current state of the mess. One line fails in the tests, and

useless copies are made when evaluating nested expressions.
Changes:
- kill LazyBit, introduce EvalBeforeNestingBit and EvalBeforeAssigningBit
- product and random don't evaluate immediately anymore
- eval() always evaluates
- change the value of Dynamic to some large positive value,
  in preparation of future simplifications

Eigen/src/Core/CommaInitializer.h

@@ -76,9 +76,10 @@ struct MatrixBase<Derived>::CommaInitializer
     ei_assert(m_col<m_matrix.cols()
       && "Too many coefficients passed to MatrixBase::operator<<");
     ei_assert(m_currentBlockRows==other.rows());
-    if (OtherDerived::RowsAtCompileTime>0 && OtherDerived::ColsAtCompileTime>0)
-      m_matrix.block< (OtherDerived::RowsAtCompileTime>0?OtherDerived::RowsAtCompileTime:1) ,
-                      (OtherDerived::ColsAtCompileTime>0?OtherDerived::ColsAtCompileTime:1) >(m_row, m_col) = other;
+    if (OtherDerived::SizeAtCompileTime != Dynamic)
+      m_matrix.block<OtherDerived::RowsAtCompileTime != Dynamic ? OtherDerived::RowsAtCompileTime : 1,
+                     OtherDerived::ColsAtCompileTime != Dynamic ? OtherDerived::ColsAtCompileTime : 1>
+                    (m_row, m_col) = other;
     else
       m_matrix.block(m_row, m_col, other.rows(), other.cols()) = other;
     m_col += other.cols();

Eigen/src/Core/ForwardDeclarations.h

@@ -69,40 +69,50 @@ template<typename Scalar> struct ei_scalar_quotient1_op;
 template<typename Scalar> struct ei_scalar_min_op;
 template<typename Scalar> struct ei_scalar_max_op;
 
-template<typename T> struct ei_xpr_copy
+template<typename T> struct ei_copy_unless_matrix
 {
   typedef T type;
 };
 
 template<typename _Scalar, int _Rows, int _Cols, unsigned int _Flags, int _MaxRows, int _MaxCols>
-struct ei_xpr_copy<Matrix<_Scalar, _Rows, _Cols, _Flags, _MaxRows, _MaxCols> >
+struct ei_copy_unless_matrix<Matrix<_Scalar, _Rows, _Cols, _Flags, _MaxRows, _MaxCols> >
 {
   typedef const Matrix<_Scalar, _Rows, _Cols, _Flags, _MaxRows, _MaxCols> & type;
 };
 
+template<typename T> struct ei_xpr_copy
+{
+  typedef typename ei_meta_if<T::Flags & TemporaryBit, 
+                              T,
+                              typename ei_copy_unless_matrix<T>::type
+                             >::ret type;
+};
+
 template<typename T> struct ei_eval
 {
   typedef Matrix<typename ei_traits<T>::Scalar,
                  ei_traits<T>::RowsAtCompileTime,
                  ei_traits<T>::ColsAtCompileTime,
-                 ei_traits<T>::Flags & ~LazyBit, // unset lazy bit after evaluation
+                 ei_traits<T>::Flags & ~(EvalBeforeNestingBit | EvalBeforeAssigningBit),
                  ei_traits<T>::MaxRowsAtCompileTime,
                  ei_traits<T>::MaxColsAtCompileTime> type;
 };
 
-template<typename T, int n> struct ei_eval_if_expensive
+template<typename T> struct ei_eval_temporary
 {
-  enum { eval = n * NumTraits<typename T::Scalar>::ReadCost < (n-1) * T::CoeffReadCost };
-  typedef typename ei_meta_if<eval, typename T::Eval, T>::ret type;
-  typedef typename ei_meta_if<eval, typename T::Eval, T&>::ret reftype;
+  typedef Matrix<typename ei_traits<T>::Scalar,
+                 ei_traits<T>::RowsAtCompileTime,
+                 ei_traits<T>::ColsAtCompileTime,
+                 (ei_traits<T>::Flags | TemporaryBit) & ~(EvalBeforeNestingBit | EvalBeforeAssigningBit),
+                 ei_traits<T>::MaxRowsAtCompileTime,
+                 ei_traits<T>::MaxColsAtCompileTime> type;
 };
 
-template<typename T> struct ei_eval_unless_lazy
+template<typename T, int n=1> struct ei_eval_if_needed_before_nesting
 {
-  typedef typename ei_meta_if<ei_traits<T>::Flags & LazyBit,
-                              T,
-                              typename ei_eval<T>::type
-                             >::ret type;
+  enum { eval = T::Flags & EvalBeforeNestingBit
+             || n * NumTraits<typename T::Scalar>::ReadCost < (n-1) * T::CoeffReadCost };
+  typedef typename ei_meta_if<eval, typename ei_eval_temporary<T>::type, T>::ret type;
 };
 
 #endif // EIGEN_FORWARDDECLARATIONS_H

Eigen/src/Core/Lazy.h

@@ -46,7 +46,7 @@ struct ei_traits<Lazy<ExpressionType> >
     ColsAtCompileTime = ExpressionType::ColsAtCompileTime,
     MaxRowsAtCompileTime = ExpressionType::MaxRowsAtCompileTime,
     MaxColsAtCompileTime = ExpressionType::MaxColsAtCompileTime,
-    Flags = ExpressionType::Flags | LazyBit,
+    Flags = ExpressionType::Flags & ~(EvalBeforeNestingBit | EvalBeforeAssigningBit),
     CoeffReadCost = ExpressionType::CoeffReadCost
   };
 };

Eigen/src/Core/MatrixBase.h

@@ -252,7 +252,8 @@ template<typename Derived> class MatrixBase
       */
     //@{
     template<typename OtherDerived>
-    const typename ei_eval_unless_lazy<Product<Derived, OtherDerived> >::type
+    const Product<typename ei_eval_if_needed_before_nesting<Derived, OtherDerived::ColsAtCompileTime>::type, 
+                  typename ei_eval_if_needed_before_nesting<OtherDerived, ei_traits<Derived>::ColsAtCompileTime>::type>
     operator*(const MatrixBase<OtherDerived> &other) const;
 
     template<typename OtherDerived>
@@ -312,9 +313,9 @@ template<typename Derived> class MatrixBase
 
     /// \name Generating special matrices
     //@{
-    static const typename ei_eval_unless_lazy<Random<Derived> >::type random(int rows, int cols);
-    static const typename ei_eval_unless_lazy<Random<Derived> >::type random(int size);
-    static const typename ei_eval_unless_lazy<Random<Derived> >::type random();
+    static const Random<Derived> random(int rows, int cols);
+    static const Random<Derived> random(int size);
+    static const Random<Derived> random();
     static const Zero<Derived> zero(int rows, int cols);
     static const Zero<Derived> zero(int size);
     static const Zero<Derived> zero();
@@ -358,9 +359,9 @@ template<typename Derived> class MatrixBase
     template<typename NewType>
     const CwiseUnaryOp<ei_scalar_cast_op<typename ei_traits<Derived>::Scalar, NewType>, Derived> cast() const;
 
-    const typename ei_eval_unless_lazy<Derived>::type eval() const EIGEN_ALWAYS_INLINE
+    const typename ei_eval<Derived>::type eval() const EIGEN_ALWAYS_INLINE
     {
-      return typename ei_eval_unless_lazy<Derived>::type(derived());
+      return typename ei_eval<Derived>::type(derived());
     }
 
     template<typename OtherDerived>

Eigen/src/Core/Product.h

@@ -74,37 +74,26 @@ struct ei_product_unroller<Index, 0, Lhs, Rhs>
   *
   * \sa class Sum, class Difference
   */
+template<typename Lhs, typename Rhs> struct ei_product_eval_mode
+{
+  enum{ value = Lhs::MaxRowsAtCompileTime >= 8 && Rhs::MaxColsAtCompileTime >= 8
+              ? CacheOptimal : UnrolledDotProduct };
+};
+
 template<typename Lhs, typename Rhs, int EvalMode>
 struct ei_traits<Product<Lhs, Rhs, EvalMode> >
 {
   typedef typename Lhs::Scalar Scalar;
-#if 0
-  typedef typename ei_meta_if<
-                     (int)NumTraits<Scalar>::ReadCost < (int)Lhs::CoeffReadCost,
-                     typename Lhs::Eval,
-                     Lhs>::ret ActualLhs;
-  typedef typename ei_meta_if<
-                     (int)NumTraits<Scalar>::ReadCost < (int)Lhs::CoeffReadCost,
-                     typename Lhs::Eval,
-                     typename Lhs::XprCopy>::ret ActualLhsXprCopy;
-
-  typedef typename ei_meta_if<
-                     (int)NumTraits<Scalar>::ReadCost < (int)Rhs::CoeffReadCost,
-                     typename Rhs::Eval,
-                     Rhs>::ret ActualRhs;
-  typedef typename ei_meta_if<
-                     (int)NumTraits<Scalar>::ReadCost < (int)Rhs::CoeffReadCost,
-                     typename Rhs::Eval,
-                     typename Rhs::XprCopy>::ret ActualRhsXprCopy;
-#endif
   enum {
     RowsAtCompileTime = Lhs::RowsAtCompileTime,
     ColsAtCompileTime = Rhs::ColsAtCompileTime,
     MaxRowsAtCompileTime = Lhs::MaxRowsAtCompileTime,
     MaxColsAtCompileTime = Rhs::MaxColsAtCompileTime,
-    Flags = (RowsAtCompileTime == Dynamic || ColsAtCompileTime == Dynamic)
+    Flags = ( (RowsAtCompileTime == Dynamic || ColsAtCompileTime == Dynamic)
               ? (unsigned int)(Lhs::Flags | Rhs::Flags)
-          : (unsigned int)(Lhs::Flags | Rhs::Flags) & ~LargeBit,
+              : (unsigned int)(Lhs::Flags | Rhs::Flags) & ~LargeBit )
+          | EvalBeforeAssigningBit
+          | (ei_product_eval_mode<Lhs, Rhs>::value == (int)CacheOptimal ? EvalBeforeNestingBit : 0),
     CoeffReadCost
       = Lhs::ColsAtCompileTime == Dynamic
       ? Dynamic
@@ -114,25 +103,15 @@ struct ei_traits<Product<Lhs, Rhs, EvalMode> >
   };
 };
 
-template<typename Lhs, typename Rhs> struct ei_product_eval_mode
-{
-  enum{ value = Lhs::MaxRowsAtCompileTime == Dynamic || Rhs::MaxColsAtCompileTime == Dynamic
-              ? CacheOptimal : UnrolledDotProduct };
-};
-
 template<typename Lhs, typename Rhs, int EvalMode> class Product : ei_no_assignment_operator,
   public MatrixBase<Product<Lhs, Rhs, EvalMode> >
 {
   public:
 
     EIGEN_GENERIC_PUBLIC_INTERFACE(Product)
-#if 0
-    typedef typename ei_traits<Product>::ActualLhs ActualLhs;
-    typedef typename ei_traits<Product>::ActualRhs ActualRhs;
-    typedef typename ei_traits<Product>::ActualLhsXprCopy ActualLhsXprCopy;
-    typedef typename ei_traits<Product>::ActualRhsXprCopy ActualRhsXprCopy;
-#endif
-    Product(const Lhs& lhs, const Rhs& rhs)
+
+    template<typename ArgLhs, typename ArgRhs>
+    Product(const ArgLhs& lhs, const ArgRhs& rhs)
       : m_lhs(lhs), m_rhs(rhs)
     {
       ei_assert(lhs.cols() == rhs.rows());
@@ -181,13 +160,14 @@ template<typename Lhs, typename Rhs, int EvalMode> class Product : ei_no_assignm
   */
 template<typename Derived>
 template<typename OtherDerived>
-const typename ei_eval_unless_lazy<Product<Derived, OtherDerived> >::type
+const Product<typename ei_eval_if_needed_before_nesting<Derived, OtherDerived::ColsAtCompileTime>::type, 
+              typename ei_eval_if_needed_before_nesting<OtherDerived, ei_traits<Derived>::ColsAtCompileTime>::type>
 MatrixBase<Derived>::operator*(const MatrixBase<OtherDerived> &other) const
 {
-  typedef ei_eval_if_expensive<Derived, OtherDerived::ColsAtCompileTime> Lhs;
-  typedef ei_eval_if_expensive<OtherDerived, Derived::RowsAtCompileTime> Rhs;
+  typedef ei_eval_if_needed_before_nesting<Derived, OtherDerived::ColsAtCompileTime> Lhs;
+  typedef ei_eval_if_needed_before_nesting<OtherDerived, Derived::RowsAtCompileTime> Rhs;
   return Product<typename Lhs::type, typename Rhs::type>
-           (typename Lhs::reftype(derived()), typename Rhs::reftype(other.derived())).eval();
+           (derived(), other.derived());
 }
 
 /** replaces \c *this by \c *this * \a other.

Eigen/src/Core/Random.h

@@ -41,7 +41,7 @@ struct ei_traits<Random<MatrixType> >
     ColsAtCompileTime = ei_traits<MatrixType>::ColsAtCompileTime,
     MaxRowsAtCompileTime = ei_traits<MatrixType>::MaxRowsAtCompileTime,
     MaxColsAtCompileTime = ei_traits<MatrixType>::MaxColsAtCompileTime,
-    Flags = ei_traits<MatrixType>::Flags,
+    Flags = ei_traits<MatrixType>::Flags | EvalBeforeNestingBit,
     CoeffReadCost = 2 * NumTraits<Scalar>::MulCost // FIXME: arbitrary value
   };
 };
@@ -93,10 +93,10 @@ template<typename MatrixType> class Random : ei_no_assignment_operator,
   * \sa ei_random(), ei_random(int)
   */
 template<typename Derived>
-const typename ei_eval_unless_lazy<Random<Derived> >::type
+const Random<Derived>
 MatrixBase<Derived>::random(int rows, int cols)
 {
-  return Random<Derived>(rows, cols).eval();
+  return Random<Derived>(rows, cols);
 }
 
 /** \returns a random vector (not an expression, the vector is immediately evaluated).
@@ -116,12 +116,12 @@ MatrixBase<Derived>::random(int rows, int cols)
   * \sa ei_random(), ei_random(int,int)
   */
 template<typename Derived>
-const typename ei_eval_unless_lazy<Random<Derived> >::type
+const Random<Derived>
 MatrixBase<Derived>::random(int size)
 {
   ei_assert(IsVectorAtCompileTime);
-  if(RowsAtCompileTime == 1) return Random<Derived>(1, size).eval();
-  else return Random<Derived>(size, 1).eval();
+  if(RowsAtCompileTime == 1) return Random<Derived>(1, size);
+  else return Random<Derived>(size, 1);
 }
 
 /** \returns a fixed-size random matrix or vector
@@ -136,10 +136,10 @@ MatrixBase<Derived>::random(int size)
   * \sa ei_random(int), ei_random(int,int)
   */
 template<typename Derived>
-const typename ei_eval_unless_lazy<Random<Derived> >::type
+const Random<Derived>
 MatrixBase<Derived>::random()
 {
-  return Random<Derived>(RowsAtCompileTime, ColsAtCompileTime).eval();
+  return Random<Derived>(RowsAtCompileTime, ColsAtCompileTime);
 }
 
 /** Sets all coefficients in this expression to random values.

Eigen/src/Core/Util.h

@@ -127,12 +127,15 @@ friend class Eigen::MatrixBase<Derived>;
 
 #define EIGEN_ENUM_MIN(a,b) (((int)a <= (int)b) ? (int)a : (int)b)
 
-const int Dynamic = -10;
+const int Dynamic = 10000;
 
 // matrix/expression flags
 const unsigned int RowMajorBit = 0x1;
-const unsigned int LazyBit = 0x2;
-const unsigned int LargeBit = 0x4;
+const unsigned int EvalBeforeNestingBit = 0x2;
+const unsigned int EvalBeforeAssigningBit = 0x4;
+const unsigned int LargeBit = 0x8;
+const unsigned int TemporaryBit = 0x10;
+
 
 enum { ConditionalJumpCost = 5 };