https://gcc.gnu.org/bugzilla/show_bug.cgi?id=126548
Bug ID: 126548
Summary: Wrong code with Bitint and value ranges
Product: gcc
Version: 17.0
Status: UNCONFIRMED
Keywords: wrong-code
Severity: normal
Priority: P3
Component: tree-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: ktkachov at gcc dot gnu.org
Target Milestone: ---
/* Entry point: operator_minus::wi_fold -> value_range_with_overflow
(gcc/range-op.cc:922). After the _BitInt lowering the top limb of a
_BitInt(193) is a <unnamed-signed:1>, a type on which the lowering emits
adds and subtracts that wrap by design, but which is
TYPE_OVERFLOW_UNDEFINED, so value_range_with_overflow returns UNDEFINED for
the wrapping half and the union comes out a singleton.
No undefined behaviour in this program: p - 4 is representable in 193 bits
for every long p, and the shift count 192 is below the precision 193. */
int printf (const char *, ...);
typedef _BitInt(193) s193;
__attribute__((noipa)) static long
f (long p)
{
s193 t = (s193) p - (s193) 4;
return (long) (t >> 192); /* sign of t: 0 or -1 */
}
int
main (void)
{
volatile long v = 81985529216486895L; /* > 4, so f must return 0 */
long r = f (v);
printf ("%ld\n", r);
if (r != 0)
__builtin_abort ();
if (f (-1L) != -1)
__builtin_abort ();
return 0;
}
on aarch64 at -O2 aborts and passes at -O0
