Average Error: 29.6 → 0.9
Time: 4.7s
Precision: binary64
\[[a, b]=\mathsf{sort}([a, b])\]
\[\log \left(e^{a} + e^{b}\right) \]
↓
\[\begin{array}{l} t_0 := 1 + e^{a}\\ t_1 := \frac{b}{t_0}\\ t_1 + \left(\left(b \cdot t_1\right) \cdot \left(0.5 + \frac{-0.5}{t_0}\right) + \log t_0\right) \end{array} \]
\log \left(e^{a} + e^{b}\right)
↓
\begin{array}{l}
t_0 := 1 + e^{a}\\
t_1 := \frac{b}{t_0}\\
t_1 + \left(\left(b \cdot t_1\right) \cdot \left(0.5 + \frac{-0.5}{t_0}\right) + \log t_0\right)
\end{array}

(FPCore (a b) :precision binary64 (log (+ (exp a) (exp b))))
↓
(FPCore (a b)
 :precision binary64
 (let* ((t_0 (+ 1.0 (exp a))) (t_1 (/ b t_0)))
   (+ t_1 (+ (* (* b t_1) (+ 0.5 (/ -0.5 t_0))) (log t_0)))))
double code(double a, double b) {
	return log(exp(a) + exp(b));
}
↓
double code(double a, double b) {
	double t_0 = 1.0 + exp(a);
	double t_1 = b / t_0;
	return t_1 + (((b * t_1) * (0.5 + (-0.5 / t_0))) + log(t_0));
}

Error

Bits error versus a

Bits error versus b

Try it out

Your Program's Arguments

Results

Enter valid numbers for all inputs

Derivation

  1. Initial program 29.6

    \[\log \left(e^{a} + e^{b}\right) \]

  2. Taylor expanded around 0 1.1

    \[\leadsto \color{blue}{\left(\log \left(e^{a} + 1\right) + \left(0.5 \cdot \frac{{b}^{2}}{e^{a} + 1} + \frac{b}{e^{a} + 1}\right)\right) - 0.5 \cdot \frac{{b}^{2}}{{\left(e^{a} + 1\right)}^{2}}} \]

  3. Simplified0.9

    \[\leadsto \color{blue}{\frac{b}{1 + e^{a}} + \left(\left(\frac{b}{1 + e^{a}} \cdot b\right) \cdot \left(0.5 + \frac{-0.5}{1 + e^{a}}\right) + \log \left(1 + e^{a}\right)\right)} \]

  4. Final simplification0.9

    \[\leadsto \frac{b}{1 + e^{a}} + \left(\left(b \cdot \frac{b}{1 + e^{a}}\right) \cdot \left(0.5 + \frac{-0.5}{1 + e^{a}}\right) + \log \left(1 + e^{a}\right)\right) \]

Reproduce

herbie shell --seed 2021190 
(FPCore (a b)
  :name "symmetry log of sum of exp"
  :precision binary64
  (log (+ (exp a) (exp b))))