Average Error: 20.5 → 5.3
Time: 35.9s
Precision: 64
\[\frac{\left(x - y\right) \cdot \left(x + y\right)}{x \cdot x + y \cdot y}\]
↓
\[\begin{array}{l} \mathbf{if}\;y \le -1.3563538085895464 \cdot 10^{+154}:\\ \;\;\;\;-1\\ \mathbf{elif}\;y \le -7.204664864552257 \cdot 10^{-157}:\\ \;\;\;\;\frac{\left(x - y\right) \cdot \left(y + x\right)}{\sqrt{y \cdot y + x \cdot x} \cdot \sqrt{y \cdot y + x \cdot x}}\\ \mathbf{elif}\;y \le 7.296256817310134 \cdot 10^{-184}:\\ \;\;\;\;1\\ \mathbf{else}:\\ \;\;\;\;\frac{x - y}{\sqrt{y \cdot y + x \cdot x}} \cdot \frac{y + x}{\sqrt{y \cdot y + x \cdot x}}\\ \end{array}\]

Error

Bits error versus x

Bits error versus y

Target

Original20.5
Target0.0
Herbie5.3
\[\begin{array}{l} \mathbf{if}\;0.5 \lt \left|\frac{x}{y}\right| \lt 2:\\ \;\;\;\;\frac{\left(x - y\right) \cdot \left(x + y\right)}{x \cdot x + y \cdot y}\\ \mathbf{else}:\\ \;\;\;\;1 - \frac{2}{1 + \frac{x}{y} \cdot \frac{x}{y}}\\ \end{array}\]

Derivation

  1. Split input into 4 regimes

  2. if y < -1.3563538085895464e+154

    1. Initial program 63.6

      \[\frac{\left(x - y\right) \cdot \left(x + y\right)}{x \cdot x + y \cdot y}\]
    2. Using strategy rm

    3. Applied add-sqr-sqrt63.6

      \[\leadsto \frac{\left(x - y\right) \cdot \left(x + y\right)}{\color{blue}{\sqrt{x \cdot x + y \cdot y} \cdot \sqrt{x \cdot x + y \cdot y}}}\]

    4. Applied times-frac62.0

      \[\leadsto \color{blue}{\frac{x - y}{\sqrt{x \cdot x + y \cdot y}} \cdot \frac{x + y}{\sqrt{x \cdot x + y \cdot y}}}\]

    5. Taylor expanded around 0 0

      \[\leadsto \color{blue}{-1}\]

    if -1.3563538085895464e+154 < y < -7.204664864552257e-157

    1. Initial program 0.0

      \[\frac{\left(x - y\right) \cdot \left(x + y\right)}{x \cdot x + y \cdot y}\]
    2. Using strategy rm

    3. Applied add-sqr-sqrt0.0

      \[\leadsto \frac{\left(x - y\right) \cdot \left(x + y\right)}{\color{blue}{\sqrt{x \cdot x + y \cdot y} \cdot \sqrt{x \cdot x + y \cdot y}}}\]

    if -7.204664864552257e-157 < y < 7.296256817310134e-184

    1. Initial program 29.1

      \[\frac{\left(x - y\right) \cdot \left(x + y\right)}{x \cdot x + y \cdot y}\]

    2. Taylor expanded around -inf 14.5

      \[\leadsto \color{blue}{1}\]

    if 7.296256817310134e-184 < y

    1. Initial program 3.9

      \[\frac{\left(x - y\right) \cdot \left(x + y\right)}{x \cdot x + y \cdot y}\]
    2. Using strategy rm

    3. Applied add-sqr-sqrt3.9

      \[\leadsto \frac{\left(x - y\right) \cdot \left(x + y\right)}{\color{blue}{\sqrt{x \cdot x + y \cdot y} \cdot \sqrt{x \cdot x + y \cdot y}}}\]

    4. Applied times-frac4.3

      \[\leadsto \color{blue}{\frac{x - y}{\sqrt{x \cdot x + y \cdot y}} \cdot \frac{x + y}{\sqrt{x \cdot x + y \cdot y}}}\]
  3. Recombined 4 regimes into one program.

  4. Final simplification5.3

    \[\leadsto \begin{array}{l} \mathbf{if}\;y \le -1.3563538085895464 \cdot 10^{+154}:\\ \;\;\;\;-1\\ \mathbf{elif}\;y \le -7.204664864552257 \cdot 10^{-157}:\\ \;\;\;\;\frac{\left(x - y\right) \cdot \left(y + x\right)}{\sqrt{y \cdot y + x \cdot x} \cdot \sqrt{y \cdot y + x \cdot x}}\\ \mathbf{elif}\;y \le 7.296256817310134 \cdot 10^{-184}:\\ \;\;\;\;1\\ \mathbf{else}:\\ \;\;\;\;\frac{x - y}{\sqrt{y \cdot y + x \cdot x}} \cdot \frac{y + x}{\sqrt{y \cdot y + x \cdot x}}\\ \end{array}\]

Reproduce

herbie shell --seed 2019100 
(FPCore (x y)
  :name "Kahan p9 Example"
  :pre (and (< 0 x 1) (< y 1))

  :herbie-target
  (if (< 0.5 (fabs (/ x y)) 2) (/ (* (- x y) (+ x y)) (+ (* x x) (* y y))) (- 1 (/ 2 (+ 1 (* (/ x y) (/ x y))))))

  (/ (* (- x y) (+ x y)) (+ (* x x) (* y y))))