From fc23e937076d01e565fac5b771f22099dee5ad6c Mon Sep 17 00:00:00 2001
From: Gael Guennebaud <g.gael@free.fr>
Date: Wed, 17 Sep 2014 09:56:07 +0200
Subject: [PATCH] Add a portable log2 function for integers

---
 Eigen/src/Core/MathFunctions.h                    | 15 +++++++++++++++
 .../SparseCore/ConservativeSparseSparseProduct.h  |  2 +-
 2 files changed, 16 insertions(+), 1 deletion(-)

diff --git a/Eigen/src/Core/MathFunctions.h b/Eigen/src/Core/MathFunctions.h
index e9fed2e52..8834f71fa 100644
--- a/Eigen/src/Core/MathFunctions.h
+++ b/Eigen/src/Core/MathFunctions.h
@@ -694,6 +694,21 @@ bool (isfinite)(const std::complex<T>& x)
   return isfinite(real(x)) && isfinite(imag(x));
 }
 
+// Log base 2 for 32 bits positive integers.
+// Conveniently returns 0 for x==0.
+int log2(int x)
+{
+  eigen_assert(x>=0);
+  unsigned int v(x);
+  static const int table[32] = { 0, 9, 1, 10, 13, 21, 2, 29, 11, 14, 16, 18, 22, 25, 3, 30, 8, 12, 20, 28, 15, 17, 24, 7, 19, 27, 23, 6, 26, 5, 4, 31 };
+  v |= v >> 1;
+  v |= v >> 2;
+  v |= v >> 4;
+  v |= v >> 8;
+  v |= v >> 16;
+  return table[(v * 0x07C4ACDDU) >> 27];
+}
+
 } // end namespace numext
 
 namespace internal {
diff --git a/Eigen/src/SparseCore/ConservativeSparseSparseProduct.h b/Eigen/src/SparseCore/ConservativeSparseSparseProduct.h
index ae8fc75e5..398008597 100644
--- a/Eigen/src/SparseCore/ConservativeSparseSparseProduct.h
+++ b/Eigen/src/SparseCore/ConservativeSparseSparseProduct.h
@@ -101,7 +101,7 @@ static void conservative_sparse_sparse_product_impl(const Lhs& lhs, const Rhs& r
       // otherwise => loop through the entire vector
       // In order to avoid to perform an expensive log2 when the
       // result is clearly very sparse we use a linear bound up to 200.
-      if((nnz<200 && nnz<t200) || nnz * log2(nnz) < t)
+      if((nnz<200 && nnz<t200) || nnz * numext::log2(int(nnz)) < t)
       {
         if(nnz>1) std::sort(indices,indices+nnz);
         for(Index k=0; k<nnz; ++k)