jmc (03/23/83)
Did anyone ever notice this bug? The C optimiser on Sys III/V7 PDP11, and I
think Sys III VAX loops with this program.
main()
{
register int a; /* Doesnt have to be register */
register int b; /* Ditto */
for (;;) {
if (b) {
while (a) ; /* More or less anything */
b = 0; /* Must be a constant, failing 'if' */
}
}
}
The fix is in the routine 'redunbr' in the C optimiser which attempts to avoid
branching to a conditional branch which it knows the outcome of. Add the
condition '&& p->labno != p1->labno' to the condtion 'if (compare.....'.
Interesting - the program would loop infinately, so does the optimiser. Does
the optimiser know something that Turing didn't?
John Collins
...!mcvax!ukc!root44!jmc
...!edcaad!root44!jmc