math: fix acoshf on negative values

acosh(x) is invalid for x<1, acoshf tried to be clever using
signed comparisions to handle all x<2 the same way, but the
formula was wrong on large negative values.

src/math/acoshf.c

@@ -3,14 +3,15 @@
 /* acosh(x) = log(x + sqrt(x*x-1)) */
 float acoshf(float x)
 {
-	union {float f; int32_t i;} u = {.f = x};
+	union {float f; uint32_t i;} u = {x};
+	uint32_t a = u.i & 0x7fffffff;
 
-	if (u.i < 0x3f800000+(1<<23))
-		/* x < 2, invalid if x < 1 or nan */
+	if (a < 0x3f800000+(1<<23))
+		/* |x| < 2, invalid if x < 1 or nan */
 		/* up to 2ulp error in [1,1.125] */
 		return log1pf(x-1 + sqrtf((x-1)*(x-1)+2*(x-1)));
-	if (u.i < 0x3f800000+(12<<23))
-		/* x < 0x1p12 */
+	if (a < 0x3f800000+(12<<23))
+		/* |x| < 0x1p12 */
 		return logf(2*x - 1/(x+sqrtf(x*x-1)));
 	/* x >= 0x1p12 */
 	return logf(x) + 0.693147180559945309417232121458176568f;

src/math/acoshl.c

@@ -13,10 +13,10 @@ long double acoshl(long double x)
 	int e = u.i.se & 0x7fff;
 
 	if (e < 0x3fff + 1)
-		/* x < 2, invalid if x < 1 or nan */
+		/* |x| < 2, invalid if x < 1 or nan */
 		return log1pl(x-1 + sqrtl((x-1)*(x-1)+2*(x-1)));
 	if (e < 0x3fff + 32)
-		/* x < 0x1p32 */
+		/* |x| < 0x1p32 */
 		return logl(2*x - 1/(x+sqrtl(x*x-1)));
 	return logl(x) + 0.693147180559945309417232121458176568L;
 }