1 files changed, 19 insertions, 0 deletions
diff --git a/lib/libm/__expo2.c b/lib/libm/__expo2.c
new file mode 100644
index 00000000..0d025457
--- /dev/null
+++ b/lib/libm/__expo2.c
@@ -0,0 +1,19 @@
+#include "libm.h"
+
+/* k is such that k*ln2 has minimal relative error and x - kln2 > log(DBL_MIN)
+ */
+static const int k = 2043;
+static const double kln2 = 0x1.62066151add8bp+10;
+
+/* exp(x)/2 for x >= log(DBL_MAX), slightly better than 0.5*exp(x/2)*exp(x/2) */
+double __expo2(double x, double sign)
+{
+	double scale;
+
+	/* note that k is odd and scale*scale overflows */
+	INSERT_WORDS(scale, (uint32_t)(0x3ff + k / 2) << 20, 0);
+	/* exp(x - k ln2) * 2**(k-1) */
+	/* in directed rounding correct sign before rounding or overflow is
+	 * important */
+	return exp(x - kln2) * (sign * scale) * scale;
+}