On Tue, Dec 10, 2013 at 9:26 AM, Tejas Belagod <tbela...@arm.com> wrote:
> H.J. Lu wrote:
>>
>> On Tue, Dec 10, 2013 at 9:04 AM, Kirill Yukhin <kirill.yuk...@gmail.com>
>> wrote:
>>>
>>> On 10 Dec 08:23, H.J. Lu wrote:
>>>>
>>>> What is wrong to pass the correct offset to
>>>> CANNOT_CHANGE_MODE_CLASS? Backends are free to
>>>> ignore it.
>>>
>>> Yes, but as fas as understand this hook as a predicate
>>> saying if it not-safe to change mode1 to mode2 for given
>>
>>
>> In many places, the macro is used with the known offset.
>> I have a follow up patch which improves x86 code generation,
>> in cases like:
>>
>> http://gcc.gnu.org/bugzilla/show_bug.cgi?id=45198
>>
>
> What is it that subreg_get_info () can't resolve that
> CANNOT_CHANGE_MODE_CLASS with an offset can?
>
We have
int
simplify_subreg_regno (unsigned int xregno, enum machine_mode xmode,
unsigned int offset, enum machine_mode ymode)
{
struct subreg_info info;
unsigned int yregno;
#ifdef CANNOT_CHANGE_MODE_CLASS
/* Give the backend a chance to disallow the mode change. */
if (GET_MODE_CLASS (xmode) != MODE_COMPLEX_INT
&& GET_MODE_CLASS (xmode) != MODE_COMPLEX_FLOAT
&& REG_CANNOT_CHANGE_MODE_P (xregno, xmode, ymode)
/* We can use mode change in LRA for some transformations. */
&& ! lra_in_progress)
return -1;
#endif
CANNOT_CHANGE_MODE_CLASS is checked before subreg_get_info is
called. When REG_CANNOT_CHANGE_MODE_P returns false,
there is nothing subreg_get_info can do.
--
H.J.
