https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66127
Bug ID: 66127
Summary: Division by zero gets folded away
Product: gcc
Version: 6.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: middle-end
Assignee: unassigned at gcc dot gnu.org
Reporter: mpolacek at gcc dot gnu.org
Target Milestone: ---
In match.pd, we have
(simplify
(mult @0 integer_zerop@1)
@1)
so anything * 0 -> 0. That seems to be undesirable in case "anything" contains
a division by zero. And a few lines below we have
/* Make sure to preserve divisions by zero. This is the reason why
we don't simplify x / x to 1 or 0 / x to 0. */
(for op (mult trunc_div ceil_div floor_div round_div exact_div)
(simplify
(op @0 integer_onep)
(non_lvalue @0)))
This means that
int
main (void)
{
int z = 0;
int a = 0 * (1 / z);
return a;
}
$ xgcc f.c; ./a.out
is "ok", but e.g.
int
main (void)
{
int z = 0;
int a = 1 * (1 / z);
return a;
}
naturally results in SIGFPE.
Yes, I know that division by zero is UB and there are no guarantees whatsoever,
but this folding is causing me a grief in the C FE because while fold doesn't
fold away "1 / 0", it folds "0 * (1 / 0)" into 0. That's bad when we find
ourselves in a situation where a constant integer expression is required.
