------- Comment #4 from rguenth at gcc dot gnu dot org 2006-11-10 14:35 -------
/* Optimize away "if (x & C) x |= C" and similar bit manipulation
transformations. */
static int
noce_try_bitop (struct noce_if_info *if_info)
{
...
/* ??? We could also handle AND here. */
if (GET_CODE (cond) == ZERO_EXTRACT)
{
if (XEXP (cond, 1) != const1_rtx
|| GET_CODE (XEXP (cond, 2)) != CONST_INT
|| ! rtx_equal_p (x, XEXP (cond, 0)))
return FALSE;
bitnum = INTVAL (XEXP (cond, 2));
mode = GET_MODE (x);
if (bitnum >= HOST_BITS_PER_WIDE_INT)
return FALSE;
... which is wrong for little-endian bitfieds ...
/* if ((x & C) == 0) x |= C; is transformed to x |= C. */
/* if ((x & C) != 0) x |= C; is transformed to nothing. */
if (GET_CODE (a) == IOR)
result = (code == NE) ? a : NULL_RTX;
else if (code == NE)
{
/* if ((x & C) == 0) x ^= C; is transformed to x |= C. */
result = gen_int_mode ((HOST_WIDE_INT) 1 << bitnum, mode);
result = simplify_gen_binary (IOR, mode, x, result);
}
else
{
/* if ((x & C) != 0) x ^= C; is transformed to x &= ~C. */
result = gen_int_mode (~((HOST_WIDE_INT) 1 << bitnum), mode);
result = simplify_gen_binary (AND, mode, x, result);
}
--
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=29797
