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Date: Fri, 13 Mar 1998 13:43:54 +0200 (EET)
From: Tuukka Toivonen <tuukkat AT ees2 DOT oulu DOT fi>
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To: Marc Lehmann <pcg AT goof DOT com>
cc: Thomas Koehler <T DOT Koehler AT pfh DOT research DOT philips DOT com>,
        beastium <beastium-list AT Desk DOT nl>
Subject: Re: paranoia & extra precision [was -fno-float-store in pgcc]
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Sender: Marc Lehmann <pcg AT goof DOT com>
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On Thu, 12 Mar 1998, Marc Lehmann wrote:

>> There seems to be annother way, but involes the unducumented functions
>> __getfpucw and __setfpucw.

Undocumented???? Those functions has nothing to do with the _hardware_ ->
they are some OS/GCC thingies I don't know about -> GCC/OS is undocumented
-> GCC/OS is broken (not really... with GNU software they're documented in
source code, at least)

These functions probably use FPU instructions fstenv/fldenv, which are well
documented in Intel Pentium manuals as well as internals of the FPU.

>- not all operations are affected

_If_ the IEEE requires transcendental functions to have a low precision,
true. Does anyone _know_ what IEEE says? It's difficult for me to believe
that if IEEE would actually require it, Intel wouldn't honor them _even if_
they honor them with add/sub/div/mul etc...

>- the compiler cannot know when to set it, i.e. the same fundamental problem
>  than with mmx, the compiler had to set the cw on almost every procedure
>  entry etc...

Yep, the user must tell it.

>x86 fpu = hopelessly broken. by design.

Except for maybe transcendental instructions, it is not. Just set low
precision at bootup and don't change it later -> you'll have IEEE
compliance without anything (else) to do. Nothing to worry about... you'll
get the poor IEEE precision then always.

--
| Tuukka Toivonen <tuukkat AT ee DOT oulu DOT fi>       [PGP public key
| Homepage: http://www.ee.oulu.fi/~tuukkat/       available]
| Try also finger -l tuukkat AT ee DOT oulu DOT fi
| Studying information engineering at the University of Oulu
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