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Date: Sat, 17 Mar 2012 16:10:52 -0400
From: Rich Felker <dalias@...ifal.cx>
To: musl@...ts.openwall.com
Subject: Re: i386 fegetround bug
On Sat, Mar 17, 2012 at 07:48:01PM +0100, Szabolcs Nagy wrote:
> * Szabolcs Nagy <nsz@...t70.net> [2012-03-17 15:42:44 +0100]:
> > fegetround had a bug: eax was not cleared properly
> >
> > while fixing it i also changed fesetround see my commit message
> >
> > http://nsz.repo.hu/git/?p=musl;a=commit;h=88cfaf8a142a8c57beccb89398a6421c4bbf121a
> >
>
> i realized my analysis was not entirely correct after rereading
> Intel 64 and IA-32 .. Manual, vol.1. 8.6 last paragraph and 8.7.1
>
> it suggests that pending unmasked exceptions can occur
> with normal operation (assuming there are unmasked exceptions)
> (i just couldn't produce such case from c as an arithmetic op
> is usually followed by an fld which traped before i could test
> my theory with an fnstenv)
>
> so if we want to be unmasked-exception-friendly then
> fnstenv/fldenv pair should be used
I'm not in favor of hurting performance or other useful
characteristics for the sake of a nonstandard feature of dubious
usefulness. Unless there's a demand to change it, I'm fine with your
new (fastest) version of the code.
> (maybe with some extra check for unmasked exceptions first:
> fnstcw x
> and $0x3f,x
> sub $0x3f,x
> jnz unmasked_exceptions
> ....
> and the unmasked_exceptions case uses fnstenv/fldenv, otherwise
> plain fldcw is used)
I'm not opposed to this solution, but also don't really find it
necessary. By the way, I think shifting right the low 6 bits and
checking carry flag would be more efficient. :)
Rich
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