Average Error: 53.5 → 0.2
Time: 4.5s
Precision: binary64
\[\log \left(x + \sqrt{x \cdot x + 1}\right) \]
↓
\[\begin{array}{l} \mathbf{if}\;x \leq -1.3291109624821573:\\ \;\;\;\;\log \left(\frac{-0.5}{x}\right)\\ \mathbf{elif}\;x \leq 0.008183541869942034:\\ \;\;\;\;\mathsf{fma}\left(0.075, {x}^{5}, x - 0.16666666666666666 \cdot {x}^{3}\right)\\ \mathbf{else}:\\ \;\;\;\;\log \left(x + \mathsf{hypot}\left(1, x\right)\right)\\ \end{array} \]
\log \left(x + \sqrt{x \cdot x + 1}\right)
↓
\begin{array}{l}
\mathbf{if}\;x \leq -1.3291109624821573:\\
\;\;\;\;\log \left(\frac{-0.5}{x}\right)\\

\mathbf{elif}\;x \leq 0.008183541869942034:\\
\;\;\;\;\mathsf{fma}\left(0.075, {x}^{5}, x - 0.16666666666666666 \cdot {x}^{3}\right)\\

\mathbf{else}:\\
\;\;\;\;\log \left(x + \mathsf{hypot}\left(1, x\right)\right)\\


\end{array}

(FPCore (x) :precision binary64 (log (+ x (sqrt (+ (* x x) 1.0)))))
↓
(FPCore (x)
 :precision binary64
 (if (<= x -1.3291109624821573)
   (log (/ -0.5 x))
   (if (<= x 0.008183541869942034)
     (fma 0.075 (pow x 5.0) (- x (* 0.16666666666666666 (pow x 3.0))))
     (log (+ x (hypot 1.0 x))))))
double code(double x) {
	return log((x + sqrt(((x * x) + 1.0))));
}
↓
double code(double x) {
	double tmp;
	if (x <= -1.3291109624821573) {
		tmp = log((-0.5 / x));
	} else if (x <= 0.008183541869942034) {
		tmp = fma(0.075, pow(x, 5.0), (x - (0.16666666666666666 * pow(x, 3.0))));
	} else {
		tmp = log((x + hypot(1.0, x)));
	}
	return tmp;
}

Error

Bits error versus x

Target

Original53.5
Target45.1
Herbie0.2
\[\begin{array}{l} \mathbf{if}\;x < 0:\\ \;\;\;\;\log \left(\frac{-1}{x - \sqrt{x \cdot x + 1}}\right)\\ \mathbf{else}:\\ \;\;\;\;\log \left(x + \sqrt{x \cdot x + 1}\right)\\ \end{array} \]

Derivation

  1. Split input into 3 regimes

  2. if x < -1.32911096248215732

    1. Initial program 63.0

      \[\log \left(x + \sqrt{x \cdot x + 1}\right) \]

    2. Taylor expanded in x around -inf 0.5

      \[\leadsto \log \color{blue}{\left(\frac{-0.5}{x}\right)} \]

    if -1.32911096248215732 < x < 0.0081835418699420341

    1. Initial program 58.9

      \[\log \left(x + \sqrt{x \cdot x + 1}\right) \]

    2. Taylor expanded in x around 0 0.1

      \[\leadsto \color{blue}{\left(0.075 \cdot {x}^{5} + x\right) - 0.16666666666666666 \cdot {x}^{3}} \]

    3. Applied egg-rr0.1

      \[\leadsto \color{blue}{\mathsf{fma}\left(0.075, {x}^{5}, x - 0.16666666666666666 \cdot {x}^{3}\right)} \]

    if 0.0081835418699420341 < x

    1. Initial program 32.7

      \[\log \left(x + \sqrt{x \cdot x + 1}\right) \]

    2. Applied egg-rr0.1

      \[\leadsto \log \left(x + \color{blue}{\mathsf{hypot}\left(1, x\right)}\right) \]
  3. Recombined 3 regimes into one program.

  4. Final simplification0.2

    \[\leadsto \begin{array}{l} \mathbf{if}\;x \leq -1.3291109624821573:\\ \;\;\;\;\log \left(\frac{-0.5}{x}\right)\\ \mathbf{elif}\;x \leq 0.008183541869942034:\\ \;\;\;\;\mathsf{fma}\left(0.075, {x}^{5}, x - 0.16666666666666666 \cdot {x}^{3}\right)\\ \mathbf{else}:\\ \;\;\;\;\log \left(x + \mathsf{hypot}\left(1, x\right)\right)\\ \end{array} \]

Reproduce

herbie shell --seed 2022129 
(FPCore (x)
  :name "Hyperbolic arcsine"
  :precision binary64

  :herbie-target
  (if (< x 0.0) (log (/ -1.0 (- x (sqrt (+ (* x x) 1.0))))) (log (+ x (sqrt (+ (* x x) 1.0)))))

  (log (+ x (sqrt (+ (* x x) 1.0)))))