merge the backing-out of the stupid RetByVal change, and implement a simple

aliasing check in inverse, that catches simple cases like x = x.inverse()

Eigen/src/Core/NoAlias.h

@@ -45,18 +45,12 @@ class NoAlias
   public:
     NoAlias(ExpressionType& expression) : m_expression(expression) {}
 
-    /* \sa MatrixBase::lazyAssign() */
+    /** Behaves like MatrixBase::lazyAssign(other)
+      * \sa MatrixBase::lazyAssign() */
     template<typename OtherDerived>
     EIGEN_STRONG_INLINE ExpressionType& operator=(const StorageBase<OtherDerived>& other)
     { return m_expression.lazyAssign(other.derived()); }
 
-    template<typename OtherDerived>
-    EIGEN_STRONG_INLINE ExpressionType& operator=(const ReturnByValue<OtherDerived>& other)
-    {
-      other.evalTo(m_expression);
-      return m_expression;
-    }
-
     /** \sa MatrixBase::operator+= */
     template<typename OtherDerived>
     EIGEN_STRONG_INLINE ExpressionType& operator+=(const StorageBase<OtherDerived>& other)

Eigen/src/Core/ReturnByValue.h

@@ -36,9 +36,9 @@ struct ei_traits<ReturnByValue<Derived> >
   enum {
     // We're disabling the DirectAccess because e.g. the constructor of
     // the Block-with-DirectAccess expression requires to have a coeffRef method.
-    // FIXME this should be fixed so we can have DirectAccessBit here.
+    // Also, we don't want to have to implement the stride stuff.
     Flags = (ei_traits<typename ei_traits<Derived>::ReturnType>::Flags
-             | EvalBeforeNestingBit | EvalBeforeAssigningBit) & ~DirectAccessBit
+             | EvalBeforeNestingBit) & ~DirectAccessBit
   };
 };
 
@@ -84,11 +84,8 @@ template<typename Derived>
 template<typename OtherDerived>
 Derived& DenseBase<Derived>::operator=(const ReturnByValue<OtherDerived>& other)
 {
-  // since we're by-passing the mechanisms in Assign.h, we implement here the EvalBeforeAssigningBit.
-  // override by using .noalias(), see corresponding operator= in NoAlias.
-  typename Derived::PlainObject result(rows(), cols());
-  other.evalTo(result);
-  return (derived() = result);
+  other.evalTo(derived());
+  return derived();
 }
 
 #endif // EIGEN_RETURNBYVALUE_H

Eigen/src/LU/Inverse.h

@@ -284,7 +284,6 @@ struct ei_inverse_impl : public ReturnByValue<ei_inverse_impl<MatrixType> >
   typedef typename MatrixType::Index Index;
   typedef typename ei_eval<MatrixType>::type MatrixTypeNested;
   typedef typename ei_cleantype<MatrixTypeNested>::type MatrixTypeNestedCleaned;
-
   const MatrixTypeNested m_matrix;
 
   ei_inverse_impl(const MatrixType& matrix)
@@ -296,6 +295,8 @@ struct ei_inverse_impl : public ReturnByValue<ei_inverse_impl<MatrixType> >
 
   template<typename Dest> inline void evalTo(Dest& dst) const
   {
+    // FIXME this is a naive aliasing check that could be improved. It only catches x = x.inverse();
+    ei_assert(&dst != &m_matrix && "Aliasing problem detected in inverse(), you need to do inverse().eval() here.");
     ei_compute_inverse<MatrixTypeNestedCleaned, Dest>::run(m_matrix, dst);
   }
 };
@@ -354,7 +355,14 @@ inline void MatrixBase<Derived>::computeInverseAndDetWithCheck(
 {
   // i'd love to put some static assertions there, but SFINAE means that they have no effect...
   ei_assert(rows() == cols());
-  ei_compute_inverse_and_det_with_check<PlainObject, ResultType>::run
+  // for 2x2, it's worth giving a chance to avoid evaluating.
+  // for larger sizes, evaluating has negligible cost and limits code size.
+  typedef typename ei_meta_if<
+    RowsAtCompileTime == 2,
+    typename ei_cleantype<typename ei_nested<Derived, 2>::type>::type,
+    PlainObject
+  >::ret MatrixType;
+  ei_compute_inverse_and_det_with_check<MatrixType, ResultType>::run
     (derived(), absDeterminantThreshold, inverse, determinant, invertible);
 }
 

test/prec_inverse_4x4.cpp

@@ -64,9 +64,7 @@ template<typename MatrixType> void inverse_general_4x4(int repeat)
   double error_avg = error_sum / repeat;
   EIGEN_DEBUG_VAR(error_avg);
   EIGEN_DEBUG_VAR(error_max);
-  // FIXME that 1.3 used to be a 1.0 until the NumTraits changes on 28 April 2010, and then a 1.2 until the ReturnByValue/Inverse changes
-  // on 30 May 2010, what's going wrong (if anything) ??
-  VERIFY(error_avg < (NumTraits<Scalar>::IsComplex ? 8.0 : 1.3));
+  VERIFY(error_avg < (NumTraits<Scalar>::IsComplex ? 8.0 : 1.2));  // FIXME that 1.2 used to be a 1.0 until the NumTraits changes on 28 April 2010, what's going wrong??
   VERIFY(error_max < (NumTraits<Scalar>::IsComplex ? 64.0 : 20.0));
 }