On Fri, Apr 30, 2021 at 5:42 AM Richard Sandiford
<richard.sandif...@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.sandif...@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.
>
> The only way to guarantee that the temporary won't be spilled is to hide
> it until after RA.
>
Let me think about it.
--
H.J.
