From: buhr@stat.wisc.edu (Kevin Buhr)
To: Alan Cox <alan@lxorguk.ukuu.org.uk>
Cc: linux-kernel@vger.kernel.org
Subject: Re: RFC: changing precision control setting in initial FPU context
Date: 03 Mar 2001 18:27:50 -0600 [thread overview]
Message-ID: <vban1b2ped5.fsf@mozart.stat.wisc.edu> (raw)
In-Reply-To: <E14ZLat-0004Js-00@the-village.bc.nu>
In-Reply-To: Alan Cox's message of "Sat, 3 Mar 2001 23:37:45 +0000 (GMT)"
Alan Cox <alan@lxorguk.ukuu.org.uk> writes:
>
> You want peoples existing applications to suddenely and magically change
> their results. Umm problem.
So, how would you feel about a mechanism whereby the kernel could be
passed a default FPU control word by the binary (with old binaries, by
default, using the old default control word)? There's already an ELF
AT_FPUCW auxv entry type. What if this was used by the kernel, rather
than the C library (as it is now), to set a default to be used in
"init_fpu()" when and if the program executed a floating point
instruction?
Then, a compiler startup-code writer would be able to specify a
default control word for binaries that was appropriate for (new)
programs generated by that compiler *WITHOUT* worrying about whether
he was accidentally turning a non-FP program into an FP program by
introducing "fnstcw" as its only FPU instruction.
The C library is already trying to do this (setting the CW based on
the AT_FPUCW vector). It just can't do it *right* because it doesn't
know if the program is really FP. It just guesses that if the
AT_FPUCW vector contains something other than the hard-coded
_FPU_DEFAULT (which is supposed to be equal to the kernel default: it
isn't, but it's close enough), it must be set; otherwise, it's left
alone.
Kevin <buhr@stat.wisc.edu>
next prev parent reply other threads:[~2001-03-04 0:28 UTC|newest]
Thread overview: 13+ messages / expand[flat|nested] mbox.gz Atom feed top
2001-03-03 10:47 RFC: changing precision control setting in initial FPU context Adam J. Richter
2001-03-03 23:29 ` Kevin Buhr
2001-03-03 23:37 ` Alan Cox
2001-03-04 0:27 ` Kevin Buhr [this message]
2001-03-04 0:45 ` Ulrich Drepper
-- strict thread matches above, loose matches on Subject: below --
2001-03-03 7:12 Kevin Buhr
2001-03-03 9:31 ` Albert D. Cahalan
2001-03-03 10:26 ` Kevin Buhr
2001-03-03 20:04 ` Albert D. Cahalan
2001-03-03 21:00 ` Jason Riedy
2001-03-03 23:17 ` Kevin Buhr
2001-03-04 4:21 ` Jason Riedy
2001-03-03 22:34 ` Kevin Buhr
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