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Disable fast psqrt for NEON.

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Accuracy is too poor - requires at least two Newton iterations, but then
it is no longer significantly faster than `vsqrt`.

Fixes #2094.
This commit is contained in:
Antonio Sanchez 2021-02-23 19:48:46 -08:00
parent aba3998278
commit 6cf0ab5e99
1 changed files with 0 additions and 34 deletions
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34
Eigen/src/Core/arch/NEON/PacketMath.h
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@ -3297,42 +3297,8 @@ template<> EIGEN_STRONG_INLINE Packet4ui psqrt(const Packet4ui& a) {
return res;
}
#if EIGEN_FAST_MATH
/* Functions for sqrt support packet2f/packet4f.*/
// The EIGEN_FAST_MATH version uses the vrsqrte_f32 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 more details see: http://www.beyond3d.com/content/articles/8/
template<> EIGEN_STRONG_INLINE Packet4f psqrt(const Packet4f& _x){
Packet4ui denormal_mask = vandq_u32(vcgeq_f32(_x, vdupq_n_f32(0.0f)),
vcltq_f32(_x, pset1<Packet4f>((std::numeric_limits<float>::min)())));
// Compute approximate reciprocal sqrt.
Packet4f x = vrsqrteq_f32(_x);
// Do one Newton's iteration for 1/sqrt(x).
x = vmulq_f32(vrsqrtsq_f32(vmulq_f32(_x, x), x), x);
// Flush results for denormals to zero.
return vreinterpretq_f32_u32(vbicq_u32(vreinterpretq_u32_f32(pmul(_x, x)), denormal_mask));
}
template<> EIGEN_STRONG_INLINE Packet2f psqrt(const Packet2f& _x) {
Packet2ui denormal_mask = vand_u32(vcge_f32(_x, vdup_n_f32(0.0f)),
vclt_f32(_x, pset1<Packet2f>((std::numeric_limits<float>::min)())));
// Compute approximate reciprocal sqrt.
Packet2f x = vrsqrte_f32(_x);
// Do one Newton's iteration for 1/sqrt(x).
x = vmul_f32(vrsqrts_f32(vmul_f32(_x, x), x), x);
// Flush results for denormals to zero.
return vreinterpret_f32_u32(vbic_u32(vreinterpret_u32_f32(pmul(_x, x)), denormal_mask));
}
#else
template<> EIGEN_STRONG_INLINE Packet4f psqrt(const Packet4f& _x){return vsqrtq_f32(_x);}
template<> EIGEN_STRONG_INLINE Packet2f psqrt(const Packet2f& _x){return vsqrt_f32(_x); }
#endif
//---------- bfloat16 ----------
// TODO: Add support for native armv8.6-a bfloat16_t