stabilize store merging

Don't let pointer randomization change the order in which we process
store chains.  This may cause SSA_NAMEs to be released in different
order, and if they're reused later, they may cause differences in SSA
partitioning, leading to differences in expand, and ultimately to
different code.

bootstrap-debug-lean (-fcompare-debug) on i686-linux-gnu has failed in
haifa-sched.c since r245196 exposed the latent ordering problem in
store merging.  In this case, the IR differences (different SSA names
selected for copies in out-of-SSA, resulting in some off-by-one
differences in pseudos) were not significant enough to be visible in
the compiler output.


for  gcc/ChangeLog

	* gimple-ssa-store-merging.c (struct imm_store_chain_info):
	Add linked-list forward and backlinks.  Insert on
	construction, remove on destruction.
	(class pass_store_merging): Add m_stores_head field.
	(pass_store_merging::terminate_and_process_all_chains):
	Iterate over m_stores_head list.
	(pass_store_merging::terminate_all_aliasing_chains):
	Likewise.
	(pass_store_merging::execute): Check for debug stmts first.
	Push new chains onto the m_stores_head stack.

From-SVN: r246213

gcc/ChangeLog

+2017-03-16  Alexandre Oliva <aoliva@redhat.com>
+
+	* gimple-ssa-store-merging.c (struct imm_store_chain_info):
+	Add linked-list forward and backlinks.  Insert on
+	construction, remove on destruction.
+	(class pass_store_merging): Add m_stores_head field.
+	(pass_store_merging::terminate_and_process_all_chains):
+	Iterate over m_stores_head list.
+	(pass_store_merging::terminate_all_aliasing_chains):
+	Likewise.
+	(pass_store_merging::execute): Check for debug stmts first.
+	Push new chains onto the m_stores_head stack.
+
 2017-03-16  Michael Meissner  <meissner@linux.vnet.ibm.com>
 
 	PR target/71294

gcc/gimple-ssa-store-merging.c

@@ -675,11 +675,34 @@ merged_store_group::apply_stores ()
 
 struct imm_store_chain_info
 {
+  /* Doubly-linked list that imposes an order on chain processing.
+     PNXP (prev's next pointer) points to the head of a list, or to
+     the next field in the previous chain in the list.
+     See pass_store_merging::m_stores_head for more rationale.  */
+  imm_store_chain_info *next, **pnxp;
   tree base_addr;
   auto_vec<struct store_immediate_info *> m_store_info;
   auto_vec<merged_store_group *> m_merged_store_groups;
 
-  imm_store_chain_info (tree b_a) : base_addr (b_a) {}
+  imm_store_chain_info (imm_store_chain_info *&inspt, tree b_a)
+  : next (inspt), pnxp (&inspt), base_addr (b_a)
+  {
+    inspt = this;
+    if (next)
+      {
+	gcc_checking_assert (pnxp == next->pnxp);
+	next->pnxp = &next;
+      }
+  }
+  ~imm_store_chain_info ()
+  {
+    *pnxp = next;
+    if (next)
+      {
+	gcc_checking_assert (&next == next->pnxp);
+	next->pnxp = pnxp;
+      }
+  }
   bool terminate_and_process_chain ();
   bool coalesce_immediate_stores ();
   bool output_merged_store (merged_store_group *);
@@ -718,6 +741,17 @@ public:
 private:
   hash_map<tree_operand_hash, struct imm_store_chain_info *> m_stores;
 
+  /* Form a doubly-linked stack of the elements of m_stores, so that
+     we can iterate over them in a predictable way.  Using this order
+     avoids extraneous differences in the compiler output just because
+     of tree pointer variations (e.g. different chains end up in
+     different positions of m_stores, so they are handled in different
+     orders, so they allocate or release SSA names in different
+     orders, and when they get reused, subsequent passes end up
+     getting different SSA names, which may ultimately change
+     decisions when going out of SSA).  */
+  imm_store_chain_info *m_stores_head;
+
   bool terminate_and_process_all_chains ();
   bool terminate_all_aliasing_chains (imm_store_chain_info **,
 				      bool, gimple *);
@@ -730,11 +764,11 @@ private:
 bool
 pass_store_merging::terminate_and_process_all_chains ()
 {
-  hash_map<tree_operand_hash, struct imm_store_chain_info *>::iterator iter
-    = m_stores.begin ();
   bool ret = false;
-  for (; iter != m_stores.end (); ++iter)
-    ret |= terminate_and_release_chain ((*iter).second);
+  while (m_stores_head)
+    ret |= terminate_and_release_chain (m_stores_head);
+  gcc_assert (m_stores.elements () == 0);
+  gcc_assert (m_stores_head == NULL);
 
   return ret;
 }
@@ -799,15 +833,14 @@ pass_store_merging::terminate_all_aliasing_chains (imm_store_chain_info
 	}
     }
 
-  hash_map<tree_operand_hash, struct imm_store_chain_info *>::iterator iter
-    = m_stores.begin ();
-
   /* Check for aliasing with all other store chains.  */
-  for (; iter != m_stores.end (); ++iter)
+  for (imm_store_chain_info *next = m_stores_head, *cur = next; cur; cur = next)
     {
+      next = cur->next;
+
       /* We already checked all the stores in chain_info and terminated the
 	 chain if necessary.  Skip it here.  */
-      if (chain_info && (*chain_info) == (*iter).second)
+      if (chain_info && (*chain_info) == cur)
 	continue;
 
       /* We can't use the base object here as that does not reliably exist.
@@ -815,11 +848,11 @@ pass_store_merging::terminate_all_aliasing_chains (imm_store_chain_info
 	 minimum and maximum offset and the maximum size we could improve
 	 things here).  */
       ao_ref chain_ref;
-      ao_ref_init_from_ptr_and_size (&chain_ref, (*iter).first, NULL_TREE);
+      ao_ref_init_from_ptr_and_size (&chain_ref, cur->base_addr, NULL_TREE);
       if (ref_maybe_used_by_stmt_p (stmt, &chain_ref)
 	  || stmt_may_clobber_ref_p_1 (stmt, &chain_ref))
 	{
-	  terminate_and_release_chain ((*iter).second);
+	  terminate_and_release_chain (cur);
 	  ret = true;
 	}
     }
@@ -1336,6 +1369,9 @@ pass_store_merging::execute (function *fun)
 	{
 	  gimple *stmt = gsi_stmt (gsi);
 
+	  if (is_gimple_debug (stmt))
+	    continue;
+
 	  if (gimple_has_volatile_ops (stmt))
 	    {
 	      /* Terminate all chains.  */
@@ -1346,9 +1382,6 @@ pass_store_merging::execute (function *fun)
 	      continue;
 	    }
 
-	  if (is_gimple_debug (stmt))
-	    continue;
-
 	  if (gimple_assign_single_p (stmt) && gimple_vdef (stmt)
 	      && !stmt_can_throw_internal (stmt)
 	      && lhs_valid_for_store_merging_p (gimple_assign_lhs (stmt)))
@@ -1456,7 +1489,7 @@ pass_store_merging::execute (function *fun)
 		  terminate_all_aliasing_chains (chain_info, false, stmt);
 		  /* Start a new chain.  */
 		  struct imm_store_chain_info *new_chain
-		    = new imm_store_chain_info (base_addr);
+		    = new imm_store_chain_info (m_stores_head, base_addr);
 		  info = new store_immediate_info (bitsize, bitpos,
 						   stmt, 0);
 		  new_chain->m_store_info.safe_push (info);