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

["H.J. Lu" <hjl.tools@gmail.com>]

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.

> Thanks,
> Richard
>
> >       * emit-rtl.c (gen_reg_rtx): Add an argument for register
> >       alignment and use it if it isn't zero.
> >       * explow.c (force_reg): Add an argument for register alignment
> >       and pass it to gen_reg_rtx.
> >       * explow.h (force_reg): Add an argument for register alignment
> >       and default it to 0.
> >       * expr.h (convert_to_mode): Likewise.
> >       (convert_modes): Likewise.
> >       * expr.c (convert_to_mode): Add an argument for register
> >       alignment and pass it to convert_modes.
> >       (convert_modes): Add an argument for register alignment and
> >       pass it to gen_reg_rtx.
> > ---
> >  gcc/emit-rtl.c |  5 +++--
> >  gcc/explow.c   |  6 +++---
> >  gcc/explow.h   |  2 +-
> >  gcc/expr.c     | 10 ++++++----
> >  gcc/expr.h     |  6 ++++--
> >  gcc/rtl.h      |  2 +-
> >  6 files changed, 18 insertions(+), 13 deletions(-)
> >
> > diff --git a/gcc/emit-rtl.c b/gcc/emit-rtl.c
> > index 07e908624a0..4accf851d23 100644
> > --- a/gcc/emit-rtl.c
> > +++ b/gcc/emit-rtl.c
> > @@ -1160,10 +1160,11 @@ subreg_memory_offset (const_rtx x)
> >     This pseudo is assigned the next sequential register number.  */
> >
> >  rtx
> > -gen_reg_rtx (machine_mode mode)
> > +gen_reg_rtx (machine_mode mode, unsigned int align)
> >  {
> >    rtx val;
> > -  unsigned int align = GET_MODE_ALIGNMENT (mode);
> > +  if (align == 0)
> > +    align = GET_MODE_ALIGNMENT (mode);
> >
> >    gcc_assert (can_create_pseudo_p ());
> >
> > diff --git a/gcc/explow.c b/gcc/explow.c
> > index b6da277f689..c8673ce512d 100644
> > --- a/gcc/explow.c
> > +++ b/gcc/explow.c
> > @@ -663,7 +663,7 @@ copy_to_mode_reg (machine_mode mode, rtx x)
> >     since we mark it as a "constant" register.  */
> >
> >  rtx
> > -force_reg (machine_mode mode, rtx x)
> > +force_reg (machine_mode mode, rtx x, unsigned int reg_align)
> >  {
> >    rtx temp, set;
> >    rtx_insn *insn;
> > @@ -673,7 +673,7 @@ force_reg (machine_mode mode, rtx x)
> >
> >    if (general_operand (x, mode))
> >      {
> > -      temp = gen_reg_rtx (mode);
> > +      temp = gen_reg_rtx (mode, reg_align);
> >        insn = emit_move_insn (temp, x);
> >      }
> >    else
> > @@ -683,7 +683,7 @@ force_reg (machine_mode mode, rtx x)
> >       insn = get_last_insn ();
> >        else
> >       {
> > -       rtx temp2 = gen_reg_rtx (mode);
> > +       rtx temp2 = gen_reg_rtx (mode, reg_align);
> >         insn = emit_move_insn (temp2, temp);
> >         temp = temp2;
> >       }
> > diff --git a/gcc/explow.h b/gcc/explow.h
> > index 698f2a2a21c..621cdd7d356 100644
> > --- a/gcc/explow.h
> > +++ b/gcc/explow.h
> > @@ -40,7 +40,7 @@ extern rtx copy_to_suggested_reg (rtx, rtx, machine_mode);
> >
> >  /* Copy a value to a register if it isn't already a register.
> >     Args are mode (in case value is a constant) and the value.  */
> > -extern rtx force_reg (machine_mode, rtx);
> > +extern rtx force_reg (machine_mode, rtx, unsigned int reg_align = 0);
> >
> >  /* Return given rtx, copied into a new temp reg if it was in memory.  */
> >  extern rtx force_not_mem (rtx);
> > diff --git a/gcc/expr.c b/gcc/expr.c
> > index b4c110f8c17..42db4ddbe0a 100644
> > --- a/gcc/expr.c
> > +++ b/gcc/expr.c
> > @@ -658,9 +658,10 @@ convert_mode_scalar (rtx to, rtx from, int unsignedp)
> >     or by copying to a new temporary with conversion.  */
> >
> >  rtx
> > -convert_to_mode (machine_mode mode, rtx x, int unsignedp)
> > +convert_to_mode (machine_mode mode, rtx x, int unsignedp,
> > +              unsigned int reg_align)
> >  {
> > -  return convert_modes (mode, VOIDmode, x, unsignedp);
> > +  return convert_modes (mode, VOIDmode, x, unsignedp, reg_align);
> >  }
> >
> >  /* Return an rtx for a value that would result
> > @@ -674,7 +675,8 @@ convert_to_mode (machine_mode mode, rtx x, int unsignedp)
> >     You can give VOIDmode for OLDMODE, if you are sure X has a nonvoid mode.  */
> >
> >  rtx
> > -convert_modes (machine_mode mode, machine_mode oldmode, rtx x, int unsignedp)
> > +convert_modes (machine_mode mode, machine_mode oldmode, rtx x,
> > +            int unsignedp, unsigned int reg_align)
> >  {
> >    rtx temp;
> >    scalar_int_mode int_mode;
> > @@ -734,7 +736,7 @@ convert_modes (machine_mode mode, machine_mode oldmode, rtx x, int unsignedp)
> >        return simplify_gen_subreg (mode, x, oldmode, 0);
> >      }
> >
> > -  temp = gen_reg_rtx (mode);
> > +  temp = gen_reg_rtx (mode, reg_align);
> >    convert_move (temp, x, unsignedp);
> >    return temp;
> >  }
> > diff --git a/gcc/expr.h b/gcc/expr.h
> > index 9a2736f69fa..2b06da1a889 100644
> > --- a/gcc/expr.h
> > +++ b/gcc/expr.h
> > @@ -66,10 +66,12 @@ extern void init_expr (void);
> >  extern void convert_move (rtx, rtx, int);
> >
> >  /* Convert an rtx to specified machine mode and return the result.  */
> > -extern rtx convert_to_mode (machine_mode, rtx, int);
> > +extern rtx convert_to_mode (machine_mode, rtx, int,
> > +                         unsigned int reg_align = 0);
> >
> >  /* Convert an rtx to MODE from OLDMODE and return the result.  */
> > -extern rtx convert_modes (machine_mode, machine_mode, rtx, int);
> > +extern rtx convert_modes (machine_mode, machine_mode, rtx, int,
> > +                       unsigned int reg_align = 0);
> >
> >  /* Expand a call to memcpy or memmove or memcmp, and return the result.  */
> >  extern rtx emit_block_op_via_libcall (enum built_in_function, rtx, rtx, rtx,
> > diff --git a/gcc/rtl.h b/gcc/rtl.h
> > index 398d745aff5..c72f7fd59b9 100644
> > --- a/gcc/rtl.h
> > +++ b/gcc/rtl.h
> > @@ -3125,7 +3125,7 @@ subreg_promoted_mode (rtx x)
> >  /* In emit-rtl.c */
> >  extern rtvec gen_rtvec_v (int, rtx *);
> >  extern rtvec gen_rtvec_v (int, rtx_insn **);
> > -extern rtx gen_reg_rtx (machine_mode);
> > +extern rtx gen_reg_rtx (machine_mode, unsigned int align = 0);
> >  extern rtx gen_rtx_REG_offset (rtx, machine_mode, unsigned int, poly_int64);
> >  extern rtx gen_reg_rtx_offset (rtx, machine_mode, int);
> >  extern rtx gen_reg_rtx_and_attrs (rtx);



-- 
H.J.