On 6/25/19, Hongtao Liu <crazy...@gmail.com> wrote:
> On Sat, Jun 22, 2019 at 3:38 PM Uros Bizjak <ubiz...@gmail.com> wrote:
>>
>> On Fri, Jun 21, 2019 at 8:38 PM H.J. Lu <hjl.to...@gmail.com> wrote:
>>
>> > > > > > > > > > >> > > +/* Register pair. */
>> > > > > > > > > > >> > > +VECTOR_MODES_WITH_PREFIX (P, INT, 2); /* P2QI
>> > > > > > > > > > >> > > */
>> > > > > > > > > > >> > > +VECTOR_MODES_WITH_PREFIX (P, INT, 4); /* P2HI
>> > > > > > > > > > >> > > P4QI */
>> > > > > > > > > > >> > >
>> > > > > > > > > > >> > > I think
>> > > > > > > > > > >> > >
>> > > > > > > > > > >> > > INT_MODE (P2QI, 16);
>> > > > > > > > > > >> > > INT_MODE (P2HI, 32);
>> > > > > Why P2QI need 16 bytes but not 2 bytes?
>> > > > > Same question with P2HI.
>> > > >
>> > > > Because we made a mistake. It should be 2 and 4, since these
>> > > > arguments
>> > > Then it will run into internal comiler error when building libgcc.
>> > > I'm still invertigating it.
>> > > > are bytes, not bits.
>> >
>> > I don't think we can have 2 integer modes with the same number of bytes
>> > since
>> > it breaks things like
>> >
>> > scalar_int_mode wider_mode = GET_MODE_WIDER_MODE (mode).require ();
>> >
>> > We can get
>> >
>> > (gdb) p mode
>> > $2 = {m_mode = E_SImode}
>> > (gdb) p wider_mode
>> > $3 = {m_mode = E_P2HImode}
>> > (gdb)
>> >
>> > Neither middle-end nor backend support it.
>>
>> Ouch... It looks we hit the limitation of the middle end (which should
>> at least warn/error out if two modes of the same width are declared).
>>
>> OTOH, we can't solve this problem by using two HI/QImode registers,
>> since a consecutive register pair has to be allocated It is also not
>> possible to overload existing SI/HImode mode with different
>> requirements w.r.t register pair allocation (e.g. sometimes the whole
>> register is allocated, and sometimes a register pair is allocated).
>>
>> I think we have to invent something like SPECIAL_INT_MODE, which would
>> avoid mode promotion functionality (basically, it should not be listed
>> in mode_wider and similar arrays). This would prevent mode promotion
>> issues, while it would still allow to have mode, having the same width
>> as existing mode, but with special properties.
>>
>> I'm adding Jeff and Jakub to the discussion about SPECIAL_INT_MODE.
>>
>> Uros.
>
> Patch from H.J using PARTIAL_INT_MODE fixed this issue.
>
> +/* Register pair. */
> +PARTIAL_INT_MODE (HI, 16, P2QI);
> +PARTIAL_INT_MODE (SI, 32, P2HI);
> +
I don't think this approach is correct (the mode is not partial), and
it could work by chance. The documentation is very brief with the
details of different mode types, so let's ask middle-end and RTL
experts.
Uros.
