Date: Mon, 5 May 1997 15:44:49 +0200 From: griepent@wias-berlin.de To: bugs@FreeBSD.org Subject: Underflow floating point exceptions under FreeBSD 2.1.5 Message-ID: <9705051344.AA16083@pascal.wias-berlin.de>
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Hello, FreeBSD experts!
I am going to use numerical applications on a X11 Window desktop and
I want to work with the nice and stable FreeBSD system at home.
To stay compatible with an institute I want to use a numerical
package written in ANSI-C which works flawlessly on DEC and SGI machines.
I was able to compile (gcc-2.6.3) the package on several Intel
platforms (from 486 DX to PentiumPro) under FreeBSD-2.1.5.
Trying to run it there always occured strange underflow floating
point exceptions in code fragments computing mathematical
functions like
sqrt, cos, log
with harmless arguments of type double in the range of 1.0e-20 to 1.0e+20,
which should be normally no big deal in the IEEE 754 world!
My first idea was to recompile the code with gnu debugger support
enabled and no optimization options. So I have got the same
underflow floating point exceptions, but now more localized, namely
in the above mathematical functions contained in the shared library
/usr/lib/libm.so
My next idea was a little bit more successful:
recompiling the libm sources with gdb support enabled.
Building the numerical package with that new math library and setting
appropriate breakpoints in the sqrt, log and cos function calls of the
library I was able to localize the underflows in the loops which
iteratively compute the values of that functions.
Remembering a similar looping problem (many years ago on other platforms)
I have decided to
recompile the libm sources with no optimization!
Again building the package with that math library the code is running now
and we get the same numerical results and performance as on the DEC or SGI
machines!
Now my questions:
(1) Let me assume that the original libm shared library was compiled
also with the gcc-2.6.3 compiler.
Is there a (known) optimization bug in the gcc-2.6.3 compiler?
(2) What about the other system libraries? Perhaps I have to be aware
to run in trouble in a similar manner!
(3) Is there any hope to get rid of that problems in one of the next
versions of FreeBSD or newer gcc versions?
It was the first serious disappointment in my two year experience with
that system. Please, let me stay trusting the FreeBSD operating system,
I want miss it for such a reason!
It would be very nice, if you could give me a hint!
Jens Griepentrog
griepent@wias-berlin.de
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