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Add a portable log2 function for integers

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This commit is contained in:
Gael Guennebaud 2014-09-17 09:56:07 +02:00
parent 0f0580b97c
commit fc23e93707
2 changed files with 16 additions and 1 deletions
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15
Eigen/src/Core/MathFunctions.h
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@ -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 {

2
Eigen/src/SparseCore/ConservativeSparseSparseProduct.h
View File

@ -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)