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Update comment for fast sqrt.

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This commit is contained in:
Rasmus Munk Larsen 2016-10-04 15:08:41 -07:00
parent 3ed67cb0bb
commit 765615609d
1 changed files with 7 additions and 2 deletions
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Eigen/src/Core/arch/SSE/MathFunctions.h
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@ -444,9 +444,14 @@ Packet4f pcos<Packet4f>(const Packet4f& _x)
#if EIGEN_FAST_MATH #if EIGEN_FAST_MATH
// This is based on Quake3's fast inverse square root. // Functions for sqrt.
// The EIGEN_FAST_MATH version uses the _mm_rsqrt_ps approximation and one step
// of Newton's method, at a cost of 1-2 bits of precision as opposed to the
// exact solution. It does not handle +inf, or denormalized numbers correctly.
// The main advantage of this approach is not just speed, but also the fact that
// it can be inlined and pipelined with other computations, further reducing its
// effective latency. This is similar to Quake3's fast inverse square root.
// For detail see here: http://www.beyond3d.com/content/articles/8/ // For detail see here: http://www.beyond3d.com/content/articles/8/
// It lacks 1 (or 2 bits in some rare cases) of precision, and does not handle negative, +inf, or denormalized numbers correctly.
template<> EIGEN_DEFINE_FUNCTION_ALLOWING_MULTIPLE_DEFINITIONS EIGEN_UNUSED template<> EIGEN_DEFINE_FUNCTION_ALLOWING_MULTIPLE_DEFINITIONS EIGEN_UNUSED
Packet4f psqrt<Packet4f>(const Packet4f& _x) Packet4f psqrt<Packet4f>(const Packet4f& _x)
{ {