[PATCH 4/4] aarch64: Fix up uses of mem following stp insert [PR113070]

[Alex Coplan <alex.coplan@arm.com>]

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As the PR shows (specifically #c7) we are missing updating uses of mem
when inserting an stp in the aarch64 load/store pair fusion pass.  This
patch fixes that.

RTL-SSA has a simple view of memory and by default doesn't allow stores
to be re-ordered w.r.t. other stores.  In the ldp fusion pass, we do our
own alias analysis and so can re-order stores over other accesses when
we deem this is safe.  If neither store can be re-purposed (moved into
the required position to form the stp while respecting the RTL-SSA
constraints), then we turn both the candidate stores into "tombstone"
insns (logically delete them) and insert a new stp insn.

As it stands, we implement the insert case separately (after dealing
with the candidate stores) in fuse_pair by inserting into the middle of
the vector of changes.  This is OK when we only have to insert one
change, but with this fix we would need to insert the change for the new
stp plus multiple changes to fix up uses of mem (note the number of
fix-ups is naturally bounded by the alias limit param to prevent
quadratic behaviour).  If we kept the code structured as is and inserted
into the middle of the vector, that would lead to repeated moving of
elements in the vector which seems inefficient.  The structure of the
code would also be a little unwieldy.

To improve on that situation, this patch introduces a helper class,
stp_change_builder, which implements a state machine that helps to build
the required changes directly in program order.  That state machine is
reponsible for deciding what changes need to be made in what order, and
the code in fuse_pair then simply follows those steps.

Together with the fix in the previous patch for installing new defs
correctly in RTL-SSA, this fixes PR113070.

We take the opportunity to rename the function decide_stp_strategy to
try_repurpose_store, as that seems more descriptive of what it actually
does, since stp_change_builder is now responsible for the overall change
strategy.

Bootstrapped/regtested as a series with/without the passes enabled on
aarch64-linux-gnu, OK for trunk?

Thanks,
Alex

gcc/ChangeLog:

	PR target/113070
	* config/aarch64/aarch64-ldp-fusion.cc (struct stp_change_builder): New.
	(decide_stp_strategy): Reanme to ...
	(try_repurpose_store): ... this.
	(ldp_bb_info::fuse_pair): Refactor to use stp_change_builder to
	construct stp changes.  Fix up uses when inserting new stp insns.
---
 gcc/config/aarch64/aarch64-ldp-fusion.cc | 248 ++++++++++++++++++-----
 1 file changed, 194 insertions(+), 54 deletions(-)


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diff --git a/gcc/config/aarch64/aarch64-ldp-fusion.cc b/gcc/config/aarch64/aarch64-ldp-fusion.cc
index 689a8c884bd..703cfb1228c 100644
--- a/gcc/config/aarch64/aarch64-ldp-fusion.cc
+++ b/gcc/config/aarch64/aarch64-ldp-fusion.cc
@@ -844,11 +844,138 @@ def_upwards_move_range (def_info *def)
   return range;
 }
 
+// Class that implements a state machine for building the changes needed to form
+// a store pair instruction.  This allows us to easily build the changes in
+// program order, as required by rtl-ssa.
+struct stp_change_builder
+{
+  enum class state
+  {
+    FIRST,
+    INSERT,
+    FIXUP_USE,
+    LAST,
+    DONE
+  };
+
+  enum class action
+  {
+    TOMBSTONE,
+    CHANGE,
+    INSERT,
+    FIXUP_USE
+  };
+
+  struct change
+  {
+    action type;
+    insn_info *insn;
+  };
+
+  bool done () const { return m_state == state::DONE; }
+
+  stp_change_builder (insn_info *insns[2],
+		      insn_info *repurpose,
+		      insn_info *dest)
+    : m_state (state::FIRST), m_insns { insns[0], insns[1] },
+      m_repurpose (repurpose), m_dest (dest), m_use (nullptr) {}
+
+  change get_change () const
+  {
+    switch (m_state)
+      {
+      case state::FIRST:
+	return {
+	  m_insns[0] == m_repurpose ? action::CHANGE : action::TOMBSTONE,
+	  m_insns[0]
+	};
+      case state::LAST:
+	return {
+	  m_insns[1] == m_repurpose ? action::CHANGE : action::TOMBSTONE,
+	  m_insns[1]
+	};
+      case state::INSERT:
+	return { action::INSERT, m_dest };
+      case state::FIXUP_USE:
+	return { action::FIXUP_USE, m_use->insn () };
+      case state::DONE:
+	break;
+      }
+
+    gcc_unreachable ();
+  }
+
+  // Transition to the next state.
+  void advance ()
+  {
+    switch (m_state)
+      {
+      case state::FIRST:
+	if (m_repurpose)
+	  m_state = state::LAST;
+	else
+	  m_state = state::INSERT;
+	break;
+      case state::INSERT:
+      {
+	def_info *def = memory_access (m_insns[0]->defs ());
+	while (*def->next_def ()->insn () <= *m_dest)
+	  def = def->next_def ();
+
+	// Now we know DEF feeds the insertion point for the new stp.
+	// Look for any uses of DEF that will consume the new stp.
+	gcc_assert (*def->insn () <= *m_dest
+		    && *def->next_def ()->insn () > *m_dest);
+
+	if (auto set = dyn_cast<set_info *> (def))
+	  for (auto use : set->nondebug_insn_uses ())
+	    if (*use->insn () > *m_dest)
+	      {
+		m_use = use;
+		break;
+	      }
+
+	if (m_use)
+	  m_state = state::FIXUP_USE;
+	else
+	  m_state = state::LAST;
+	break;
+      }
+      case state::FIXUP_USE:
+	m_use = m_use->next_nondebug_insn_use ();
+	if (!m_use)
+	  m_state = state::LAST;
+	break;
+      case state::LAST:
+	m_state = state::DONE;
+	break;
+      case state::DONE:
+	gcc_unreachable ();
+      }
+  }
+
+private:
+  state m_state;
+
+  // Original candidate stores.
+  insn_info *m_insns[2];
+
+  // If non-null, this is a candidate insn to change into an stp.  Otherwise we
+  // are deleting both original insns and inserting a new insn for the stp.
+  insn_info *m_repurpose;
+
+  // Destionation of the stp, it will be placed immediately after m_dest.
+  insn_info *m_dest;
+
+  // Current nondebug use that needs updating due to stp insertion.
+  use_info *m_use;
+};
+
 // Given candidate store insns FIRST and SECOND, see if we can re-purpose one
 // of them (together with its def of memory) for the stp insn.  If so, return
 // that insn.  Otherwise, return null.
 static insn_info *
-decide_stp_strategy (insn_info *first,
+try_repurpose_store (insn_info *first,
 		     insn_info *second,
 		     const insn_range_info &move_range)
 {
@@ -1253,7 +1380,7 @@ ldp_bb_info::fuse_pair (bool load_p,
 
   insn_info *insns[2] = { first, second };
 
-  auto_vec<insn_change *, 4> changes (4);
+  auto_vec<insn_change *> changes;
   auto_vec<int, 2> tombstone_uids (2);
 
   rtx pats[2] = {
@@ -1455,9 +1582,9 @@ ldp_bb_info::fuse_pair (bool load_p,
 
   if (load_p)
     {
-      changes.quick_push (make_delete (first));
+      changes.safe_push (make_delete (first));
       pair_change = make_change (second);
-      changes.quick_push (pair_change);
+      changes.safe_push (pair_change);
 
       pair_change->move_range = move_range;
       pair_change->new_defs = merge_access_arrays (attempt,
@@ -1474,18 +1601,22 @@ ldp_bb_info::fuse_pair (bool load_p,
     }
   else
     {
-      insn_info *store_to_change = decide_stp_strategy (first, second,
+      using Action = stp_change_builder::action;
+      insn_info *store_to_change = try_repurpose_store (first, second,
 							move_range);
-
-      if (store_to_change && dump_file)
-	fprintf (dump_file, "  stp: re-purposing store %d\n",
-		 store_to_change->uid ());
-
+      insn_info *stp_dest = move_range.singleton ();
+      gcc_assert (stp_dest);
+      stp_change_builder builder (insns, store_to_change, stp_dest);
       insn_change *change;
-      for (int i = 0; i < 2; i++)
+      set_info *new_set = nullptr;
+      for (; !builder.done (); builder.advance ())
 	{
-	  change = make_change (insns[i]);
-	  if (insns[i] == store_to_change)
+	  auto action = builder.get_change ();
+	  change = (action.type == Action::INSERT)
+	    ? nullptr : make_change (action.insn);
+	  switch (action.type)
+	    {
+	    case Action::CHANGE:
 	    {
 	      set_pair_pat (change);
 	      change->new_uses = merge_access_arrays (attempt,
@@ -1495,67 +1626,76 @@ ldp_bb_info::fuse_pair (bool load_p,
 	      auto d2 = drop_memory_access (input_defs[1]);
 	      change->new_defs = merge_access_arrays (attempt, d1, d2);
 	      gcc_assert (change->new_defs.is_valid ());
-	      def_info *stp_def = memory_access (store_to_change->defs ());
+	      def_info *stp_def = memory_access (change->insn ()->defs ());
 	      change->new_defs = insert_access (attempt,
 						stp_def,
 						change->new_defs);
 	      gcc_assert (change->new_defs.is_valid ());
 	      change->move_range = move_range;
 	      pair_change = change;
+	      break;
 	    }
-	  else
+	    case Action::TOMBSTONE:
 	    {
-	      // Note that we are turning this insn into a tombstone,
-	      // we need to keep track of these if we go ahead with the
-	      // change.
-	      tombstone_uids.quick_push (insns[i]->uid ());
-	      rtx_insn *rti = insns[i]->rtl ();
+	      tombstone_uids.quick_push (change->insn ()->uid ());
+	      rtx_insn *rti = change->insn ()->rtl ();
 	      validate_change (rti, &PATTERN (rti), gen_tombstone (), true);
 	      validate_change (rti, &REG_NOTES (rti), NULL_RTX, true);
 	      change->new_uses = use_array (nullptr, 0);
+	      break;
 	    }
-	  gcc_assert (change->new_uses.is_valid ());
-	  changes.quick_push (change);
-	}
+	    case Action::INSERT:
+	    {
+	      if (dump_file)
+		fprintf (dump_file,
+			 "  stp: cannot re-purpose candidate stores\n");
 
-      if (!store_to_change)
-	{
-	  // Tricky case.  Cannot re-purpose existing insns for stp.
-	  // Need to insert new insn.
-	  if (dump_file)
-	    fprintf (dump_file,
-		     "  stp fusion: cannot re-purpose candidate stores\n");
-
-	  auto new_insn = crtl->ssa->create_insn (attempt, INSN, pair_pat);
-	  change = make_change (new_insn);
-	  change->move_range = move_range;
-	  change->new_uses = merge_access_arrays (attempt,
-						  input_uses[0],
-						  input_uses[1]);
-	  gcc_assert (change->new_uses.is_valid ());
-
-	  auto d1 = drop_memory_access (input_defs[0]);
-	  auto d2 = drop_memory_access (input_defs[1]);
-	  change->new_defs = merge_access_arrays (attempt, d1, d2);
-	  gcc_assert (change->new_defs.is_valid ());
-
-	  auto new_set = crtl->ssa->create_set (attempt, new_insn, memory);
-	  change->new_defs = insert_access (attempt, new_set,
-					    change->new_defs);
-	  gcc_assert (change->new_defs.is_valid ());
-	  changes.safe_insert (1, change);
-	  pair_change = change;
+	      auto new_insn = crtl->ssa->create_insn (attempt, INSN, pair_pat);
+	      change = make_change (new_insn);
+	      change->move_range = move_range;
+	      change->new_uses = merge_access_arrays (attempt,
+						      input_uses[0],
+						      input_uses[1]);
+	      gcc_assert (change->new_uses.is_valid ());
+
+	      auto d1 = drop_memory_access (input_defs[0]);
+	      auto d2 = drop_memory_access (input_defs[1]);
+	      change->new_defs = merge_access_arrays (attempt, d1, d2);
+	      gcc_assert (change->new_defs.is_valid ());
+
+	      new_set = crtl->ssa->create_set (attempt, new_insn, memory);
+	      change->new_defs = insert_access (attempt, new_set,
+						change->new_defs);
+	      gcc_assert (change->new_defs.is_valid ());
+	      pair_change = change;
+	      break;
+	    }
+	    case Action::FIXUP_USE:
+	    {
+	      // This use now needs to consume memory from our stp.
+	      if (dump_file)
+		fprintf (dump_file,
+			 "  stp: changing i%d to use mem from new stp "
+			 "(after i%d)\n",
+			 action.insn->uid (), stp_dest->uid ());
+	      change->new_uses = drop_memory_access (change->new_uses);
+	      gcc_assert (new_set);
+	      auto new_use = crtl->ssa->create_use (attempt, action.insn,
+						    new_set);
+	      change->new_uses = insert_access (attempt, new_use,
+						change->new_uses);
+	      break;
+	    }
+	    }
+	  changes.safe_push (change);
 	}
     }
 
   if (trailing_add)
     changes.quick_push (make_delete (trailing_add));
 
-  auto n_changes = changes.length ();
-  gcc_checking_assert (n_changes >= 2 && n_changes <= 4);
-
   auto is_changing = insn_is_changing (changes);
-  for (unsigned i = 0; i < n_changes; i++)
+  for (unsigned i = 0; i < changes.length (); i++)
     gcc_assert (rtl_ssa::restrict_movement_ignoring (*changes[i], is_changing));
 
   // Check the pair pattern is recog'd.