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From: Jeff Johnston <jjohnstn@sourceware.org> To: newlib-cvs@sourceware.org Subject: [newlib-cygwin] Fix error in powf for x close to 1 and large y Date: Fri, 11 Dec 2020 21:00:24 +0000 (GMT) [thread overview] Message-ID: <20201211210024.960683857C4F@sourceware.org> (raw) https://sourceware.org/git/gitweb.cgi?p=newlib-cygwin.git;h=cf1ef2dc5b95342fa337141b3b1ec85aac73229f commit cf1ef2dc5b95342fa337141b3b1ec85aac73229f Author: Fabian Schriever <fabian.schriever@gtd-gmbh.de> Date: Fri Dec 11 16:57:35 2020 +0100 Fix error in powf for x close to 1 and large y This patch fixes the error found by Paul Zimmermann (see https://homepages.loria.fr/PZimmermann/papers/#accuracy) regarding x close to 1 and rather large y (specifically he found the case powf(0x1.ffffeep-1,-0x1.000002p+27) which returns +Inf instead of the correct value). We found 2 more values for x which show the same faulty behaviour, and all 3 are fixed with this patch. We have tested all combinations for x in [+1.fffdfp-1, +1.00020p+0] and y in [-1.000007p+27, -1.000002p+27] and [1.000002p+27,1.000007p+27]. Diff: --- newlib/libm/math/ef_pow.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/newlib/libm/math/ef_pow.c b/newlib/libm/math/ef_pow.c index e3579f071..07b225f8c 100644 --- a/newlib/libm/math/ef_pow.c +++ b/newlib/libm/math/ef_pow.c @@ -138,7 +138,7 @@ ivln2_l = 7.0526075433e-06; /* 0x36eca570 =1/ln2 tail*/ /* |y| is huge */ if(iy>0x4d000000) { /* if |y| > 2**27 */ /* over/underflow if x is not close to one */ - if(ix<0x3f7ffff8) return (hy<0)? __math_oflowf(0):__math_uflowf(0); + if(ix<0x3f7ffff4) return (hy<0)? __math_oflowf(0):__math_uflowf(0); if(ix>0x3f800007) return (hy>0)? __math_oflowf(0):__math_uflowf(0); /* now |1-x| is tiny <= 2**-20, suffice to compute log(x) by x-x^2/2+x^3/3-x^4/4 */
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