Hello,
I am currently using gcc 3.3.5 (on gentoo gnu/linux on athlonxp arch)
to compile a sizable, computationally intensive but not otherwise
large program (~6000 lines) of Gnu99 C code (-std=gnu99), and in some
functions in particular it appears that otherwise valid floating point
operations return NaN instead of the desired computation.
For example, the operation:
long double x = (some valid float value, like 0.22532);
long double x0 = (some other valid float like 0.6364);
long double result = x - x0;
printf("%Lf\n", result);
would sometimes print NaN, sometimes the correct value. If I inspect
the value of result in the gdb debugger following the computation it
also says NaN. If I perform the computation in gdb (with command
"print x - x0") it prints the valid result, which is why this bug is
so painful.
The program uses a combination of long doubles, doubles, and floats.
The problem does not seem to distinguish or change itself by
selecting a different type. I have been unable to replicate the
problem however, in a small test program, which makes me suspect the
size of the program(with many function calls and stack variables) to
be the culprit, but there is not stack overflow error.
It seems that the location of the problems is random, though mostly
concentrated on subtraction operations for some reason. However it is
deterministic (same or similar code causes same problem in same
location), though it is not the same when changing compiler options.
(i.e., changing from -g -ggdb to -O2 changes location of problem, not
fixing it)
If anybody has a theory of what is triggering this behavior, please
let me know. Also, please include my email in the reply as I am not
subscribed. Perhaps I need to upgrade the compiler, but first I would
like to see if it is my error which caused this.
Thank you
JB
Here is the list of compiler options tried:
-lpthread -W -Wall -Wfloat-equal -std=gnu99 -g -ggdb -march=i686
-lpthread -W -Wall -Wfloat-equal -std=gnu99 -g -ggdb -march=athlon-xp
-lpthread -W -Wall -Wfloat-equal -std=gnu99 -g -ggdb
-lpthread -W -Wall -Wfloat-equal -std=gnu99 -O2
with various other custom static libraries linked, etc.
