Re: [PATCH v2 2/2] RISC-V: Add instruction fusion (for ventana-vt1)

[Philipp Tomsich <philipp.tomsich@vrull.eu>]

On Mon, 14 Nov 2022 at 17:06, Jeff Law <jeffreyalaw@gmail.com> wrote:
>
>
> On 11/13/22 13:48, Philipp Tomsich wrote:
> > The Ventana VT1 core supports quad-issue and instruction fusion.
> > This implemented TARGET_SCHED_MACRO_FUSION_P to keep fusible sequences
> > together and adds idiom matcheing for the supported fusion cases.
> >
> > gcc/ChangeLog:
> >
> >       * config/riscv/riscv.cc (enum riscv_fusion_pairs): Add symbolic
> >       constants to identify supported fusion patterns.
> >       (struct riscv_tune_param): Add fusible_op field.
> >       (riscv_macro_fusion_p): Implement.
> >       (riscv_fusion_enabled_p): Implement.
> >       (riscv_macro_fusion_pair_p): Implement and recoginze fusible
> >       idioms for Ventana VT1.
> >       (TARGET_SCHED_MACRO_FUSION_P): Point to riscv_macro_fusion_p.
> >       (TARGET_SCHED_MACRO_FUSION_PAIR_P): Point to riscv_macro_fusion_pair_p.
>
> You know the fusion rules for VT1 better than I...  I'm happy to largely
> defer to you on this.
>
> I do wonder if going forward hand matching RTL like this is going to be
> an unmaintainable mess and whether or not we would be better served
> using insn attributes to describe instruction fusion.

I had thought about that, too.
In the end our team decided to stay away from it for the time being:
fusion frequently needs to look at second-level properties and whether
the first instruction's output register is overwritten by the second
instruction.  So we kept with the same stereotype of idiom-matching
that is also used for AArch64 today.

That said, both the RISC-V and the AArch64 implementations of this are
on my list of things to refactor in a quiet hour.

>
>
>
> >
> > Signed-off-by: Philipp Tomsich <philipp.tomsich@vrull.eu>
> > ---
> >
> > Changes in v2:
> > - Update fusion patterns and catch some missing idioms/fusion pairs.
> >
> >   gcc/config/riscv/riscv.cc | 219 ++++++++++++++++++++++++++++++++++++++
> >   1 file changed, 219 insertions(+)
> >
> > diff --git a/gcc/config/riscv/riscv.cc b/gcc/config/riscv/riscv.cc
> > index 31d651f8744..43ba520885c 100644
> > --- a/gcc/config/riscv/riscv.cc
> > +++ b/gcc/config/riscv/riscv.cc
> >
> > +static bool
> > +riscv_macro_fusion_pair_p (rtx_insn *prev, rtx_insn *curr)
> > +{
> > +  rtx prev_set = single_set (prev);
> > +  rtx curr_set = single_set (curr);
> > +  /* prev and curr are simple SET insns i.e. no flag setting or branching.  */
> > +  bool simple_sets_p = prev_set && curr_set && !any_condjump_p (curr);
> > +
> > +  if (!riscv_macro_fusion_p ())
> > +    return false;
> > +
> > +  if (simple_sets_p && (riscv_fusion_enabled_p (RISCV_FUSE_ZEXTW) ||
> > +                     riscv_fusion_enabled_p (RISCV_FUSE_ZEXTH)))
>
> Formatting nit.  Bring the && down to a new line and if you still need a
> line break for the "||",  then the "||" should be on a new line as
> well.  Something like this...
>
>
> if (simple_sets_p
>        && (riscv_fusion_enabled_p (RISCV_FUSE_ZEXTW
>
>            || riscv_fusion_enabled_p (RISCV_FUSE_ZEXTH)))
>
>
> > +       && REGNO (XEXP (SET_SRC (curr_set), 0)) == REGNO(SET_DEST (curr_set))
>
> Space before open paren on this line.
>
>
> >
> > +       && (( INTVAL (XEXP (SET_SRC (curr_set), 1)) == 32
> > +             && riscv_fusion_enabled_p(RISCV_FUSE_ZEXTW) )
> > +           || ( INTVAL (XEXP (SET_SRC (curr_set), 1)) < 32
> > +                && riscv_fusion_enabled_p(RISCV_FUSE_ZEXTWS))))
>
> Extraneous spaces after the open parens before INTVALs above.
>
>
> > +       && REGNO (XEXP (SET_SRC (curr_set), 0)) == REGNO(SET_DEST (curr_set))
>
> Missing whitespace before open paren on this line.
>
>
> OK with the nits fixed.

Applied to master with these fixes (and a fix for the typo in the
commit message that Jakub spotted).
Thanks!

Philipp.