math: fix atanh (overflow and underflow issues)

in atanh exception handling was left to the called log functions,
but the argument to those functions could underflow or overflow.

use double_t and float_t to avoid some useless stores on x86

1 files changed, 13 insertions, 5 deletions

diff --git a/src/math/atanh.c b/src/math/atanh.c
index 84a84c69..63a035d7 100644
--- a/src/math/atanh.c
+++ b/src/math/atanh.c
@@ -6,16 +6,24 @@ double atanh(double x)
 	union {double f; uint64_t i;} u = {.f = x};
 	unsigned e = u.i >> 52 & 0x7ff;
 	unsigned s = u.i >> 63;
+	double_t y;
 
 	/* |x| */
 	u.i &= (uint64_t)-1/2;
-	x = u.f;
+	y = u.f;
 
 	if (e < 0x3ff - 1) {
-		/* |x| < 0.5, up to 1.7ulp error */
-		x = 0.5*log1p(2*x + 2*x*x/(1-x));
+		if (e < 0x3ff - 32) {
+			/* handle underflow */
+			if (e == 0)
+				FORCE_EVAL((float)y);
+		} else {
+			/* |x| < 0.5, up to 1.7ulp error */
+			y = 0.5*log1p(2*y + 2*y*y/(1-y));
+		}
 	} else {
-		x = 0.5*log1p(2*x/(1-x));
+		/* avoid overflow */
+		y = 0.5*log1p(2*(y/(1-y)));
 	}
-	return s ? -x : x;
+	return s ? -y : y;
 }