Re: [PATCH v2] ifcvt: Handle multiple rewired regs and refactor noce_convert_multiple_sets

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

Manolis Tsamis <manolis.tsamis@vrull.eu> writes:
> On Thu, Nov 23, 2023 at 11:01 PM Richard Sandiford
> <richard.sandiford@arm.com> wrote:
>>
>> Manolis Tsamis <manolis.tsamis@vrull.eu> writes:
>> > The existing implementation of need_cmov_or_rewire and
>> > noce_convert_multiple_sets_1 assumes that sets are either REG or SUBREG.
>> > This commit enchances them so they can handle/rewire arbitrary set statements.
>> >
>> > To do that a new helper struct noce_multiple_sets_info is introduced which is
>> > used by noce_convert_multiple_sets and its helper functions. This results in
>> > cleaner function signatures, improved efficientcy (a number of vecs and hash
>> > set/map are replaced with a single vec of struct) and simplicity.
>> >
>> > gcc/ChangeLog:
>> >
>> >       * ifcvt.cc (need_cmov_or_rewire): Renamed init_noce_multiple_sets_info.
>> >       (init_noce_multiple_sets_info): Initialize noce_multiple_sets_info.
>> >       (noce_convert_multiple_sets_1): Use noce_multiple_sets_info and handle
>> >       rewiring of multiple registers.
>> >       (noce_convert_multiple_sets): Updated to use noce_multiple_sets_info.
>> >       * ifcvt.h (struct noce_multiple_sets_info): Introduce new struct
>> >       noce_multiple_sets_info to store info for noce_convert_multiple_sets.
>> >
>> > Signed-off-by: Manolis Tsamis <manolis.tsamis@vrull.eu>
>> > ---
>>
>> Thanks, this looks like a really nice clean-up.  One comment below:
>>
> Thanks!
>
>> >
>> > Changes in v2:
>> >         - Made standalone patch.
>> >         - Better comments, some more checks.
>> >
>> >  gcc/ifcvt.cc | 252 +++++++++++++++++++++++----------------------------
>> >  gcc/ifcvt.h  |  16 ++++
>> >  2 files changed, 129 insertions(+), 139 deletions(-)
>> >
>> > diff --git a/gcc/ifcvt.cc b/gcc/ifcvt.cc
>> > index a0af553b9ff..9486d54de34 100644
>> > --- a/gcc/ifcvt.cc
>> > +++ b/gcc/ifcvt.cc
>> > @@ -98,14 +98,10 @@ static bool dead_or_predicable (basic_block, basic_block, basic_block,
>> >                               edge, bool);
>> >  static void noce_emit_move_insn (rtx, rtx);
>> >  static rtx_insn *block_has_only_trap (basic_block);
>> > -static void need_cmov_or_rewire (basic_block, hash_set<rtx_insn *> *,
>> > -                              hash_map<rtx_insn *, int> *);
>> > +static void init_noce_multiple_sets_info (basic_block,
>> > +  auto_delete_vec<noce_multiple_sets_info> &);
>> >  static bool noce_convert_multiple_sets_1 (struct noce_if_info *,
>> > -                                       hash_set<rtx_insn *> *,
>> > -                                       hash_map<rtx_insn *, int> *,
>> > -                                       auto_vec<rtx> *,
>> > -                                       auto_vec<rtx> *,
>> > -                                       auto_vec<rtx_insn *> *, int *);
>> > +  auto_delete_vec<noce_multiple_sets_info> &, int *);
>> >
>> >  /* Count the number of non-jump active insns in BB.  */
>> >
>> > @@ -3270,24 +3266,13 @@ noce_convert_multiple_sets (struct noce_if_info *if_info)
>> >    rtx x = XEXP (cond, 0);
>> >    rtx y = XEXP (cond, 1);
>> >
>> > -  /* The true targets for a conditional move.  */
>> > -  auto_vec<rtx> targets;
>> > -  /* The temporaries introduced to allow us to not consider register
>> > -     overlap.  */
>> > -  auto_vec<rtx> temporaries;
>> > -  /* The insns we've emitted.  */
>> > -  auto_vec<rtx_insn *> unmodified_insns;
>> > -
>> > -  hash_set<rtx_insn *> need_no_cmov;
>> > -  hash_map<rtx_insn *, int> rewired_src;
>> > -
>> > -  need_cmov_or_rewire (then_bb, &need_no_cmov, &rewired_src);
>> > +  auto_delete_vec<noce_multiple_sets_info> insn_info;
>> > +  init_noce_multiple_sets_info (then_bb, insn_info);
>> >
>> >    int last_needs_comparison = -1;
>> >
>> >    bool ok = noce_convert_multiple_sets_1
>> > -    (if_info, &need_no_cmov, &rewired_src, &targets, &temporaries,
>> > -     &unmodified_insns, &last_needs_comparison);
>> > +    (if_info, insn_info, &last_needs_comparison);
>> >    if (!ok)
>> >        return false;
>> >
>> > @@ -3302,8 +3287,7 @@ noce_convert_multiple_sets (struct noce_if_info *if_info)
>> >        end_sequence ();
>> >        start_sequence ();
>> >        ok = noce_convert_multiple_sets_1
>> > -     (if_info, &need_no_cmov, &rewired_src, &targets, &temporaries,
>> > -      &unmodified_insns, &last_needs_comparison);
>> > +     (if_info, insn_info, &last_needs_comparison);
>> >        /* Actually we should not fail anymore if we reached here,
>> >        but better still check.  */
>> >        if (!ok)
>> > @@ -3312,12 +3296,12 @@ noce_convert_multiple_sets (struct noce_if_info *if_info)
>> >
>> >    /* We must have seen some sort of insn to insert, otherwise we were
>> >       given an empty BB to convert, and we can't handle that.  */
>> > -  gcc_assert (!unmodified_insns.is_empty ());
>> > +  gcc_assert (!insn_info.is_empty ());
>> >
>> >    /* Now fixup the assignments.  */
>> > -  for (unsigned i = 0; i < targets.length (); i++)
>> > -    if (targets[i] != temporaries[i])
>> > -      noce_emit_move_insn (targets[i], temporaries[i]);
>> > +  for (unsigned i = 0; i < insn_info.length (); i++)
>> > +    if (insn_info[i]->target != insn_info[i]->temporary)
>> > +      noce_emit_move_insn (insn_info[i]->target, insn_info[i]->temporary);
>> >
>> >    /* Actually emit the sequence if it isn't too expensive.  */
>> >    rtx_insn *seq = get_insns ();
>> > @@ -3332,10 +3316,10 @@ noce_convert_multiple_sets (struct noce_if_info *if_info)
>> >      set_used_flags (insn);
>> >
>> >    /* Mark all our temporaries and targets as used.  */
>> > -  for (unsigned i = 0; i < targets.length (); i++)
>> > +  for (unsigned i = 0; i < insn_info.length (); i++)
>> >      {
>> > -      set_used_flags (temporaries[i]);
>> > -      set_used_flags (targets[i]);
>> > +      set_used_flags (insn_info[i]->temporary);
>> > +      set_used_flags (insn_info[i]->target);
>> >      }
>> >
>> >    set_used_flags (cond);
>> > @@ -3354,7 +3338,7 @@ noce_convert_multiple_sets (struct noce_if_info *if_info)
>> >        return false;
>> >
>> >    emit_insn_before_setloc (seq, if_info->jump,
>> > -                        INSN_LOCATION (unmodified_insns.last ()));
>> > +                        INSN_LOCATION (insn_info.last ()->unmodified_insn));
>> >
>> >    /* Clean up THEN_BB and the edges in and out of it.  */
>> >    remove_edge (find_edge (test_bb, join_bb));
>> > @@ -3389,21 +3373,13 @@ check_for_cc_cmp_clobbers (rtx dest, const_rtx, void *p0)
>> >      p[0] = NULL_RTX;
>> >  }
>> >
>> > -/* This goes through all relevant insns of IF_INFO->then_bb and tries to
>> > -   create conditional moves.  In case a simple move sufficis the insn
>> > -   should be listed in NEED_NO_CMOV.  The rewired-src cases should be
>> > -   specified via REWIRED_SRC.  TARGETS, TEMPORARIES and UNMODIFIED_INSNS
>> > -   are specified and used in noce_convert_multiple_sets and should be passed
>> > -   to this function..  */
>> > +/* This goes through all relevant insns of IF_INFO->then_bb and tries to create
>> > +   conditional moves.  Information for the insns is kept in INSN_INFO.  */
>> >
>> >  static bool
>> >  noce_convert_multiple_sets_1 (struct noce_if_info *if_info,
>> > -                           hash_set<rtx_insn *> *need_no_cmov,
>> > -                           hash_map<rtx_insn *, int> *rewired_src,
>> > -                           auto_vec<rtx> *targets,
>> > -                           auto_vec<rtx> *temporaries,
>> > -                           auto_vec<rtx_insn *> *unmodified_insns,
>> > -                           int *last_needs_comparison)
>> > +                     auto_delete_vec<noce_multiple_sets_info> &insn_info,
>> > +                     int *last_needs_comparison)
>> >  {
>> >    basic_block then_bb = if_info->then_bb;
>> >    rtx_insn *jump = if_info->jump;
>> > @@ -3421,11 +3397,6 @@ noce_convert_multiple_sets_1 (struct noce_if_info *if_info,
>> >
>> >    rtx_insn *insn;
>> >    int count = 0;
>> > -
>> > -  targets->truncate (0);
>> > -  temporaries->truncate (0);
>> > -  unmodified_insns->truncate (0);
>> > -
>> >    bool second_try = *last_needs_comparison != -1;
>> >
>> >    FOR_BB_INSNS (then_bb, insn)
>> > @@ -3434,6 +3405,8 @@ noce_convert_multiple_sets_1 (struct noce_if_info *if_info,
>> >        if (!active_insn_p (insn))
>> >       continue;
>> >
>> > +      noce_multiple_sets_info *info = insn_info[count];
>> > +
>> >        rtx set = single_set (insn);
>> >        gcc_checking_assert (set);
>> >
>> > @@ -3441,9 +3414,12 @@ noce_convert_multiple_sets_1 (struct noce_if_info *if_info,
>> >        rtx temp;
>> >
>> >        rtx new_val = SET_SRC (set);
>> > -      if (int *ii = rewired_src->get (insn))
>> > -     new_val = simplify_replace_rtx (new_val, (*targets)[*ii],
>> > -                                     (*temporaries)[*ii]);
>> > +
>> > +      int i, ii;
>> > +      FOR_EACH_VEC_ELT (info->rewired_src, i, ii)
>> > +     new_val = simplify_replace_rtx (new_val, insn_info[ii]->target,
>> > +                                     insn_info[ii]->temporary);
>> > +
>> >        rtx old_val = target;
>> >
>> >        /* As we are transforming
>> > @@ -3484,7 +3460,7 @@ noce_convert_multiple_sets_1 (struct noce_if_info *if_info,
>> >        /* We have identified swap-style idioms before.  A normal
>> >        set will need to be a cmov while the first instruction of a swap-style
>> >        idiom can be a regular move.  This helps with costing.  */
>> > -      bool need_cmov = !need_no_cmov->contains (insn);
>> > +      bool need_cmov = info->need_cmov;
>> >
>> >        /* If we had a non-canonical conditional jump (i.e. one where
>> >        the fallthrough is to the "else" case) we need to reverse
>> > @@ -3632,21 +3608,102 @@ noce_convert_multiple_sets_1 (struct noce_if_info *if_info,
>> >
>> >        /* Bookkeeping.  */
>> >        count++;
>> > -      targets->safe_push (target);
>> > -      temporaries->safe_push (temp_dest);
>> > -      unmodified_insns->safe_push (insn);
>> > +
>> > +      info->target = target;
>> > +      info->temporary = temp_dest;
>> > +      info->unmodified_insn = insn;
>> >      }
>> >
>> > +  /* The number of processed insns must match init_noce_multiple_sets_info.  */
>> > +  gcc_assert (count == (int) insn_info.length ());
>> > +
>> >    /* Even if we did not actually need the comparison, we want to make sure
>> >       to try a second time in order to get rid of the temporaries.  */
>> >    if (*last_needs_comparison == -1)
>> >      *last_needs_comparison = 0;
>> >
>> > -
>> >    return true;
>> >  }
>> >
>> > +/* Find local swap-style idioms in BB and mark the first insn (1)
>> > +   that is only a temporary as not needing a conditional move as
>> > +   it is going to be dead afterwards anyway.
>> > +
>> > +     (1) int tmp = a;
>> > +      a = b;
>> > +      b = tmp;
>> > +
>> > +      ifcvt
>> > +      -->
>> > +
>> > +      tmp = a;
>> > +      a = cond ? b : a_old;
>> > +      b = cond ? tmp : b_old;
>> > +
>> > +    Additionally, store the index of insns like (2) when a subsequent
>> > +    SET reads from their destination.
>> >
>> > +    (2) int c = a;
>> > +     int d = c;
>> > +
>> > +     ifcvt
>> > +     -->
>> > +
>> > +     c = cond ? a : c_old;
>> > +     d = cond ? d : c;     // Need to use c rather than c_old here.
>> > +*/
>> > +
>> > +static void
>> > +init_noce_multiple_sets_info (basic_block bb,
>> > +                  auto_delete_vec<noce_multiple_sets_info> &insn_info)
>> > +{
>> > +  rtx_insn *insn;
>> > +  int count = 0;
>> > +  auto_vec<rtx> dests;
>> > +
>> > +  /* Iterate over all SETs, storing the destinations
>> > +     in DEST.
>> > +     - If we hit a SET that reads from a destination
>> > +       that we have seen before and the corresponding register
>> > +       is dead afterwards, the register does not need to be
>> > +       moved conditionally.
>> > +     - If we encounter a previously changed register,
>> > +       rewire the read to the original source.  */
>> > +  FOR_BB_INSNS (bb, insn)
>> > +    {
>> > +      if (!active_insn_p (insn))
>> > +     continue;
>> > +
>> > +      noce_multiple_sets_info *info = new noce_multiple_sets_info;
>> > +      info->target = NULL_RTX;
>> > +      info->temporary = NULL_RTX;
>> > +      info->unmodified_insn = NULL;
>> > +      info->need_cmov = true;
>> > +      insn_info.safe_push (info);
>> > +
>> > +      rtx set = single_set (insn);
>> > +      gcc_checking_assert (set);
>> > +
>> > +      rtx src = SET_SRC (set);
>> > +      rtx dest = SET_DEST (set);
>> > +
>> > +      /* Check if the current SET's source is the same
>> > +      as any previously seen destination.
>> > +      This is quadratic but the number of insns in BB
>> > +      is bounded by PARAM_MAX_RTL_IF_CONVERSION_INSNS.  */
>> > +      for (int i = count - 1; i >= 0; --i)
>> > +     if (reg_mentioned_p (dests[i], src))
>> > +       {
>> > +         if (find_reg_note (insn, REG_DEAD, src) != NULL_RTX)
>>
>> This is pre-existing, but is a REG_DEAD note enough?  I think we also
>> need to check that the register isn't live on exit from the condition
>> block (or, alternatively, isn't live on entry to the join block).
>>
>> E.g. for:
>>
>>   if (cond)
>>     {
>>       tmp = a;
>>       a = b;
>>       b = tmp;
>>       tmp = ...;
>>     }
>>   ...tmp...;
>>
>> we need to preserve the original value of tmp when !cond, even though
>> tmp is temporarily dead in the "then" block.
>>
>> Thanks,
>> Richard
>>
> Good question. I played around with some tests and looked at the
> original code again and I believe it's fine.
> In your example, when insn is at `b = tmp;` then `tmp = a;` will be
> marked as need_no_cmov because tmp will have a REG_DEAD at insn. `tmp
> = ...;` will be emitted as a conditional set and the use outside the
> conditional will be just fine. I don't see any issue.

But I think we end up with:

  tmp = a;
  a = cond ? b : a;
  b = cond ? tmp : b;
  tmp = cond ? ... : tmp;

So the net effect for !cond will be to set tmp to a, rather than
preserve the original value of tmp.

Thanks,
Richard

>
> But it made me realize that my updated code doesn't really make sense
> on that part. Since I allow src to be an arbitrary expression and not
> a single reg, the code
>
>   if (find_reg_note (insn, REG_DEAD, src) != NULL_RTX)
>
> is nonsensical. Apart from the fact that the check is loop invariant
> it also doesn't properly handle src anyway. I'll have to think again
> about how to refactor this check now that src is not just a reg.
>
> Thanks,
> Manolis