Average Error: 30.1 → 0.7
Time: 15.2s
Precision: 64
\[\left(e^{x} - 2\right) + e^{-x}\]
↓
\[\mathsf{fma}\left(x, x, \mathsf{fma}\left(\frac{1}{360}, {x}^{6}, \frac{1}{12} \cdot {x}^{4}\right)\right)\]
\left(e^{x} - 2\right) + e^{-x}
↓
\mathsf{fma}\left(x, x, \mathsf{fma}\left(\frac{1}{360}, {x}^{6}, \frac{1}{12} \cdot {x}^{4}\right)\right)
double f(double x) {
        double r70069 = x;
        double r70070 = exp(r70069);
        double r70071 = 2.0;
        double r70072 = r70070 - r70071;
        double r70073 = -r70069;
        double r70074 = exp(r70073);
        double r70075 = r70072 + r70074;
        return r70075;
}
↓
double f(double x) {
        double r70076 = x;
        double r70077 = 0.002777777777777778;
        double r70078 = 6.0;
        double r70079 = pow(r70076, r70078);
        double r70080 = 0.08333333333333333;
        double r70081 = 4.0;
        double r70082 = pow(r70076, r70081);
        double r70083 = r70080 * r70082;
        double r70084 = fma(r70077, r70079, r70083);
        double r70085 = fma(r70076, r70076, r70084);
        return r70085;
}

Error

Bits error versus x

Target

Original30.1
Target0.0
Herbie0.7
\[4 \cdot {\left(\sinh \left(\frac{x}{2}\right)\right)}^{2}\]

Derivation

  1. Initial program 30.1

    \[\left(e^{x} - 2\right) + e^{-x}\]

  2. Taylor expanded around 0 0.7

    \[\leadsto \color{blue}{{x}^{2} + \left(\frac{1}{360} \cdot {x}^{6} + \frac{1}{12} \cdot {x}^{4}\right)}\]

  3. Simplified0.7

    \[\leadsto \color{blue}{\mathsf{fma}\left(x, x, \mathsf{fma}\left(\frac{1}{360}, {x}^{6}, \frac{1}{12} \cdot {x}^{4}\right)\right)}\]

  4. Final simplification0.7

    \[\leadsto \mathsf{fma}\left(x, x, \mathsf{fma}\left(\frac{1}{360}, {x}^{6}, \frac{1}{12} \cdot {x}^{4}\right)\right)\]

Reproduce

herbie shell --seed 2019303 +o rules:numerics
(FPCore (x)
  :name "exp2 (problem 3.3.7)"
  :precision binary64

  :herbie-target
  (* 4 (pow (sinh (/ x 2)) 2))

  (+ (- (exp x) 2) (exp (- x))))