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Use more FMA in reciprocal iteration for precision

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libeigen/eigen!2073
This commit is contained in:
Chaofan Qiu
2025-11-10 18:36:11 +00:00
committed by Rasmus Munk Larsen
parent 1133aa82c7
commit 943fdc71c6
1 changed files with 4 additions and 3 deletions
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7
Eigen/src/Core/MathFunctionsImpl.h
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@@ -37,15 +37,16 @@ struct generic_reciprocal_newton_step {
static_assert(Steps > 0, "Steps must be at least 1.");
EIGEN_DEVICE_FUNC static EIGEN_STRONG_INLINE Packet run(const Packet& a, const Packet& approx_a_recip) {
using Scalar = typename unpacket_traits<Packet>::type;
const Packet two = pset1<Packet>(Scalar(2));
const Packet one = pset1<Packet>(Scalar(1));
// Refine the approximation using one Newton-Raphson step:
// x_{i} = x_{i-1} * (2 - a * x_{i-1})
const Packet x = generic_reciprocal_newton_step<Packet, Steps - 1>::run(a, approx_a_recip);
const Packet tmp = pnmadd(a, x, two);
const Packet tmp = pnmadd(a, x, one);
// If tmp is NaN, it means that a is either +/-0 or +/-Inf.
// In this case return the approximation directly.
const Packet is_not_nan = pcmp_eq(tmp, tmp);
return pselect(is_not_nan, pmul(x, tmp), x);
// Use two FMAs instead of FMA+FMUL to improve precision.
return pselect(is_not_nan, pmadd(x, tmp, x), x);
}
};