if (CE->getCallReturnType()->isRecordType())
+      return CE;
+
+    return 0;
+  }
+
+  // Explicit temporary object constructors create temporaries.
+  return dyn_cast<CXXTemporaryObjectExpr>(E);
+}
+
 /// hasAnyTypeDependentArguments - Determines if any of the expressions
 /// in Exprs is type-dependent.
 bool Expr::hasAnyTypeDependentArguments(Expr** Exprs, unsigned NumExprs) {