From: Alexander Monakov <amonakov@ispras.ru>
To: musl@lists.openwall.com
Subject: Re: FE Exception triggered by comparison
Date: Wed, 27 Feb 2019 17:14:55 +0300 (MSK) [thread overview]
Message-ID: <alpine.LNX.2.20.13.1902271651450.30425@monopod.intra.ispras.ru> (raw)
In-Reply-To: <alpine.LRH.2.02.1902261023110.27472@key0.esi.com.au>
On Tue, 26 Feb 2019, Damian McGuckin wrote:
> > Yet another problem brought to you by premature optimization. What you
> > want to know is if a certain number is NaN. There's a perfectly
> > acceptable idiom for that:
> >
> > if (isnan(x)) {
> > }
>
> I understand this. But, at least on an X86 architecture, that involves a
> write-to+read-from memory and I want to avoid that.
I don't understand this claim. With both SSE and x87 this should amount to
comparing a register with itself and testing flags. Can you elaborate?
> > Well, sorry, but these instructions do not work that way. Any operation
> > on a NaN, including comparison, even comparison for equality, will raise
> > IE, at least if it is an sNaN.
(for sNaNs, sure, but notably for qNaNs comparisons for equality, as opposed
to ordered comparisons, should not raise "invalid" in C and C++)
> However, on experiments on GCC, the if/else/else/else above does not raise an
> IE. Is this an optimizer bug?
You're probably seeing the long-standing bug in gcc x86 backend where it would
emit 'ucom'-kind instructions for both ordered and equality comparisons. It
appears to be finally fixed starting from GCC 8.
Alexander
next prev parent reply other threads:[~2019-02-27 14:14 UTC|newest]
Thread overview: 27+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-02-24 13:28 Damian McGuckin
2019-02-24 17:12 ` Markus Wichmann
2019-02-24 19:25 ` Szabolcs Nagy
2019-02-24 20:04 ` Jens Gustedt
2019-02-24 21:50 ` Damian McGuckin
2019-02-25 5:21 ` Damian McGuckin
2019-02-25 15:51 ` Markus Wichmann
2019-02-26 3:55 ` Damian McGuckin
2019-02-27 14:14 ` Alexander Monakov [this message]
2019-02-27 15:38 ` Damian McGuckin
2019-02-27 16:00 ` Alexander Monakov
2019-02-27 16:09 ` Damian McGuckin
2019-02-27 16:14 ` Markus Wichmann
2019-02-27 16:20 ` Damian McGuckin
2019-02-28 1:07 ` Damian McGuckin
2019-02-28 1:27 ` Rich Felker
2019-02-28 2:28 ` Damian McGuckin
2019-02-27 16:32 ` Alexander Monakov
2019-02-27 16:42 ` Rich Felker
2019-02-27 17:08 ` Szabolcs Nagy
2019-02-27 17:14 ` Alexander Monakov
2019-02-27 17:26 ` Rich Felker
2019-02-27 19:36 ` Szabolcs Nagy
2019-02-27 19:48 ` Alexander Monakov
2019-02-27 20:16 ` Szabolcs Nagy
2019-02-27 20:35 ` Rich Felker
2019-02-27 21:03 ` Szabolcs Nagy
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