1 #include "libm.h"
2 
3 /* k is such that k*ln2 has minimal relative error and x - kln2 > log(DBL_MIN) */
4 static const int k = 2043;
5 static const double kln2 = 0x1.62066151add8bp+10;
6 
7 /* exp(x)/2 for x >= log(DBL_MAX), slightly better than 0.5*exp(x/2)*exp(x/2) */
8 #ifdef __wasilibc_unmodified_upstream // Wasm doesn't have alternate rounding modes
__expo2(double x,double sign)9 double __expo2(double x, double sign)
10 #else
11 double __expo2(double x)
12 #endif
13 {
14 	double scale;
15 
16 	/* note that k is odd and scale*scale overflows */
17 	INSERT_WORDS(scale, (uint32_t)(0x3ff + k/2) << 20, 0);
18 	/* exp(x - k ln2) * 2**(k-1) */
19 #ifdef __wasilibc_unmodified_upstream // Wasm doesn't have alternate rounding modes
20 	/* in directed rounding correct sign before rounding or overflow is important */
21 	return exp(x - kln2) * (sign * scale) * scale;
22 #else
23 	return exp(x - kln2) * scale * scale;
24 #endif
25 }
26