--jeremy
Jim Wright wrote:
>
> 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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