https://gcc.gnu.org/bugzilla/show_bug.cgi?id=87817
--- Comment #2 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
E.g. in simplify-rtx.c we have:
case SIGN_EXTRACT:
case ZERO_EXTRACT:
if (CONST_INT_P (op0)
&& CONST_INT_P (op1)
&& CONST_INT_P (op2)
&& is_a <scalar_int_mode> (mode, &int_mode)
&& INTVAL (op1) + INTVAL (op2) <= GET_MODE_PRECISION (int_mode)
&& HWI_COMPUTABLE_MODE_P (int_mode))
{
/* Extracting a bit-field from a constant */
unsigned HOST_WIDE_INT val = UINTVAL (op0);
HOST_WIDE_INT op1val = INTVAL (op1);
HOST_WIDE_INT op2val = INTVAL (op2);
if (!BITS_BIG_ENDIAN)
val >>= op2val;
else if (is_a <scalar_int_mode> (op0_mode, &int_op0_mode))
val >>= GET_MODE_PRECISION (int_op0_mode) - op2val - op1val;
else
/* Not enough information to calculate the bit position. */
break;
if (HOST_BITS_PER_WIDE_INT != op1val)
{
/* First zero-extend. */
val &= (HOST_WIDE_INT_1U << op1val) - 1;
/* If desired, propagate sign bit. */
if (code == SIGN_EXTRACT
&& (val & (HOST_WIDE_INT_1U << (op1val - 1)))
!= 0)
val |= ~ ((HOST_WIDE_INT_1U << op1val) - 1);
}
return gen_int_mode (val, int_mode);
}
where for ZERO_EXTRACT we actually return CONST0_RTX (int_mode) if op1val == 0,
but can invoke UB before that in the big endian shift. For SIGN_EXTRACT we
invoke UB in any case, HOST_WIDE_INT_1U << (-1).
Note, the bzhi patterns aren't the only ones that rely on ZERO_EXTRACT being
defined for size 0, e.g. bextr does as well.
