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

[Manolis Tsamis <manolis.tsamis@vrull.eu>]

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:
>
> >
> > 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.
>

Something else that I noticed about the current code is that the condition below

             if (find_reg_note (insn, REG_DEAD, src) != NULL_RTX)
               need_no_cmov->add (insns[i]);
             else
               rewired_src->put (insn, i);

considers rewiring and no_cmov exclusive, but they are not since the
same register can appear in SRC and as a REG_DEAD note in a single
instruction.
For example if we had something like

 (insn 1 (set (reg:SI 107) (reg:SI 105)))
 (insn 2 (set (reg:SI 102) (reg:SI 107))
    (expr_list:REG_DEAD (reg:SI 107))

Then the existing code would not rewrite insn 2 because reg 107 is
also dead at insn 2.

But although we have identified these issues I haven't been able to
trigger them in either x86 or aarch64.
One reason looks to be that although there is coded support for SUBREG
in noce_convert_multiple_sets, which would make it possible to write
some more complicated testcases, it does not trigger.
On aarch64 (and (reg X) (const_int 0xfff...)) is used and on x86 the
SUBREG is nested within zero_extend or sign_extend so SUBREG_P is
false in both cases. So we're left with only simple register moves
which look to work fine (there may be related issues in other targets
though).

Manolis

> Thanks,
> Richard
>
>
> > +           insn_info[i]->need_cmov = false;
> > +         else
> > +           insn_info[count]->rewired_src.safe_push (i);
> > +       }
> > +
> > +      dests.safe_push (dest);
> > +      count++;
> > +    }
> > +}
> >
> >  /* Return true iff basic block TEST_BB is comprised of only
> >     (SET (REG) (REG)) insns suitable for conversion to a series
> > @@ -4121,89 +4178,6 @@ check_cond_move_block (basic_block bb,
> >    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
> > -need_cmov_or_rewire (basic_block bb,
> > -                  hash_set<rtx_insn *> *need_no_cmov,
> > -                  hash_map<rtx_insn *, int> *rewired_src)
> > -{
> > -  rtx_insn *insn;
> > -  int count = 0;
> > -  auto_vec<rtx_insn *> insns;
> > -  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)
> > -    {
> > -      rtx set, src, dest;
> > -
> > -      if (!active_insn_p (insn))
> > -     continue;
> > -
> > -      set = single_set (insn);
> > -      if (set == NULL_RTX)
> > -     continue;
> > -
> > -      src = SET_SRC (set);
> > -      if (SUBREG_P (src))
> > -     src = SUBREG_REG (src);
> > -      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.  */
> > -      if (REG_P (src))
> > -     for (int i = count - 1; i >= 0; --i)
> > -       if (reg_overlap_mentioned_p (src, dests[i]))
> > -         {
> > -           if (find_reg_note (insn, REG_DEAD, src) != NULL_RTX)
> > -             need_no_cmov->add (insns[i]);
> > -           else
> > -             rewired_src->put (insn, i);
> > -         }
> > -
> > -      insns.safe_push (insn);
> > -      dests.safe_push (dest);
> > -
> > -      count++;
> > -    }
> > -}
> > -
> >  /* Given a basic block BB suitable for conditional move conversion,
> >     a condition COND, and pointer maps THEN_VALS and ELSE_VALS containing
> >     the register values depending on COND, emit the insns in the block as
> > diff --git a/gcc/ifcvt.h b/gcc/ifcvt.h
> > index be1385aabe4..83420fe1207 100644
> > --- a/gcc/ifcvt.h
> > +++ b/gcc/ifcvt.h
> > @@ -40,6 +40,22 @@ struct ce_if_block
> >    int pass;                          /* Pass number.  */
> >  };
> >
> > +struct noce_multiple_sets_info
> > +{
> > +  /* A list of indices to instructions that we need to rewire into this
> > +     instruction when we replace them with temporary conditional moves.  */
> > +  auto_vec<int> rewired_src;
> > +  /* The true targets for a conditional move.  */
> > +  rtx target;
> > +  /* The temporary introduced for this conditional move.  */
> > +  rtx temporary;
> > +  /* The original instruction that we're replacing.  */
> > +  rtx_insn *unmodified_insn;
> > +  /* True if a conditional move is needed.
> > +     False if a simple move can be used instead.  */
> > +  bool need_cmov;
> > +};
> > +
> >  /* Used by noce_process_if_block to communicate with its subroutines.
> >
> >     The subroutines know that A and B may be evaluated freely.  They