Optimize the product of a householder-sequence with the identity, and optimize the evaluation of a HouseholderSequence to a dense matrix using faster blocked product.

CTestConfig.cmake

diff --git a/CTestConfig.cmake b/CTestConfig.cmake
index 4c00278..8b4cd79 100644
--- a/CTestConfig.cmake
+++ b/CTestConfig.cmake
@@ -11,7 +11,7 @@
 set(CTEST_DROP_SITE "manao.inria.fr")
 set(CTEST_DROP_LOCATION "/CDash/submit.php?project=Eigen")
 set(CTEST_DROP_SITE_CDASH TRUE)
-set(CTEST_PROJECT_SUBPROJECTS
-Official
-Unsupported
-)
+#set(CTEST_PROJECT_SUBPROJECTS
+#Official
+#Unsupported
+#)

Eigen/src/Householder/HouseholderSequence.h

diff --git a/Eigen/src/Householder/HouseholderSequence.h b/Eigen/src/Householder/HouseholderSequence.h
index a4f40b7..af19701 100644
--- a/Eigen/src/Householder/HouseholderSequence.h
+++ b/Eigen/src/Householder/HouseholderSequence.h
@@ -295,6 +295,14 @@
         for(Index k = 0; k<cols()-vecs ; ++k)
           dst.col(k).tail(rows()-k-1).setZero();
       }
+      else if(m_length>BlockSize)
+      {
+        dst.setIdentity(rows(), rows());
+        if(m_reverse)
+          applyThisOnTheLeft(dst,workspace,true);
+        else
+          applyThisOnTheLeft(dst,workspace,true);
+      }
       else
       {
         dst.setIdentity(rows(), rows());
@@ -332,24 +340,27 @@
     }
 
     /** \internal */
-    template<typename Dest> inline void applyThisOnTheLeft(Dest& dst) const
+    template<typename Dest> inline void applyThisOnTheLeft(Dest& dst, bool inputIsIdentity = false) const
     {
       Matrix<Scalar,1,Dest::ColsAtCompileTime,RowMajor,1,Dest::MaxColsAtCompileTime> workspace;
-      applyThisOnTheLeft(dst, workspace);
+      applyThisOnTheLeft(dst, workspace, inputIsIdentity);
     }
 
     /** \internal */
     template<typename Dest, typename Workspace>
-    inline void applyThisOnTheLeft(Dest& dst, Workspace& workspace) const
+    inline void applyThisOnTheLeft(Dest& dst, Workspace& workspace, bool inputIsIdentity = false) const
     {
-      const Index BlockSize = 48;
+      if(inputIsIdentity && m_reverse)
+        inputIsIdentity = false;
       // if the entries are large enough, then apply the reflectors by block
       if(m_length>=BlockSize && dst.cols()>1)
       {
-        for(Index i = 0; i < m_length; i+=BlockSize)
+        // Make sure we have at least 2 useful blocks, otherwise it is point-less:
+        Index blockSize = m_length<2*BlockSize ? (m_length+1)/2 : BlockSize;
+        for(Index i = 0; i < m_length; i+=blockSize)
         {
-          Index end = m_reverse ? (std::min)(m_length,i+BlockSize) : m_length-i;
-          Index k = m_reverse ? i : (std::max)(Index(0),end-BlockSize);
+          Index end = m_reverse ? (std::min)(m_length,i+blockSize) : m_length-i;
+          Index k = m_reverse ? i : (std::max)(Index(0),end-blockSize);
           Index bs = end-k;
           Index start = k + m_shift;
           
@@ -359,7 +370,14 @@
                                                                    Side==OnTheRight ? bs : m_vectors.rows()-start,
                                                                    Side==OnTheRight ? m_vectors.cols()-start : bs);
           typename internal::conditional<Side==OnTheRight, Transpose<SubVectorsType>, SubVectorsType&>::type sub_vecs(sub_vecs1);
-          Block<Dest,Dynamic,Dynamic> sub_dst(dst,dst.rows()-rows()+m_shift+k,0, rows()-m_shift-k,dst.cols());
+
+          Index dstStart = dst.rows()-rows()+m_shift+k;
+          Index dstRows  = rows()-m_shift-k;
+          Block<Dest,Dynamic,Dynamic> sub_dst(dst,
+                                              dstStart,
+                                              inputIsIdentity ? dstStart : 0,
+                                              dstRows,
+                                              inputIsIdentity ? dstRows : dst.cols());
           apply_block_householder_on_the_left(sub_dst, sub_vecs, m_coeffs.segment(k, bs), !m_reverse);
         }
       }
@@ -369,7 +387,8 @@
         for(Index k = 0; k < m_length; ++k)
         {
           Index actual_k = m_reverse ? k : m_length-k-1;
-          dst.bottomRows(rows()-m_shift-actual_k)
+          Index dstStart = rows()-m_shift-actual_k;
+          dst.bottomRightCorner(dstStart, inputIsIdentity ? dstStart : dst.cols())
             .applyHouseholderOnTheLeft(essentialVector(actual_k), m_coeffs.coeff(actual_k), workspace.data());
         }
       }
@@ -387,7 +406,7 @@
     {
       typename internal::matrix_type_times_scalar_type<Scalar, OtherDerived>::Type
         res(other.template cast<typename internal::matrix_type_times_scalar_type<Scalar,OtherDerived>::ResultScalar>());
-      applyThisOnTheLeft(res);
+      applyThisOnTheLeft(res, internal::is_identity<OtherDerived>::value && res.rows()==res.cols());
       return res;
     }
 
@@ -461,6 +480,7 @@
     bool m_reverse;
     Index m_length;
     Index m_shift;
+    enum { BlockSize = 48 };
 };
 
 /** \brief Computes the product of a matrix with a Householder sequence.

bench/bench_gemm.cpp

diff --git a/bench/bench_gemm.cpp b/bench/bench_gemm.cpp
index 8528c55..688d99c 100644
--- a/bench/bench_gemm.cpp
+++ b/bench/bench_gemm.cpp
@@ -129,7 +129,7 @@
 template<typename A, typename B, typename C>
 EIGEN_DONT_INLINE void gemm(const A& a, const B& b, C& c)
 {
- c.noalias() += a * b;
+  c.noalias() += a * b;
 }
 
 int main(int argc, char ** argv)