Re: What problems are there with floating point emulation in the kernel?

Conn Clark wrote:

Its not the use of floating point in the kernel thats the problem.

	I would like to "emulate" the fpu in the kernel so I can
compile glibc and everything else with fpu instructions to save
ram. I have been told however that there are "issues" with this
approach, and the only way to go is to emulate the fpu in the
kernel and compile everything with soft float. What I want to
know is what these issues are? (accuracy,segment_fault,etc..)

Oh.  Right.  Well, google reports at least one issue with the code;
see http://groups.google.com/groups?selm=fa.h1feiov.9jqg8k%40ifi.uio.no

Mikael Pettersson (mikpe@csd.uu.se)
Subject: Re: Kernel bugs found using inspect tool
Newsgroups: fa.linux.kernel
Date: 2000/02/25

Ricky Beam writes:
 > On Tue, 22 Feb 2000, Rupp, Ed J wrote:
 > >the AT&T toolchest called 'inspect'.  I found 10 near-certain
 > >bugs which may or may not be causing any observable problems
 > >and 13 probable bugs which I couldn't immediately tell if they ...
 > >===== Questionable ====
 > >arch/ppc/math-emu/op-4.h: comparison takes precedence over assignment near
 > >line 215
 > >  (x3 += ((x2 += ((x1 += ((x0 += i) < x0)) < x1) < x2)))

This whole expression is broken.
The evaluation order for the "<" operator is unspecified, which means that
subexpressions like "((x0 += i) < x0)" do not have well-defined values.
They also violate the ANSI-C rules about permissible accesses and updates
of variables between sequence points.

- Dan


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