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Subjectbug in redhat gcc 2.96
We believe we have found a bug in gcc.  We have been trying to track
down why the .../drivers/scsi/sym53c8xx.c driver oopses with a divide
by zero when initializing at line 5265, which reads:

period = (4 * div_10M[0] + np->clock_khz - 1) / np->clock_khz;

We believe the bug is that gcc is generating incorrect code for this:

if (f1 < 55000) f1 = 40000;
else f1 = 80000;
Here is the test code to demonstrate this:

% cat bug.c
int main (int argc, char *argv[])
{
unsigned f1;
f1 = (unsigned)argc;

if (f1 < 5) {
f1 = 4;
} else {
f1 = 8;
}
exit (f1);
}
And here are commands to exhibit the problem.

% for i in 0 1 2 3 4 5 6 ; do ln bug.c bug$i.c ; done
% for i in 0 1 2 3 4 5 6 ; do gcc -save-temps -O$i -o bug$i bug$i.c ; done
% for i in 0 1 2 3 4 5 6 ; do ./bug$i 1 2 ; echo $? ; done
% for i in 0 1 2 3 4 5 6 ; do ./bug$i 1 2 3 4 5 6 7 ; echo $? ; done

The level 0 optimization assembly code appears correct. For level 1 and
above, the compiler emits a long-subtract-with-borrow statement which
leaves EAX either 0 filled or 1 filled, based on the carry flag.

As this is with Red Hat's version of gcc, I'm not sending
this to the gcc folks. RPMs of gcc with this problem
include gcc-2.96-69 and gcc-2.96-81. This has been logged
as http://bugzilla.redhat.com/bugzilla/show_bug.cgi?id=39764.
Any suggestions for a way to cope with this? We have a
customer who's system fails due to this.


--
Jim Wright Software Engineer Penguin Computing
jwright@penguincomputing.com v:415-358-2609 f:415-358-2646

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