From: Wilco Dijkstra <Wilco.Dijkstra@arm.com>
To: Giuliano Augusto Faulin Belinassi <giuliano.belinassi@usp.br>
Cc: GCC Patches <gcc-patches@gcc.gnu.org>,
"giuliano.belinassi@gmail.com" <giuliano.belinassi@gmail.com>,
nd <nd@arm.com>, Jeff Law <law@redhat.com>
Subject: Re: [PATCH] Add sinh(tanh(x)) and cosh(tanh(x)) rules
Date: Fri, 19 Oct 2018 13:46:00 -0000 [thread overview]
Message-ID: <DB5PR08MB103010C39AAFAE5766A73F3A83F90@DB5PR08MB1030.eurprd08.prod.outlook.com> (raw)
In-Reply-To: <CAEFO=4CV1P23fmb_kAv8u=ubJ5PejVFC59gFGnbNm2TrO-P38Q@mail.gmail.com>
Hi,
>> Did you enable FMA? I'd expect 1 - x*x to be accurate with FMA, so the relative error
>> should be much better. If there is no FMA, 2*(1-fabs(x)) - (1-fabs(x))^2 should be
>> more accurate when abs(x)>0.5 and still much faster.
>
>No, but I will check how to enable it if FMA is available.
> I did a minor test with your formula and the precision improved a lot.
> But now I am puzzled about how did you come up with that formula :-).
> I am able to proof equality, but how did you know it was going to be
> more precise?
Basically when x is close to 1, x the top N bits in the mantissa will be ones.
Then x*x has one bits in the top 2*N bits in the mantissa. Ie. we lose N bits of
useful information in the multiply - problematic when N gets close to the number
of mantissa bits. In contrast FMA computes the fully accurate result due to
cancellation of the top 2*N one-bits in the subtract.
If we can use (1-x) instead of x in the evaluation, we avoid losing accuracy in the
multiply when x is close to 1. Then it's basic algebra to find an equivalent formula
that can produce 1-x^2 using 1-x. For example (1+x)*(1-x) will work fine too
(using 1+x loses 1 low bit of x).
Note that these alternative evaluations lose accuracy close to 0 in exactly the
same way, so if no FMA is available you'd need to select between the 2 cases.
Wilco
next prev parent reply other threads:[~2018-10-19 13:39 UTC|newest]
Thread overview: 37+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-10-18 23:22 Wilco Dijkstra
2018-10-19 13:39 ` Giuliano Augusto Faulin Belinassi
2018-10-19 13:46 ` Wilco Dijkstra [this message]
2018-10-19 14:49 ` Jakub Jelinek
2018-10-19 15:35 ` Wilco Dijkstra
2018-10-19 14:03 ` Segher Boessenkool
2018-10-19 15:10 ` Giuliano Augusto Faulin Belinassi
2018-10-19 17:28 ` Wilco Dijkstra
2018-10-20 19:06 ` Giuliano Augusto Faulin Belinassi
2018-10-22 20:58 ` Jeff Law
2018-10-23 9:52 ` Richard Biener
2018-10-23 11:31 ` Wilco Dijkstra
2018-10-23 11:35 ` Jakub Jelinek
2018-10-23 12:02 ` Wilco Dijkstra
2018-11-07 22:21 ` Jeff Law
2018-11-07 22:34 ` Wilco Dijkstra
2018-11-08 1:44 ` Segher Boessenkool
2018-11-08 13:33 ` Wilco Dijkstra
2018-11-09 20:04 ` Jeff Law
2018-11-09 22:04 ` Giuliano Augusto Faulin Belinassi
2018-11-10 5:36 ` Segher Boessenkool
2018-11-12 14:17 ` Richard Biener
2018-12-02 14:37 ` Segher Boessenkool
-- strict thread matches above, loose matches on Subject: below --
2018-08-07 20:00 Giuliano Augusto Faulin Belinassi
2018-08-07 20:29 ` Paul Koning
2018-08-07 20:42 ` Giuliano Augusto Faulin Belinassi
2018-08-08 13:59 ` Paul Koning
2018-08-08 18:57 ` Giuliano Augusto Faulin Belinassi
2018-08-08 19:55 ` Paul Koning
2018-10-12 4:55 ` Jeff Law
2018-10-12 14:55 ` Giuliano Augusto Faulin Belinassi
2018-10-16 21:16 ` Jeff Law
2018-10-17 21:43 ` Giuliano Augusto Faulin Belinassi
2018-10-17 22:45 ` Jeff Law
2018-10-18 5:38 ` Giuliano Augusto Faulin Belinassi
2018-10-18 17:10 ` Jeff Law
2018-10-18 22:46 ` Giuliano Belinassi
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