tree-optimization/99912 - delete trivially dead stmts during DSE

DSE performs a backwards walk over stmts removing stores but it
leaves removing resulting dead SSA defs to later passes.  This
eats into its own alias walking budget if the removed stores kept
loads live.  The following patch adds removal of trivially dead
SSA defs which helps in this situation and reduces the amount of
garbage followup passes need to deal with.

2021-04-28  Richard Biener  <rguenther@suse.de>

	PR tree-optimization/99912
	* tree-ssa-dse.c (dse_dom_walker::m_need_cfg_cleanup): New.
	(dse_dom_walker::todo): Likewise.
	(dse_dom_walker::dse_optimize_stmt): Move VDEF check to the
	caller.
	(dse_dom_walker::before_dom_children): Remove trivially
	dead SSA defs and schedule CFG cleanup if we removed all
	PHIs in a block.
	(pass_dse::execute): Get TODO as computed by the DOM walker
	and return it.  Wipe dominator info earlier.

	* gcc.dg/pr95580.c: Disable DSE.
	* gcc.dg/Wrestrict-8.c: Place a use after each memcpy.
	* c-c++-common/ubsan/overflow-negate-3.c: Make asms volatile
	to prevent them from being removed.
	* c-c++-common/ubsan/overflow-sub-4.c: Likewise.

1 files changed, 59 insertions, 12 deletions

diff --git a/gcc/tree-ssa-dse.c b/gcc/tree-ssa-dse.c
index 4967a5a..aecf6ab 100644
--- a/gcc/tree-ssa-dse.c
+++ b/gcc/tree-ssa-dse.c
@@ -963,16 +963,25 @@ public:
   dse_dom_walker (cdi_direction direction)
     : dom_walker (direction),
     m_live_bytes (param_dse_max_object_size),
-    m_byte_tracking_enabled (false) {}
+    m_byte_tracking_enabled (false),
+    m_need_cfg_cleanup (false) {}
 
   virtual edge before_dom_children (basic_block);
+  unsigned todo () const;
 
 private:
   auto_sbitmap m_live_bytes;
   bool m_byte_tracking_enabled;
+  bool m_need_cfg_cleanup;
   void dse_optimize_stmt (gimple_stmt_iterator *);
 };
 
+unsigned
+dse_dom_walker::todo () const
+{
+  return m_need_cfg_cleanup ? TODO_cleanup_cfg : 0;
+}
+
 /* Delete a dead call at GSI, which is mem* call of some kind.  */
 static void
 delete_dead_or_redundant_call (gimple_stmt_iterator *gsi, const char *type)
@@ -1049,11 +1058,6 @@ dse_dom_walker::dse_optimize_stmt (gimple_stmt_iterator *gsi)
 {
   gimple *stmt = gsi_stmt (*gsi);
 
-  /* If this statement has no virtual defs, then there is nothing
-     to do.  */
-  if (!gimple_vdef (stmt))
-    return;
-
   /* Don't return early on *this_2(D) ={v} {CLOBBER}.  */
   if (gimple_has_volatile_ops (stmt)
       && (!gimple_clobber_p (stmt)
@@ -1180,12 +1184,53 @@ dse_dom_walker::before_dom_children (basic_block bb)
 
   for (gsi = gsi_last_bb (bb); !gsi_end_p (gsi);)
     {
-      dse_optimize_stmt (&gsi);
+      gimple *stmt = gsi_stmt (gsi);
+
+      if (gimple_vdef (stmt))
+	dse_optimize_stmt (&gsi);
+      else if (def_operand_p def_p = single_ssa_def_operand (stmt, SSA_OP_DEF))
+	{
+	  /* When we remove dead stores make sure to also delete trivially
+	     dead SSA defs.  */
+	  if (has_zero_uses (DEF_FROM_PTR (def_p))
+	      && !gimple_has_side_effects (stmt))
+	    {
+	      if (dump_file && (dump_flags & TDF_DETAILS))
+		{
+		  fprintf (dump_file, "  Deleted trivially dead stmt: ");
+		  print_gimple_stmt (dump_file, stmt, 0, dump_flags);
+		  fprintf (dump_file, "\n");
+		}
+	      if (gsi_remove (&gsi, true) && need_eh_cleanup)
+		bitmap_set_bit (need_eh_cleanup, bb->index);
+	      release_defs (stmt);
+	    }
+	}
       if (gsi_end_p (gsi))
 	gsi = gsi_last_bb (bb);
       else
 	gsi_prev (&gsi);
     }
+  bool removed_phi = false;
+  for (gphi_iterator si = gsi_start_phis (bb); !gsi_end_p (si);)
+    {
+      gphi *phi = si.phi ();
+      if (has_zero_uses (gimple_phi_result (phi)))
+	{
+	  if (dump_file && (dump_flags & TDF_DETAILS))
+	    {
+	      fprintf (dump_file, "  Deleted trivially dead PHI: ");
+	      print_gimple_stmt (dump_file, phi, 0, dump_flags);
+	      fprintf (dump_file, "\n");
+	    }
+	  remove_phi_node (&si, true);
+	  removed_phi = true;
+	}
+      else
+	gsi_next (&si);
+    }
+  if (removed_phi && gimple_seq_empty_p (phi_nodes (bb)))
+    m_need_cfg_cleanup = true;
   return NULL;
 }
 
@@ -1234,21 +1279,23 @@ pass_dse::execute (function *fun)
 
   /* Dead store elimination is fundamentally a walk of the post-dominator
      tree and a backwards walk of statements within each block.  */
-  dse_dom_walker (CDI_POST_DOMINATORS).walk (fun->cfg->x_exit_block_ptr);
+  dse_dom_walker walker (CDI_POST_DOMINATORS);
+  walker.walk (fun->cfg->x_exit_block_ptr);
+  free_dominance_info (CDI_POST_DOMINATORS);
+
+  unsigned todo = walker.todo ();
 
   /* Removal of stores may make some EH edges dead.  Purge such edges from
      the CFG as needed.  */
   if (!bitmap_empty_p (need_eh_cleanup))
     {
       gimple_purge_all_dead_eh_edges (need_eh_cleanup);
-      cleanup_tree_cfg ();
+      todo |= TODO_cleanup_cfg;
     }
 
   BITMAP_FREE (need_eh_cleanup);
 
-  /* For now, just wipe the post-dominator information.  */
-  free_dominance_info (CDI_POST_DOMINATORS);
-  return 0;
+  return todo;
 }
 
 } // anon namespace