Re: [PATCH 02/12] Allow generating pseudo register with specific alignment

[Richard Sandiford <richard.sandiford@arm.com>]

Richard Biener via Gcc-patches <gcc-patches@gcc.gnu.org> writes:
> On Fri, Apr 30, 2021 at 8:30 PM Richard Sandiford via Gcc-patches
> <gcc-patches@gcc.gnu.org> wrote:
>>
>> "H.J. Lu via Gcc-patches" <gcc-patches@gcc.gnu.org> writes:
>> > On Fri, Apr 30, 2021 at 5:49 AM H.J. Lu <hjl.tools@gmail.com> wrote:
>> >>
>> >> On Fri, Apr 30, 2021 at 5:42 AM Richard Sandiford
>> >> <richard.sandiford@arm.com> wrote:
>> >> >
>> >> > "H.J. Lu via Gcc-patches" <gcc-patches@gcc.gnu.org> writes:
>> >> > > On Fri, Apr 30, 2021 at 2:06 AM Richard Sandiford
>> >> > > <richard.sandiford@arm.com> wrote:
>> >> > >>
>> >> > >> "H.J. Lu via Gcc-patches" <gcc-patches@gcc.gnu.org> writes:
>> >> > >> > gen_reg_rtx tracks stack alignment needed for pseudo registers so that
>> >> > >> > associated hard registers can be properly spilled onto stack.  But there
>> >> > >> > are cases where associated hard registers will never be spilled onto
>> >> > >> > stack.  gen_reg_rtx is changed to take an argument for register alignment
>> >> > >> > so that stack realignment can be avoided when not needed.
>> >> > >>
>> >> > >> How is it guaranteed that they will never be spilled though?
>> >> > >> I don't think that that guarantee exists for any kind of pseudo,
>> >> > >> except perhaps for the temporary pseudos that the RA creates to
>> >> > >> replace (match_scratch …)es.
>> >> > >>
>> >> > >
>> >> > > The caller of creating pseudo registers with specific alignment must
>> >> > > guarantee that they will never be spilled.   I am only using it in
>> >> > >
>> >> > >   /* Make operand1 a register if it isn't already.  */
>> >> > >   if (can_create_pseudo_p ()
>> >> > >       && !register_operand (op0, mode)
>> >> > >       && !register_operand (op1, mode))
>> >> > >     {
>> >> > >       /* NB: Don't increase stack alignment requirement when forcing
>> >> > >          operand1 into a pseudo register to copy data from one memory
>> >> > >          location to another since it doesn't require a spill.  */
>> >> > >       emit_move_insn (op0,
>> >> > >                       force_reg (GET_MODE (op0), op1,
>> >> > >                                  (UNITS_PER_WORD * BITS_PER_UNIT)));
>> >> > >       return;
>> >> > >     }
>> >> > >
>> >> > > for vector moves.  RA shouldn't spill it.
>> >> >
>> >> > But this is the point: it's a case of hoping that the RA won't spill it,
>> >> > rather than having a guarantee that it won't.
>> >> >
>> >> > Even if the moves start out adjacent, they could be separated by later
>> >> > RTL optimisations, particularly scheduling.  (I realise pre-RA scheduling
>> >> > isn't enabled by default for x86, but it can still be enabled explicitly.)
>> >> > Or if the same data is being copied to two locations, we might reuse
>> >> > values loaded by the first copy for the second copy as well.
>> >
>> > There are cases where pseudo vector registers are created as pure
>> > temporary registers in the backend and they shouldn't ever be spilled
>> > to stack.   They will be spilled to stack only if there are other non-temporary
>> > vector register usage in which case stack will be properly re-aligned.
>> > Caller of creating pseudo registers with specific alignment guarantees
>> > that they are used only as pure temporary registers.
>>
>> I don't think there's really a distinct category of pure temporary
>> registers though.  The things I mentioned above can happen for any
>> kind of pseudo register.
>
> I wonder if for the cases HJ thinks of it is appropriate to use hardregs?
> Do we generally handle those well?  That is, are they again subject
> to be allocated by RA when no longer live?

Yeah, using hard registers should work.  Of course, any given fixed choice
of hard register has the potential to be suboptimal in some situation,
but it should be safe.

Thanks,
Richard