math: fix asin, atan, log1p, tanh to raise underflow on subnormal

for these functions f(x)=x for small inputs, because f(0)=0 and
f'(0)=1, but for subnormal values they should raise the underflow
flag (required by annex F), if they are approximated by a polynomial
around 0 then spurious underflow should be avoided (not required by
annex F)

all these functions should raise inexact flag for small x if x!=0,
but it's not required by the standard and it does not seem a worthy
goal, so support for it is removed in some cases.

raising underflow:
- x*x may not raise underflow for subnormal x if FLT_EVAL_METHOD!=0
- x*x may raise spurious underflow for normal x if FLT_EVAL_METHOD==0
- in case of double subnormal x, store x as float
- in case of float subnormal x, store x*x as float

1 files changed, 3 insertions, 2 deletions

diff --git a/src/math/atan.c b/src/math/atan.c
index 5a1d33e6..63b0ab25 100644
--- a/src/math/atan.c
+++ b/src/math/atan.c
@@ -77,8 +77,9 @@ double atan(double x)
 	}
 	if (ix < 0x3fdc0000) {    /* |x| < 0.4375 */
 		if (ix < 0x3e400000) {  /* |x| < 2^-27 */
-			/* raise inexact if x!=0 */
-			FORCE_EVAL(x + 0x1p120f);
+			if (ix < 0x00100000)
+				/* raise underflow for subnormal x */
+				FORCE_EVAL((float)x);
 			return x;
 		}
 		id = -1;