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Message-ID: <20210510205303.GO2799122@port70.net>
Date: Mon, 10 May 2021 22:53:03 +0200
From: Szabolcs Nagy <nsz@...t70.net>
To: musl@...ts.openwall.com
Subject: [PATCH] math: avoid internal overflow in expm1l.c
the ld80 expm1l implementation in c tries to compute
2^k q + 2^k - 1
but 2^k can overflow (when k==16384) while q is small and negative.
then the result should be finite, but the code gives nan. to avoid the
overflow use
2 (2^(k-1) q + 2^(k-1) - 0.5).
note: this implementation is not used on x86 currently.
---
src/math/expm1l.c | 6 +++---
1 file changed, 3 insertions(+), 3 deletions(-)
diff --git a/src/math/expm1l.c b/src/math/expm1l.c
index d1715078..79795dc0 100644
--- a/src/math/expm1l.c
+++ b/src/math/expm1l.c
@@ -109,9 +109,9 @@ long double expm1l(long double x)
/* exp(x) = exp(k ln 2) exp(remainder ln 2) = 2^k exp(remainder ln 2).
We have qx = exp(remainder ln 2) - 1, so
- exp(x) - 1 = 2^k (qx + 1) - 1 = 2^k qx + 2^k - 1. */
- px = scalbnl(1.0, k);
- x = px * qx + (px - 1.0);
+ exp(x) - 1 = 2^k (qx + 1) - 1 = 2 (2^(k-1) qx + 2^(k-1) - 0.5). */
+ px = scalbnl(1.0, k-1);
+ x = 2*(px * qx + (px - 0.5));
return x;
}
#elif LDBL_MANT_DIG == 113 && LDBL_MAX_EXP == 16384
--
2.29.2
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