Re: [PATCH 3/3] aarch64: Fix up debug uses in ldp/stp pass [PR113089]

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

Sorry for the earlier review comment about debug insns.  I hadn't
looked far enough into the queue to see this patch.

Alex Coplan <alex.coplan@arm.com> writes:
> As the PR shows, we were missing code to update debug uses in the
> load/store pair fusion pass.  This patch fixes that.
>
> Note that this patch depends on the following patch to create new uses
> in RTL-SSA, submitted as part of the fixes for PR113070:
> https://gcc.gnu.org/pipermail/gcc-patches/2024-January/642919.html
>
> The patch tries to give a complete treatment of the debug uses that will
> be affected by the changes we make, and in particular makes an effort to
> preserve debug info where possible, e.g. when re-ordering an update of
> a base register by a constant over a debug use of that register.  When
> re-ordering loads over a debug use of a transfer register, we reset the
> debug insn.  Likewise when re-ordering stores over debug uses of mem.
>
> While doing this I noticed that try_promote_writeback used a strange
> choice of move_range for the pair insn, in that it chose the previous
> nondebug insn instead of the insn itself.  Since the insn is being
> changed, these move ranges are equivalent (at least in terms of nondebug
> insn placement as far as RTL-SSA is concerned), but I think it is more
> natural to choose the pair insn itself.  This is needed to avoid
> incorrectly updating some debug uses.
>
> Notes on testing:
>  - The series was bootstrapped/regtested on top of the fixes for
>    PR113070 and PR113356.  It seemed to make more sense to test with
>    correct use/def info, and as mentioned above, this patch depends on
>    one of the PR113070 patches.
>  - I also ran the testsuite with -g -funroll-loops -mearly-ldp-fusion
>    -mlate-ldp-fusion to try and flush out more issues, and worked
>    through some examples where writeback updates were triggered to
>    make sure it was doing the right thing.
>  - The patches also survived an LTO+PGO bootstrap with
>    --enable-languages=all (with the passes enabled).
>
> Bootstrapped/regtested as a series on aarch64-linux-gnu (with/without
> the pass enabled).  OK for trunk?
>
> Thanks,
> Alex
>
> gcc/ChangeLog:
>
> 	PR target/113089
> 	* config/aarch64/aarch64-ldp-fusion.cc (reset_debug_use): New.
> 	(fixup_debug_use): New.
> 	(fixup_debug_uses_trailing_add): New.
> 	(fixup_debug_uses): New. Use it ...
> 	(ldp_bb_info::fuse_pair): ... here.
> 	(try_promote_writeback): Call fixup_debug_uses_trailing_add to
> 	fix up debug uses of the base register that are affected by
> 	folding in the trailing add insn.
>
> gcc/testsuite/ChangeLog:
>
> 	PR target/113089
> 	* gcc.c-torture/compile/pr113089.c: New test.
> ---
>  gcc/config/aarch64/aarch64-ldp-fusion.cc      | 332 +++++++++++++++++-
>  .../gcc.c-torture/compile/pr113089.c          |  26 ++
>  2 files changed, 351 insertions(+), 7 deletions(-)
>  create mode 100644 gcc/testsuite/gcc.c-torture/compile/pr113089.c
>
> diff --git a/gcc/config/aarch64/aarch64-ldp-fusion.cc b/gcc/config/aarch64/aarch64-ldp-fusion.cc
> index 4d7fd72c6b1..fd0278e7acf 100644
> --- a/gcc/config/aarch64/aarch64-ldp-fusion.cc
> +++ b/gcc/config/aarch64/aarch64-ldp-fusion.cc
> @@ -1342,6 +1342,309 @@ ldp_bb_info::track_tombstone (int uid)
>      gcc_unreachable (); // Bit should have changed.
>  }
>  
> +// Reset the debug insn containing USE (the debug insn has been
> +// optimized away).
> +static void
> +reset_debug_use (use_info *use)
> +{
> +  auto use_insn = use->insn ();
> +  auto use_rtl = use_insn->rtl ();
> +  insn_change change (use_insn);
> +  change.new_uses = {};
> +  INSN_VAR_LOCATION_LOC (use_rtl) = gen_rtx_UNKNOWN_VAR_LOC ();
> +  crtl->ssa->change_insn (change);
> +}
> +
> +// USE is a debug use that needs updating because DEF (a def of the same
> +// register) is being re-ordered over it.  If BASE is non-null, then DEF
> +// is an update of the register BASE by a constant, given by WB_OFFSET,
> +// and we can preserve debug info by accounting for the change in side
> +// effects.
> +static void
> +fixup_debug_use (obstack_watermark &attempt,
> +		 use_info *use,
> +		 def_info *def,
> +		 rtx base,
> +		 poly_int64 wb_offset)
> +{
> +  auto use_insn = use->insn ();
> +  if (base)
> +    {
> +      auto use_rtl = use_insn->rtl ();
> +      insn_change change (use_insn);
> +
> +      gcc_checking_assert (REG_P (base) && use->regno () == REGNO (base));
> +      change.new_uses = check_remove_regno_access (attempt,
> +						   change.new_uses,
> +						   use->regno ());
> +
> +      // The effect of the writeback is to add WB_OFFSET to BASE.  If
> +      // we're re-ordering DEF below USE, then we update USE by adding
> +      // WB_OFFSET to it.  Otherwise, if we're re-ordering DEF above
> +      // USE, we update USE by undoing the effect of the writeback
> +      // (subtracting WB_OFFSET).
> +      use_info *new_use;
> +      if (*def->insn () > *use_insn)
> +	{
> +	  // We now need USE_INSN to consume DEF.  Create a new use of DEF.
> +	  //
> +	  // N.B. this means until we call change_insns for the main change
> +	  // group we will temporarily have a debug use consuming a def that
> +	  // comes after it, but RTL-SSA doesn't currently support updating
> +	  // debug insns as part of the main change group (together with
> +	  // nondebug changes), so we will have to live with this update
> +	  // leaving the IR being temporarily inconsistent.  It seems to
> +	  // work out OK once the main change group is applied.
> +	  wb_offset *= -1;
> +	  new_use = crtl->ssa->create_use (attempt,
> +					   use_insn,
> +					   as_a<set_info *> (def));
> +	}
> +      else
> +	new_use = find_access (def->insn ()->uses (), use->regno ());
> +
> +      change.new_uses = insert_access (attempt, new_use, change.new_uses);
> +
> +      if (dump_file)
> +	{
> +	  const char *dir = (*def->insn () < *use_insn) ? "down" : "up";
> +	  pretty_printer pp;
> +	  pp_string (&pp, "[");
> +	  pp_access (&pp, use, 0);
> +	  pp_string (&pp, "]");
> +	  pp_string (&pp, " due to wb def ");
> +	  pp_string (&pp, "[");
> +	  pp_access (&pp, def, 0);
> +	  pp_string (&pp, "]");
> +	  fprintf (dump_file,
> +		   "  i%d: fix up debug use %s re-ordered %s, "
> +		   "sub r%u -> r%u + ",
> +		   use_insn->uid (), pp_formatted_text (&pp),
> +		   dir, REGNO (base), REGNO (base));
> +	  print_dec (wb_offset, dump_file);
> +	  fprintf (dump_file, "\n");
> +	}
> +
> +      insn_propagation prop (use_rtl, base,
> +			     plus_constant (GET_MODE (base), base, wb_offset));
> +      if (prop.apply_to_pattern (&INSN_VAR_LOCATION_LOC (use_rtl)))
> +	crtl->ssa->change_insn (change);
> +      else
> +	{
> +	  if (dump_file)
> +	    fprintf (dump_file, "  i%d: RTL substitution failed (%s)"
> +		     ", resetting debug insn", use_insn->uid (),
> +		     prop.failure_reason);
> +	  reset_debug_use (use);
> +	}
> +    }
> +  else
> +    {
> +      if (dump_file)
> +	{
> +	  pretty_printer pp;
> +	  pp_string (&pp, "[");
> +	  pp_access (&pp, use, 0);
> +	  pp_string (&pp, "] due to re-ordered load def [");
> +	  pp_access (&pp, def, 0);
> +	  pp_string (&pp, "]");
> +	  fprintf (dump_file, "  i%d: resetting debug use %s\n",
> +		   use_insn->uid (), pp_formatted_text (&pp));
> +	}
> +      reset_debug_use (use);
> +    }
> +}
> +
> +// Update debug uses when folding in a trailing add insn to form a
> +// writeback pair.
> +//
> +// ATTEMPT is used to allocate RTL-SSA temporaries for the changes,
> +// the final pair is placed immediately after PAIR_DST, TRAILING_ADD
> +// is a trailing add insn which is being folded into the pair to make it
> +// use writeback addressing, and WRITEBACK_EFFECT is the pattern for
> +// TRAILING_ADD.
> +static void
> +fixup_debug_uses_trailing_add (obstack_watermark &attempt,
> +			       insn_info *pair_dst,
> +			       insn_info *trailing_add,
> +			       rtx writeback_effect)
> +{
> +  rtx base = SET_DEST (writeback_effect);
> +
> +  poly_int64 wb_offset;
> +  rtx base2 = strip_offset (SET_SRC (writeback_effect), &wb_offset);
> +  gcc_checking_assert (rtx_equal_p (base, base2));
> +
> +  auto defs = trailing_add->defs ();
> +  gcc_checking_assert (defs.size () == 1);
> +  def_info *def = defs[0];
> +
> +  if (auto set = safe_dyn_cast<set_info *> (def->prev_def ()))
> +    for (auto use : set->debug_insn_uses ())
> +      if (*use->insn () > *pair_dst)
> +	// DEF is getting re-ordered above USE, fix up USE accordingly.
> +	fixup_debug_use (attempt, use, def, base, wb_offset);
> +}
> +
> +// Called from fuse_pair, fixes up any debug uses that will be affected
> +// by the changes.
> +//
> +// ATTEMPT is the obstack watermark used to allocate RTL-SSA temporaries for
> +// the changes, INSNS gives the candidate insns: at this point the use/def
> +// information should still be as on entry to fuse_pair, but the patterns may
> +// have changed, hence we pass ORIG_RTL which contains the original patterns
> +// for the candidate insns.
> +//
> +// The final pair will be placed immediately after PAIR_DST, LOAD_P is true if
> +// it is a load pair, bit I of WRITEBACK is set if INSNS[I] originally had
> +// writeback, and WRITEBACK_EFFECT is an rtx describing the overall update to
> +// the base register in the final pair (if any).  BASE_REGNO gives the register
> +// number of the base register used in the final pair.
> +static void
> +fixup_debug_uses (obstack_watermark &attempt,
> +		  insn_info *insns[2],
> +		  rtx orig_rtl[2],
> +		  insn_info *pair_dst,
> +		  insn_info *trailing_add,
> +		  bool load_p,
> +		  int writeback,
> +		  rtx writeback_effect,
> +		  unsigned base_regno)
> +{
> +  // USE is a debug use that needs updating because DEF (a def of the
> +  // resource) is being re-ordered over it.  If WRITEBACK_PAT is non-NULL,
> +  // then it gives the original RTL pattern for DEF's insn, and DEF is a
> +  // writeback update of the base register.
> +  //
> +  // This simply unpacks WRITEBACK_PAT if needed and calls fixup_debug_use.
> +  auto update_debug_use = [&](use_info *use, def_info *def,
> +			      rtx writeback_pat)
> +    {
> +      poly_int64 offset = 0;
> +      rtx base = NULL_RTX;
> +      if (writeback_pat)
> +	{
> +	  rtx mem = XEXP (writeback_pat, load_p);
> +	  gcc_checking_assert (GET_RTX_CLASS (GET_CODE (XEXP (mem, 0)))
> +			       == RTX_AUTOINC);
> +
> +	  base = ldp_strip_offset (mem, &offset);
> +	  gcc_checking_assert (REG_P (base) && REGNO (base) == base_regno);
> +	}
> +      fixup_debug_use (attempt, use, def, base, offset);
> +    };
> +
> +  // Reset any debug uses of mem over which we re-ordered a store.
> +  //
> +  // It would be nice to try and preserve debug info here, but it seems that
> +  // would require doing alias analysis to see if the store aliases with the
> +  // debug use, which seems a little extravagant just to preserve debug info.
> +  if (!load_p)
> +    {
> +      auto def = memory_access (insns[0]->defs ());
> +      auto last_def = memory_access (insns[1]->defs ());
> +      for (; def != last_def; def = def->next_def ())
> +	if (auto set = dyn_cast<set_info *> (def))

Similarly to the review of the other patch, I think we should use
an unconditional as_a here.

> +	  for (auto use : set->debug_insn_uses ())
> +	    {
> +	      if (dump_file)
> +		fprintf (dump_file, "  i%d: resetting debug use of mem\n",
> +			 use->insn ()->uid ());
> +	      reset_debug_use (use);
> +	    }
> +    }
> +
> +  // Now let's take care of register uses, starting with debug uses
> +  // attached to defs from our first insn.
> +  for (auto def : insns[0]->defs ())
> +    {
> +      auto set = dyn_cast<set_info *> (def);
> +      if (!set || set->is_mem () || !set->first_debug_insn_use ())
> +	continue;
> +
> +      def_info *defs[2] = {
> +	def,
> +	find_access (insns[1]->defs (), def->regno ())
> +      };
> +
> +      rtx writeback_pats[2] = {};
> +      if (def->regno () == base_regno)
> +	for (int i = 0; i < 2; i++)
> +	  if (defs[i] && (writeback & (1 << i)))

Are both checks needed?  If so, what does it mean for the writeback
bit to be set but defs[i] to be null?  Could we assert the defs[i]
part instead?

> +	    writeback_pats[i] = orig_rtl[i];
> +
> +      // Now that we've characterized the defs involved, go through the
> +      // debug uses and determine how to update them (if needed).
> +      for (auto use : set->debug_insn_uses ())
> +	{
> +	  if (*pair_dst < *use->insn () && defs[1])
> +	    // We're re-ordering defs[1] above a previous use of the
> +	    // same resource.
> +	    update_debug_use (use, defs[1], writeback_pats[1]);
> +	  else if (*pair_dst >= *use->insn ())
> +	    // We're re-ordering defs[0] below its use.
> +	    update_debug_use (use, defs[0], writeback_pats[0]);
> +	}
> +    }
> +
> +  // Now let's look at registers which are def'd by the second insn
> +  // but not by the first insn, there may still be debug uses of a
> +  // previous def which can be affected by moving the second insn up.
> +  for (auto def : insns[1]->defs ())
> +    {
> +      // This should be M log N where N is the number of defs in
> +      // insns[0] and M is the number of defs in insns[1].
> +      if (def->is_mem () || find_access (insns[0]->defs (), def->regno ()))
> +	  continue;
> +
> +      auto prev_set = safe_dyn_cast<set_info *> (def->prev_def ());
> +      if (!prev_set)
> +	continue;
> +
> +      rtx writeback_pat = NULL_RTX;
> +      if (def->regno () == base_regno && (writeback & 2))
> +	writeback_pat = orig_rtl[1];
> +
> +      // We have a def in insns[1] which isn't def'd by the first insn.
> +      // Look to the previous def and see if it has any debug uses.
> +      for (auto use : prev_set->debug_insn_uses ())
> +	if (*pair_dst < *use->insn ())
> +	  // We're ordering DEF above a previous use of the same register.
> +	  update_debug_use (use, def, writeback_pat);

This might be more efficient as a reverse walk that breaks as soon as
*pair_dst >= *use->insn (), since the previous def could be arbitrarily
far away and have arbitrarily many leading uses.  But it probably doesn't
matter much in practice.

> +    }
> +
> +  if ((writeback & 2) && !writeback_effect)
> +    {
> +      // If the second insn initially had writeback but the final
> +      // pair does not, then there may be trailing debug uses of the
> +      // second writeback def which need re-parenting: do that.
> +      auto def = find_access (insns[1]->defs (), base_regno);
> +      gcc_assert (def);
> +      for (auto use : as_a<set_info *> (def)->debug_insn_uses ())
> +	{
> +	  insn_change change (use->insn ());
> +	  change.new_uses = check_remove_regno_access (attempt,
> +						       change.new_uses,
> +						       base_regno);
> +	  auto new_use = find_access (insns[0]->uses (), base_regno);
> +
> +	  // N.B. insns must have already shared a common base due to writeback.
> +	  gcc_assert (new_use);
> +
> +	  if (dump_file)
> +	    fprintf (dump_file,
> +		     "  i%d: cancelling wb, re-parenting trailing debug use\n",
> +		     use->insn ()->uid ());
> +
> +	  change.new_uses = insert_access (attempt, new_use, change.new_uses);
> +	  crtl->ssa->change_insn (change);
> +	}
> +    }
> +  else if (trailing_add)
> +    fixup_debug_uses_trailing_add (attempt, pair_dst, trailing_add,
> +				   writeback_effect);
> +}
> +
>  // Try and actually fuse the pair given by insns I1 and I2.
>  //
>  // Here we've done enough analysis to know this is safe, we only
> @@ -1378,6 +1681,9 @@ ldp_bb_info::fuse_pair (bool load_p,
>    insn_info *first = (*i1 < *i2) ? i1 : i2;
>    insn_info *second = (first == i1) ? i2 : i1;
>  
> +  insn_info *pair_dst = move_range.singleton ();
> +  gcc_assert (pair_dst);
> +
>    insn_info *insns[2] = { first, second };
>  
>    auto_vec<insn_change *> changes;
> @@ -1388,6 +1694,13 @@ ldp_bb_info::fuse_pair (bool load_p,
>      PATTERN (second->rtl ())
>    };
>  
> +  // Make copies of the patterns as we might need to refer to the original RTL
> +  // later, for example when updating debug uses (which is after we've updated
> +  // one or both of the patterns in the candidate insns).
> +  rtx orig_rtl[2];
> +  for (int i = 0; i < 2; i++)
> +    orig_rtl[i] = copy_rtx (pats[i]);
> +

FWIW, an alternative (that avoids RTL allocation) would be to use
temporarily_undo_changes and redo_changes.  But this is fine too.

The patch is OK from my POV with the as_a change above, but I'd be
interested in the answer to the writeback question above.

Thanks,
Richard

>    use_array input_uses[2] = { first->uses (), second->uses () };
>    def_array input_defs[2] = { first->defs (), second->defs () };
>  
> @@ -1604,9 +1917,7 @@ ldp_bb_info::fuse_pair (bool load_p,
>        using Action = stp_change_builder::action;
>        insn_info *store_to_change = try_repurpose_store (first, second,
>  							move_range);
> -      insn_info *stp_dest = move_range.singleton ();
> -      gcc_assert (stp_dest);
> -      stp_change_builder builder (insns, store_to_change, stp_dest);
> +      stp_change_builder builder (insns, store_to_change, pair_dst);
>        insn_change *change;
>        set_info *new_set = nullptr;
>        for (; !builder.done (); builder.advance ())
> @@ -1677,7 +1988,7 @@ ldp_bb_info::fuse_pair (bool load_p,
>  		fprintf (dump_file,
>  			 "  stp: changing i%d to use mem from new stp "
>  			 "(after i%d)\n",
> -			 action.insn->uid (), stp_dest->uid ());
> +			 action.insn->uid (), pair_dst->uid ());
>  	      change->new_uses = drop_memory_access (change->new_uses);
>  	      gcc_assert (new_set);
>  	      auto new_use = crtl->ssa->create_use (attempt, action.insn,
> @@ -1741,6 +2052,11 @@ ldp_bb_info::fuse_pair (bool load_p,
>  
>    gcc_assert (crtl->ssa->verify_insn_changes (changes));
>  
> +  // Fix up any debug uses that will be affected by the changes.
> +  if (MAY_HAVE_DEBUG_INSNS)
> +    fixup_debug_uses (attempt, insns, orig_rtl, pair_dst, trailing_add,
> +		      load_p, writeback, writeback_effect, base_regno);
> +
>    confirm_change_group ();
>    crtl->ssa->change_insns (changes);
>  
> @@ -2807,7 +3123,7 @@ try_promote_writeback (insn_info *insn)
>  
>    rtx wb_effect = NULL_RTX;
>    def_info *add_def;
> -  const insn_range_info pair_range (insn->prev_nondebug_insn ());
> +  const insn_range_info pair_range (insn);
>    insn_info *insns[2] = { nullptr, insn };
>    insn_info *trailing_add = find_trailing_add (insns, pair_range, 0, &wb_effect,
>  					       &add_def, base_def, offset,
> @@ -2830,14 +3146,16 @@ try_promote_writeback (insn_info *insn)
>  					pair_change.new_defs);
>    gcc_assert (pair_change.new_defs.is_valid ());
>  
> -  pair_change.move_range = insn_range_info (insn->prev_nondebug_insn ());
> -
>    auto is_changing = insn_is_changing (changes);
>    for (unsigned i = 0; i < ARRAY_SIZE (changes); i++)
>      gcc_assert (rtl_ssa::restrict_movement_ignoring (*changes[i], is_changing));
>  
>    gcc_assert (rtl_ssa::recog_ignoring (attempt, pair_change, is_changing));
>    gcc_assert (crtl->ssa->verify_insn_changes (changes));
> +
> +  if (MAY_HAVE_DEBUG_INSNS)
> +    fixup_debug_uses_trailing_add (attempt, insn, trailing_add, wb_effect);
> +
>    confirm_change_group ();
>    crtl->ssa->change_insns (changes);
>  }
> diff --git a/gcc/testsuite/gcc.c-torture/compile/pr113089.c b/gcc/testsuite/gcc.c-torture/compile/pr113089.c
> new file mode 100644
> index 00000000000..70c71f23f1c
> --- /dev/null
> +++ b/gcc/testsuite/gcc.c-torture/compile/pr113089.c
> @@ -0,0 +1,26 @@
> +/* { dg-do compile } */
> +/* { dg-options "-g -funroll-loops" } */
> +
> +typedef unsigned short uint16;
> +
> +void intrapred_chroma_plane(uint16 ***mb_preds, int* max_imgpel_values, int crx, int cry, int px) {
> +  for (int uv = 0; uv < 2; uv++) {
> +    uint16 **mb_pred = mb_preds[uv + 1];
> +    uint16 **predU2 = &mb_pred[px - 2];
> +    uint16 *upPred = &mb_pred[px][px];
> +    int max_imgpel_value = max_imgpel_values[uv];
> +
> +    int ih = upPred[crx - 1];
> +    for (int i = 0; i < crx*3; ++i)
> +      ih += upPred[crx*3];
> +
> +    int iv = (mb_pred[cry - 1][px+1]);
> +    for (int i = 0; i < cry - 1; ++i) {
> +      iv += (i + 1) * (*(mb_preds[uv][0]) - *(*predU2--));
> +    }
> +
> +    for (int j = 0; j < cry; ++j)
> +      for (int i = 0; i < crx; ++i)
> +        mb_pred[j][i] = (uint16) (max_imgpel_value * ((i * ih + iv)));
> +  }
> +}