Bug 6778 – expm1() raises bogus underflow exception for -large

Bug 6778 - expm1() raises bogus underflow exception for -large


Summary:	expm1() raises bogus underflow exception for -large

Status:	RESOLVED FIXED

Product:	glibc
Classification:	Unclassified
Component:	math
Version:	unspecified

Importance:	P2 normal
Target Milestone:	---
Assigned To:	Not yet assigned to anyone

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Depends on:
Blocks:
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Reported:	2008-07-30 11:05 UTC by Michael Kerrisk
Modified:	2012-07-06 11:19 UTC (History)
CC List:	1 user (show)

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test program (11.48 KB, text/plain) 2008-07-31 08:29 UTC, Michael Kerrisk	Details
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Description Michael Kerrisk 2008-07-30 11:05:27 UTC

(glibc 2.8)
When expm1(x) is given a large negative x value (e.g., in the range -1e5 through
to -1e308), it correctly returns -1.

However, it also raises an underflow exception.  This is bogus.  The result has
not underflowed: the result is asymptotically close to 1 (not zero).

Comment 1 Michael Kerrisk 2008-07-31 08:29:06 UTC

Created attachment 2843 [details]
test program

Example runs showing problem:

$ /tmp/mt_expm1 -- -1e5
errno == 0
fetestexcept() says:  FE_UNDERFLOW FE_INEXACT
expm1(-1.00000000000000000e+05)=-1.00000000000000000e+00
0 FE_UNDERFLOW normal

$ /tmp/mt_expm1 -- -1e308
errno == 0
fetestexcept() says:  FE_UNDERFLOW FE_INEXACT
expm1(-1.00000000000000001e+308)=-1.00000000000000000e+00
0 FE_UNDERFLOW normal

Comment 2 Joseph Myers 2012-02-29 20:59:11 UTC

Confirmed on x86 with current sources.  Does not appear on x86_64.  This is a
quality-of-implementation issue only, since C99 and C11 permit spurious
underflow exceptions (as well as spurious inexact exceptions, and raising
inexact even when the result is exact).

Comment 3 Joseph Myers 2012-07-06 11:19:45 UTC

Fixed for 2.17 by:

commit f17ac40d7cb8e8c462476b6ab703262f6b8f6da8
Author: Joseph Myers <joseph@codesourcery.com>
Date:   Fri Jul 6 11:17:41 2012 +0000

    Fix expm1 spurious underflow exceptions (bug 6778).