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From: "andreas_roever at web dot de" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug libstdc++/103005] experimental simd sin and cos with big arguments returns values bigger than 1
Date: Sat, 06 Nov 2021 16:51:28 +0000	[thread overview]
Message-ID: <bug-103005-4-d4rbUsm5Ui@http.gcc.gnu.org/bugzilla/> (raw)
In-Reply-To: <bug-103005-4@http.gcc.gnu.org/bugzilla/>

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103005

--- Comment #2 from Andreas Röver <andreas_roever at web dot de> ---
I think the source of the problem is function

__fold_input(const simd<double, _Abi>& __x)

in simd_math.h

this function should return values between -pi/4 and pi/4 but doesn't for the
big argument. E.g for 

2985064393126969344

it returns

27.678747

according to wolfram alpha it should return 

0.189812

This "big" value throws off the calculation of the power series that is
performed after this


the 27.678646 is the result of a rounding error

2985064393126969344 / (pi/2) is (according to wolfram alpha again) 

1900351014455063569.620838

but the CPU gets

1900351014455063552

This seems to be the closest value that the CPU can get to the real value.

the difference between those 2 times pi/2 is the above mentioned 27.67...

... now my numerics skills are not good enough to find a cure for that problem.

  parent reply	other threads:[~2021-11-06 16:51 UTC|newest]

Thread overview: 4+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-10-30 17:15 [Bug libstdc++/103005] New: " andreas_roever at web dot de
2021-11-01 10:32 ` [Bug libstdc++/103005] " marxin at gcc dot gnu.org
2021-11-06 16:51 ` andreas_roever at web dot de [this message]
2021-11-06 16:52 ` andreas_roever at web dot de

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