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math: fix ld80 acoshl(x) for x < 0

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acosh(x) is nan for x < 1, but x < 0 cases were not handled specially
and acoshl gave wrong result for some -0x1p32 < x < -2 values, e.g.:

acoshl(-0x1p20) returned -inf,
acoshl(-0x1.4p20) returned -0x1.db365758403aa9acp+0L,

fixed by checking the sign bit and handling it specially.

reported by Paul Zimmermann.
This commit is contained in:
Szabolcs Nagy 2023-08-18 23:16:00 +02:00 • committed by Rich Felker
parent 83b858f83b
commit 6d10102709
1 changed files with 7 additions and 3 deletions
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10
src/math/acoshl.c
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@ -10,14 +10,18 @@ long double acoshl(long double x)
long double acoshl(long double x)
{
union ldshape u = {x};
int e = u.i.se & 0x7fff;
int e = u.i.se;
if (e < 0x3fff + 1)
/* |x| < 2, invalid if x < 1 or nan */
/* 0 <= x < 2, invalid if x < 1 */
return log1pl(x-1 + sqrtl((x-1)*(x-1)+2*(x-1)));
if (e < 0x3fff + 32)
/* |x| < 0x1p32 */
/* 2 <= x < 0x1p32 */
return logl(2*x - 1/(x+sqrtl(x*x-1)));
if (e & 0x8000)
/* x < 0 or x = -0, invalid */
return (x - x) / (x - x);
/* 0x1p32 <= x or nan */
return logl(x) + 0.693147180559945309417232121458176568L;
}
#elif LDBL_MANT_DIG == 113 && LDBL_MAX_EXP == 16384