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Re: [RFA][PATCH] Isolate erroneous paths optimization


On 11/04/13 06:19, Richard Biener wrote:
On Thu, Oct 31, 2013 at 7:11 AM, Jeff Law <law@redhat.com> wrote:

I've incorporated the various suggestions from Marc and Richi, except for
Richi's to integrate this into jump threading.

I've also made the following changes since the last version:

   1. Added more testcases.

   2. Use infer_nonnull_range, moving it from tree-vrp.c
   into gimple.c.  Minor improvements to infer_nonnull_range
   to make it handle more cases we care about and avoid using
   unnecessary routines from tree-ssa.c (which can now be removed)

   3. Multiple undefined statements in a block are handled in the
   logical way.

Bootstrapped and regression tested on x86_64-unknown-linux-gnu.  OK for the
trunk?

Comments inline
Thanks, always appreciated.

index deeb3f2..6db9f56 100644
--- a/gcc/common.opt
+++ b/gcc/common.opt
@@ -2104,6 +2104,12 @@ foptimize-strlen
  Common Report Var(flag_optimize_strlen) Optimization
  Enable string length optimizations on trees

+fisolate-erroneous-paths
+Common Report Var(flag_isolate_erroneous_paths) Init(1) Optimization

Drop Init(1) (see below)
Probably a cut-n-paste. As I've mentioned in another thread, the option stuff is a huge black box that I haven't really looked at. I'm pretty sure I just took something and hacked it. Fixed.



+
+  /* If we did not run to the end of DUPLICATE, then SI points to STMT and
+     SI2 points to the duplicate of STMT in DUPLICATE.  */
+  if (!gsi_end_p (si2))
+    {
+      /* SI2 is the iterator in the duplicate block and it now points
+        to our victim statement.  */
+      gimple_seq seq = NULL;
+      gimple stmt
+       = gimple_build_call (builtin_decl_explicit (BUILT_IN_TRAP), 0);
+      gimple_seq_add_stmt (&seq, stmt);
+      gsi_insert_before (&si2, seq, GSI_SAME_STMT);
+      /* Now delete all remaining statements in this block.  */
+      for (; !gsi_end_p (si2);)
+       gsi_remove (&si2, true);

Please do

    stmt = gsi_stmt (si2);
    unlink_stmt_vdef (stmt);
    gsi_remove (&si2, true);
    release_defs (stmt);

to "completely" remove the stmts correctly (you've left SSA names
unreleased and virtual SSA form broken).
I had to think about this for a while -- we have to be a bit careful here because of the limitations of the name manager. But I think this case is OK. Specifically any SSA_NAMEs we release here won't have any dangling references due to unreachable blocks. Thus we don't run afoul of the limitations of the name manager (yes, that problem is still on my todo list to fix up).


+             next_i = i + 1;
+             if (integer_zerop (op))
+               {

I always prefer

                    if (!integer_zerop (op))
                      continue;

to reduce indentation of following code (but that's personal
preference).
No strong opinions here.  Changed per your request.


+                 FOR_EACH_IMM_USE_STMT (use_stmt, iter, lhs)
+                   {
+                     /* We only care about uses in BB which are
assignements
+                        with memory operands.
+
+                        We could see if any uses are as function arguments
+                        when the callee has marked the argument as being
+                        non-null.  */
+                     if (gimple_bb (use_stmt) != bb
+                         || (!is_gimple_assign (use_stmt)
+                             && !is_gimple_call (use_stmt)
+                             && gimple_code (use_stmt) != GIMPLE_RETURN))

any reason for this restrictions on use_stmt?
Historical when this used to open-code the check for NULL pointer dereferences. infer_nonnull_range should do the right thing. Redundant checks bits removed, comment updated.






+
+      /* Now look at the statements in the block and see if any of
+        them explicitly dereference a NULL pointer.  Believe it or
+        not, this does happen from time to time.  */

"happens because of constant propagation."
"because of jump threading and constant propagation" I went through the trouble of tracking this down a little while ago to ensure this code was still useful and didn't update the comment. FWIW, the included tests show instances where we can get explicit dereferences of NULL due to jump threading + constant propagation.






+      for (si = gsi_start_bb (bb); !gsi_end_p (si); gsi_next (&si))
+       {
+         gimple stmt = gsi_stmt (si);
+
+

extra vertical space
Fixed.


+         /* By passing null_pointer_node, we can use infer_nonnull_range
+            to detect explicit NULL pointer dereferences and other uses
+            where a non-NULL value is required.  */
+         if (infer_nonnull_range (stmt, null_pointer_node))
+           {
+             /* First insert a TRAP before this statement.  */
+             gimple_seq seq = NULL;
+             tree t
+               = build_call_expr_loc (0,

Use the location of 'stmt'?

+                                      builtin_decl_explicit
(BUILT_IN_TRAP),
+                                      0);
+             gimplify_and_add (t, &seq);
+             gsi_insert_before (&si, seq, GSI_SAME_STMT);

and please build GIMPLE directly here as well.
Hmm, thought I had already fixed this in both stanzas.



+             /* Now delete all remaining statements in this block.  */
+             for (gimple_stmt_iterator si2 = si; !gsi_end_p (si2);)
+               gsi_remove (&si2, true);

See above.

Maybe you can split this common functionality out into a helper.
I'd been considering this already.  Done.

+static bool
+gate_isolate_erroneous_paths (void)
+{
+  /* If we do not have a suitable builtin function for the trap statement,
+     then do not perform the optimization.  */
+  return (flag_isolate_erroneous_paths != 0
+         && builtin_decl_explicit (BUILT_IN_TRAP) != NULL);

I don't think this can happen.
Fortran front-end doesn't provide this IIRC.


+/* Callback for walk_stmt_load_store_ops.
+
+   Return TRUE if OP will dereference the tree stored in DATA, FALSE
+   otherwise.
+
+   This routine only makes a superficial check for a dereference.  Thus
+   it must only be used if it is safe to return a false negative.  */
+static bool
+check_loadstore (gimple stmt ATTRIBUTE_UNUSED, tree op, void *data)
+{
+  if ((TREE_CODE (op) == MEM_REF || TREE_CODE (op) == TARGET_MEM_REF)
+      && operand_equal_p (TREE_OPERAND (op, 0), (tree)data, 0))

As you are interested in pointer dereferences and we are in SSA form
you can use pointer equality:

            && TREE_OPERAND (op, 0) == (tree) data
We're also interested in explicit NULL dereferences, explicit uses of NULL for parameters marked as being non-null and explicit NULL return values in functions marked as never returning NULL. So DATA could be 0B here. And those aren't unique. Thus pointer equality is not sufficient. The included tests show examples that fail if we strictly use pointer equality.



+         /* If "nonnull" applies to all the arguments, then ARG
+            is non-null if it's in the argument list.  */
+         if (TREE_VALUE (attrs) == NULL_TREE)
+           {
+             for (unsigned int i = 0; i < gimple_call_num_args (stmt); i++)
+               {
+                 if (operand_equal_p (op, gimple_call_arg (stmt, i), 0)

See above (pointer comparison).
Same comment applies. We want to catch 0B for explicit NULL pointer arguments in an arglist.


+                     && POINTER_TYPE_P (TREE_TYPE (gimple_call_arg (stmt,
i))))
+                   return true;
+               }
+             return false;
+           }
+
+         /* Now see if op appears in the nonnull list.  */
+         for (tree t = TREE_VALUE (attrs); t; t = TREE_CHAIN (t))
+           {
+             int idx = TREE_INT_CST_LOW (TREE_VALUE (t)) - 1;
+             tree arg = gimple_call_arg (stmt, idx);
+             if (operand_equal_p (op, arg, 0))

See above.
Similarly.


+               return true;
+           }
+       }
+    }
+
+  /* If this function is marked as returning non-null, then we can
+     infer OP is non-null if it is used in the return statement.  */
+  if (gimple_code (stmt) == GIMPLE_RETURN
+      && gimple_return_retval (stmt)
+      && operand_equal_p (gimple_return_retval (stmt), op, 0)

See above.
Similarly.

@@ -453,6 +453,7 @@ static const struct default_options
default_options_table[] =
      { OPT_LEVELS_1_PLUS, OPT_ftree_ch, NULL, 1 },
      { OPT_LEVELS_1_PLUS, OPT_fcombine_stack_adjustments, NULL, 1 },
      { OPT_LEVELS_1_PLUS, OPT_fcompare_elim, NULL, 1 },
+    { OPT_LEVELS_1_PLUS, OPT_fisolate_erroneous_paths, NULL, 1 },

Why enable this at -O1?  We have -fno-strict-overflow, -fno-strict-aliasing
at -O1 so I'd rather defer this to -O2 and above (including -Os).
No particular reason. -O2 and above is fine with me. Changed to OPT_LEVELS_2_PLUS. Removes the need to change 20030711-1.c as that test only runs at -O1.

This patch also doesn't need to add gsi_start_nondebug_after_labels as Andrew P. added that function recently.



Otherwise the patch looks ok to me.
Attached is the updated patch. Bootstrapped and regression tested on x86_64-unknown-linux-gnu.


jeff

	* Makefile.in (OBJS): Add gimple-ssa-isolate-paths.o
	* common.opt (-fisolate-erroneous-paths): Add option and
	documentation.
	* gimple-ssa-isolate-paths.c: New file.
	* gimple.c (check_loadstore): New function.
	(infer_nonnull_range): Moved into gimple.c from tree-vrp.c
	Verify OP is in the argument list and the argument corresponding
	to OP is a pointer type.  Use operand_equal_p rather than
	pointer equality when testing if OP is on the nonnull list.
	Use check_loadstore rather than count_ptr_derefs.  Handle
	GIMPLE_RETURN statements.
	* tree-vrp.c (infer_nonnull_range): Remove.
	* gimple.h (infer_nonnull_range): Declare.
	* opts.c (default_options_table): Add OPT_fisolate_erroneous_paths.
	* passes.def: Add pass_isolate_erroneous_paths.
	* timevar.def (TV_ISOLATE_ERRONEOUS_PATHS): New timevar.
	* tree-pass.h (make_pass_isolate_erroneous_paths): Declare.
	* tree-ssa.c (struct count_ptr_d): Remove.
	(count_ptr_derefs, count_uses_and_derefs): Remove.
	* tree-ssa.h (count_uses_and_derefs): Remove.
	


	* gcc.dg/pr38984.c: Add -fno-isolate-erroneous-paths.
	* gcc.dg/tree-ssa/isolate-1.c: New test.
	* gcc.dg/tree-ssa/isolate-2.c: New test.
	* gcc.dg/tree-ssa/isolate-3.c: New test.
	* gcc.dg/tree-ssa/isolate-4.c: New test.

diff --git a/gcc/Makefile.in b/gcc/Makefile.in
index cc88fb8..9dd5dbe 100644
--- a/gcc/Makefile.in
+++ b/gcc/Makefile.in
@@ -1234,6 +1234,7 @@ OBJS = \
 	gimple-fold.o \
 	gimple-low.o \
 	gimple-pretty-print.o \
+	gimple-ssa-isolate-paths.o \
 	gimple-ssa-strength-reduction.o \
 	gimple-streamer-in.o \
 	gimple-streamer-out.o \
diff --git a/gcc/common.opt b/gcc/common.opt
index 3a40db2..bda4790 100644
--- a/gcc/common.opt
+++ b/gcc/common.opt
@@ -2109,6 +2109,12 @@ foptimize-strlen
 Common Report Var(flag_optimize_strlen) Optimization
 Enable string length optimizations on trees
 
+fisolate-erroneous-paths
+Common Report Var(flag_isolate_erroneous_paths) Optimization
+Detect paths which trigger erroneous or undefined behaviour.  Isolate those
+paths from the main control flow and turn the statement with erroneous or
+undefined behaviour into a trap.
+
 ftree-loop-distribution
 Common Report Var(flag_tree_loop_distribution) Optimization
 Enable loop distribution on trees
diff --git a/gcc/gimple-ssa-isolate-paths.c b/gcc/gimple-ssa-isolate-paths.c
new file mode 100644
index 0000000..4868867
--- /dev/null
+++ b/gcc/gimple-ssa-isolate-paths.c
@@ -0,0 +1,325 @@
+/* Detect paths through the CFG which can never be executed in a conforming
+   program and isolate them.
+
+   Copyright (C) 2013
+   Free Software Foundation, Inc.
+
+This file is part of GCC.
+
+GCC is free software; you can redistribute it and/or modify
+it under the terms of the GNU General Public License as published by
+the Free Software Foundation; either version 3, or (at your option)
+any later version.
+
+GCC is distributed in the hope that it will be useful,
+but WITHOUT ANY WARRANTY; without even the implied warranty of
+MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+GNU General Public License for more details.
+
+You should have received a copy of the GNU General Public License
+along with GCC; see the file COPYING3.  If not see
+<http://www.gnu.org/licenses/>.  */
+
+#include "config.h"
+#include "system.h"
+#include "coretypes.h"
+#include "tree.h"
+#include "flags.h"
+#include "basic-block.h"
+#include "gimple.h"
+#include "tree-ssa.h"
+#include "tree-ssanames.h"
+#include "gimple-ssa.h"
+#include "tree-ssa-operands.h"
+#include "tree-phinodes.h"
+#include "ssa-iterators.h"
+#include "cfgloop.h"
+#include "tree-pass.h"
+
+
+static bool cfg_altered;
+
+/* Insert a trap before SI and remove SI and all statements after SI.  */
+
+static void
+insert_trap_and_remove_trailing_statements (gimple_stmt_iterator *si_p)
+{
+  gimple_seq seq = NULL;
+  gimple stmt = gimple_build_call (builtin_decl_explicit (BUILT_IN_TRAP), 0);
+  gimple_seq_add_stmt (&seq, stmt);
+  gsi_insert_before (si_p, seq, GSI_SAME_STMT);
+
+  /* Now delete all remaining statements in this block.  */
+  for (; !gsi_end_p (*si_p);)
+    {
+      stmt = gsi_stmt (*si_p);
+      unlink_stmt_vdef (stmt);
+      gsi_remove (si_p, true);
+      release_defs (stmt);
+    }
+}
+
+/* BB when reached via incoming edge E will exhibit undefined behaviour
+   at STMT.  Isolate and optimize the path which exhibits undefined
+   behaviour.
+
+   Isolation is simple.  Duplicate BB and redirect E to BB'.
+
+   Optimization is simple as well.  Replace STMT in BB' with an
+   unconditional trap and remove all outgoing edges from BB'.
+
+   DUPLICATE is a pre-existing duplicate, use it as BB' if it exists.
+
+   Return BB'.  */
+
+basic_block
+isolate_path (basic_block bb, basic_block duplicate, edge e, gimple stmt)
+{
+  gimple_stmt_iterator si, si2;
+  edge_iterator ei;
+  edge e2;
+  
+
+  /* First duplicate BB if we have not done so already and remove all
+     the duplicate's outgoing edges as duplicate is going to unconditionally
+     trap.  Removing the outgoing edges is both an optimization and ensures
+     we don't need to do any PHI node updates.  */
+  if (!duplicate)
+    {
+      duplicate = duplicate_block (bb, NULL, NULL);
+      for (ei = ei_start (duplicate->succs); (e2 = ei_safe_edge (ei)); )
+	remove_edge (e2);
+    }
+
+  /* Complete the isolation step by redirecting E to reach DUPLICATE.  */
+  e2 = redirect_edge_and_branch (e, duplicate);
+  if (e2)
+    flush_pending_stmts (e2);
+
+
+  /* There may be more than one statement in DUPLICATE which exhibits
+     undefined behaviour.  Ultimately we want the first such statement in
+     DUPLCIATE so that we're able to delete as much code as possible.
+
+     So each time we discover undefined behaviour in DUPLICATE, search for
+     the statement which triggers undefined behaviour.  If found, then
+     transform the statement into a trap and delete everything after the
+     statement.  If not found, then this particular instance was subsumed by
+     an earlier instance of undefined behaviour and there's nothing to do. 
+
+     This is made more complicated by the fact that we have STMT, which is in
+     BB rather than in DUPLICATE.  So we set up two iterators, one for each
+     block and walk forward looking for STMT in BB, advancing each iterator at
+     each step.
+
+     When we find STMT the second iterator should point to STMT's equivalent in
+     duplicate.  If DUPLICATE ends before STMT is found in BB, then there's
+     nothing to do. 
+
+     Ignore labels and debug statements.  */
+  si = gsi_start_nondebug_after_labels_bb (bb);
+  si2 = gsi_start_nondebug_after_labels_bb (duplicate);
+  while (!gsi_end_p (si) && !gsi_end_p (si2) && gsi_stmt (si) != stmt)
+    {
+      gsi_next_nondebug (&si);
+      gsi_next_nondebug (&si2);
+    }
+
+  /* This would be an indicator that we never found STMT in BB, which should
+     never happen.  */
+  gcc_assert (!gsi_end_p (si));
+
+  /* If we did not run to the end of DUPLICATE, then SI points to STMT and
+     SI2 points to the duplicate of STMT in DUPLICATE.  Insert a trap
+     before SI2 and remove SI2 and all trailing statements.  */
+  if (!gsi_end_p (si2))
+    insert_trap_and_remove_trailing_statements (&si2);
+
+  return duplicate;
+}
+
+/* Search the function for statements which, if executed, would cause
+   the program to fault such as a dereference of a NULL pointer.
+
+   Such a program can't be valid if such a statement was to execute
+   according to ISO standards.
+
+   We detect explicit NULL pointer dereferences as well as those implied
+   by a PHI argument having a NULL value which unconditionally flows into
+   a dereference in the same block as the PHI.
+
+   In the former case we replace the offending statement with an
+   unconditional trap and eliminate the outgoing edges from the statement's
+   basic block.  This may expose secondary optimization opportunities.
+
+   In the latter case, we isolate the path(s) with the NULL PHI 
+   feeding the dereference.  We can then replace the offending statement
+   and eliminate the outgoing edges in the duplicate.  Again, this may
+   expose secondary optimization opportunities.
+
+   A warning for both cases may be advisable as well.
+
+   Other statically detectable violations of the ISO standard could be
+   handled in a similar way, such as out-of-bounds array indexing.  */
+
+static unsigned int
+gimple_ssa_isolate_erroneous_paths (void)
+{
+  basic_block bb;
+
+  initialize_original_copy_tables ();
+
+  /* Search all the blocks for edges which, if traversed, will
+     result in undefined behaviour.  */
+  cfg_altered = false;
+  FOR_EACH_BB (bb)
+    {
+      gimple_stmt_iterator si;
+
+      /* First look for a PHI which sets a pointer to NULL and which
+ 	 is then dereferenced within BB.  This is somewhat overly
+	 conservative, but probably catches most of the interesting
+	 cases.   */
+      for (si = gsi_start_phis (bb); !gsi_end_p (si); gsi_next (&si))
+	{
+	  gimple phi = gsi_stmt (si);
+	  tree lhs = gimple_phi_result (phi);
+
+	  /* If the result is not a pointer, then there is no need to
+ 	     examine the arguments.  */
+	  if (!POINTER_TYPE_P (TREE_TYPE (lhs)))
+	    continue;
+
+	  /* PHI produces a pointer result.  See if any of the PHI's
+	     arguments are NULL. 
+
+	     When we remove an edge, we want to reprocess the current
+	     index, hence the ugly way we update I for each iteration.  */
+	  basic_block duplicate = NULL;
+	  for (unsigned i = 0, next_i = 0;
+	       i < gimple_phi_num_args (phi);
+	       i = next_i)
+	    {
+	      tree op = gimple_phi_arg_def (phi, i);
+
+	      next_i = i + 1;
+	
+	      if (!integer_zerop (op))
+		continue;
+
+	      edge e = gimple_phi_arg_edge (phi, i);
+	      imm_use_iterator iter;
+	      gimple use_stmt;
+
+	      /* We've got a NULL PHI argument.  Now see if the
+ 	         PHI's result is dereferenced within BB.  */
+	      FOR_EACH_IMM_USE_STMT (use_stmt, iter, lhs)
+	        {
+	          /* We only care about uses in BB.  Catching cases in
+		     in other blocks would require more complex path
+		     isolation code.  */
+		  if (gimple_bb (use_stmt) != bb)
+		    continue;
+
+		  if (infer_nonnull_range (use_stmt, lhs))
+		    {
+		      duplicate = isolate_path (bb, duplicate,
+						e, use_stmt);
+
+		      /* When we remove an incoming edge, we need to
+			 reprocess the Ith element.  */
+		      next_i = i;
+		      cfg_altered = true;
+		    }
+		}
+	    }
+	}
+
+      /* Now look at the statements in the block and see if any of
+	 them explicitly dereference a NULL pointer.  This happens
+	 because of jump threading and constant propagation.  */
+      for (si = gsi_start_bb (bb); !gsi_end_p (si); gsi_next (&si))
+	{
+	  gimple stmt = gsi_stmt (si);
+
+	  /* By passing null_pointer_node, we can use infer_nonnull_range
+	     to detect explicit NULL pointer dereferences and other uses
+	     where a non-NULL value is required.  */
+	  if (infer_nonnull_range (stmt, null_pointer_node))
+	    {
+	      insert_trap_and_remove_trailing_statements (&si);
+
+	      /* And finally, remove all outgoing edges from BB.  */
+	      edge e;
+	      for (edge_iterator ei = ei_start (bb->succs);
+		   (e = ei_safe_edge (ei)); )
+		remove_edge (e);
+
+	      /* Ignore any more operands on this statement and
+		 continue the statement iterator (which should
+		 terminate its loop immediately.  */
+	      cfg_altered = true;
+	      break;
+	    }
+	}
+    }
+  free_original_copy_tables ();
+
+  /* We scramble the CFG and loop structures a bit, clean up 
+     appropriately.  We really should incrementally update the
+     loop structures, in theory it shouldn't be that hard.  */
+  if (cfg_altered)
+    {
+      free_dominance_info (CDI_DOMINATORS);
+      free_dominance_info (CDI_POST_DOMINATORS);
+      loops_state_set (LOOPS_NEED_FIXUP);
+      return TODO_cleanup_cfg | TODO_update_ssa;
+    }
+  return 0;
+}
+
+static bool
+gate_isolate_erroneous_paths (void)
+{
+  /* If we do not have a suitable builtin function for the trap statement,
+     then do not perform the optimization.  */
+  return (flag_isolate_erroneous_paths != 0
+	  && builtin_decl_explicit (BUILT_IN_TRAP) != NULL);
+}
+
+namespace {
+const pass_data pass_data_isolate_erroneous_paths =
+{
+  GIMPLE_PASS, /* type */
+  "isolate-paths", /* name */
+  OPTGROUP_NONE, /* optinfo_flags */
+  true, /* has_gate */
+  true, /* has_execute */
+  TV_ISOLATE_ERRONEOUS_PATHS, /* tv_id */
+  ( PROP_cfg | PROP_ssa ), /* properties_required */
+  0, /* properties_provided */
+  0, /* properties_destroyed */
+  0, /* todo_flags_start */
+  TODO_verify_ssa, /* todo_flags_finish */
+};
+
+class pass_isolate_erroneous_paths : public gimple_opt_pass
+{
+public:
+  pass_isolate_erroneous_paths (gcc::context *ctxt)
+    : gimple_opt_pass (pass_data_isolate_erroneous_paths, ctxt)
+  {}
+
+  /* opt_pass methods: */
+  opt_pass * clone () { return new pass_isolate_erroneous_paths (m_ctxt); }
+  bool gate () { return gate_isolate_erroneous_paths (); }
+  unsigned int execute () { return gimple_ssa_isolate_erroneous_paths (); }
+
+}; // class pass_uncprop
+}
+
+gimple_opt_pass *
+make_pass_isolate_erroneous_paths (gcc::context *ctxt)
+{
+  return new pass_isolate_erroneous_paths (ctxt);
+}
diff --git a/gcc/gimple.c b/gcc/gimple.c
index 20f6010..15688af 100644
--- a/gcc/gimple.c
+++ b/gcc/gimple.c
@@ -4103,6 +4103,87 @@ nonfreeing_call_p (gimple call)
   return false;
 }
 
+/* Callback for walk_stmt_load_store_ops.
+ 
+   Return TRUE if OP will dereference the tree stored in DATA, FALSE
+   otherwise.
+
+   This routine only makes a superficial check for a dereference.  Thus
+   it must only be used if it is safe to return a false negative.  */
+static bool
+check_loadstore (gimple stmt ATTRIBUTE_UNUSED, tree op, void *data)
+{
+  if ((TREE_CODE (op) == MEM_REF || TREE_CODE (op) == TARGET_MEM_REF)
+      && operand_equal_p (TREE_OPERAND (op, 0), (tree)data, 0))
+    return true;
+  return false;
+}
+
+/* If OP can be inferred to be non-zero after STMT executes, return true.  */
+
+bool
+infer_nonnull_range (gimple stmt, tree op)
+{
+  /* We can only assume that a pointer dereference will yield
+     non-NULL if -fdelete-null-pointer-checks is enabled.  */
+  if (!flag_delete_null_pointer_checks
+      || !POINTER_TYPE_P (TREE_TYPE (op))
+      || gimple_code (stmt) == GIMPLE_ASM)
+    return false;
+
+  if (walk_stmt_load_store_ops (stmt, (void *)op,
+				check_loadstore, check_loadstore))
+    return true;
+
+  if (is_gimple_call (stmt) && !gimple_call_internal_p (stmt))
+    {
+      tree fntype = gimple_call_fntype (stmt);
+      tree attrs = TYPE_ATTRIBUTES (fntype);
+      for (; attrs; attrs = TREE_CHAIN (attrs))
+	{
+	  attrs = lookup_attribute ("nonnull", attrs);
+
+	  /* If "nonnull" wasn't specified, we know nothing about
+	     the argument.  */
+	  if (attrs == NULL_TREE)
+	    return false;
+
+	  /* If "nonnull" applies to all the arguments, then ARG
+	     is non-null if it's in the argument list.  */
+	  if (TREE_VALUE (attrs) == NULL_TREE)
+	    {
+	      for (unsigned int i = 0; i < gimple_call_num_args (stmt); i++)
+		{
+		  if (operand_equal_p (op, gimple_call_arg (stmt, i), 0)
+		      && POINTER_TYPE_P (TREE_TYPE (gimple_call_arg (stmt, i))))
+		    return true;
+		}
+	      return false;
+	    }
+
+	  /* Now see if op appears in the nonnull list.  */
+	  for (tree t = TREE_VALUE (attrs); t; t = TREE_CHAIN (t))
+	    {
+	      int idx = TREE_INT_CST_LOW (TREE_VALUE (t)) - 1;
+	      tree arg = gimple_call_arg (stmt, idx);
+	      if (operand_equal_p (op, arg, 0))
+		return true;
+	    }
+	}
+    }
+
+  /* If this function is marked as returning non-null, then we can
+     infer OP is non-null if it is used in the return statement.  */
+  if (gimple_code (stmt) == GIMPLE_RETURN
+      && gimple_return_retval (stmt)
+      && operand_equal_p (gimple_return_retval (stmt), op, 0)
+      && lookup_attribute ("returns_nonnull",
+			   TYPE_ATTRIBUTES (TREE_TYPE (current_function_decl))))
+    return true;
+
+  return false;
+}
+
 /* Create a new VAR_DECL and copy information from VAR to it.  */
 
 tree
diff --git a/gcc/gimple.h b/gcc/gimple.h
index b34424c..430b50c 100644
--- a/gcc/gimple.h
+++ b/gcc/gimple.h
@@ -1089,6 +1089,7 @@ extern void dump_decl_set (FILE *, bitmap);
 extern bool gimple_can_coalesce_p (tree, tree);
 extern bool nonfreeing_call_p (gimple);
 extern tree copy_var_decl (tree, tree, tree);
+extern bool infer_nonnull_range (gimple, tree);
 
 /* In trans-mem.c.  */
 extern void diagnose_tm_safe_errors (tree);
diff --git a/gcc/opts.c b/gcc/opts.c
index 4db20f0..3a939ac 100644
--- a/gcc/opts.c
+++ b/gcc/opts.c
@@ -493,6 +493,7 @@ static const struct default_options default_options_table[] =
     { OPT_LEVELS_2_PLUS, OPT_fvect_cost_model_, NULL, VECT_COST_MODEL_CHEAP },
     { OPT_LEVELS_2_PLUS_SPEED_ONLY, OPT_foptimize_strlen, NULL, 1 },
     { OPT_LEVELS_2_PLUS, OPT_fhoist_adjacent_loads, NULL, 1 },
+    { OPT_LEVELS_2_PLUS, OPT_fisolate_erroneous_paths, NULL, 1 },
 
     /* -O3 optimizations.  */
     { OPT_LEVELS_3_PLUS, OPT_ftree_loop_distribute_patterns, NULL, 1 },
diff --git a/gcc/passes.def b/gcc/passes.def
index 31ce113..0dabba4 100644
--- a/gcc/passes.def
+++ b/gcc/passes.def
@@ -166,9 +166,16 @@ along with GCC; see the file COPYING3.  If not see
 	 is removed, and this place fits nicely.  Remember this when
 	 trying to move or duplicate pass_dominator somewhere earlier.  */
       NEXT_PASS (pass_dominator);
+      /* At this point the majority of const/copy propagations
+	 are exposed.  Go ahead and identify paths that should never
+	 be executed in a conforming program and isolate those paths.
+
+	 This will expose more degenerate PHIs in the main path and
+	 expose more PRE/DOM optimization opportunities.  */
+      NEXT_PASS (pass_isolate_erroneous_paths);
       /* The only const/copy propagation opportunities left after
-	 DOM should be due to degenerate PHI nodes.  So rather than
-	 run the full propagators, run a specialized pass which
+	 DOM and erroneous path isolation should be due to degenerate PHI nodes.
+	 So rather than run the full propagators, run a specialized pass which
 	 only examines PHIs to discover const/copy propagation
 	 opportunities.  */
       NEXT_PASS (pass_phi_only_cprop);
diff --git a/gcc/testsuite/gcc.dg/pr38984.c b/gcc/testsuite/gcc.dg/pr38984.c
index 11f1e7f..0c03180 100644
--- a/gcc/testsuite/gcc.dg/pr38984.c
+++ b/gcc/testsuite/gcc.dg/pr38984.c
@@ -1,5 +1,5 @@
 /* { dg-do compile } */
-/* { dg-options "-O2 -fno-delete-null-pointer-checks -fdump-tree-optimized" }
+/* { dg-options "-O2 -fno-delete-null-pointer-checks -fdump-tree-optimized -fno-isolate-erroneous-paths" }
  * */
 
 int f(int *p)
diff --git a/gcc/testsuite/gcc.dg/tree-ssa/isolate-1.c b/gcc/testsuite/gcc.dg/tree-ssa/isolate-1.c
new file mode 100644
index 0000000..6b779b4
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/tree-ssa/isolate-1.c
@@ -0,0 +1,58 @@
+
+/* { dg-do compile } */ 
+/* { dg-options "-O2 -fdump-tree-isolate-paths" } */
+
+
+struct demangle_component
+{
+
+  int type;
+  int zzz;
+
+};
+
+
+struct d_info
+{
+  struct demangle_component *comps;
+  int next_comp;
+  int num_comps;
+};
+
+
+static struct demangle_component *
+d_make_empty (struct d_info *di)
+{
+  struct demangle_component *p;
+
+  if (di->next_comp >= di->num_comps)
+    return ((void *)0);
+  p = &di->comps[di->next_comp];
+  return p;
+}
+
+
+
+struct demangle_component *
+d_type (struct d_info *di)
+{
+   struct demangle_component *ret;
+   ret = d_make_empty (di);
+   ret->type = 42;
+   ret->zzz = -1;
+   return ret;
+}
+
+/* We're testing two aspects of isolation here.  First that isolation
+   occurs, second that if we have two null dereferences in a block that
+   that we delete everything from the first dereferece to the end of the
+   block, regardless of which comes first in the immediate use iterator.  */
+/* { dg-final { scan-tree-dump-times "__builtin_trap" 1 "isolate-paths"} } */
+/* { dg-final { scan-tree-dump-times "->type" 1 "isolate-paths"} } */
+/* { dg-final { scan-tree-dump-times "->zzz" 1 "isolate-paths"} } */
+/* { dg-final { cleanup-tree-dump "isolate-paths" } } */
+
+
+
+
+
diff --git a/gcc/testsuite/gcc.dg/tree-ssa/isolate-2.c b/gcc/testsuite/gcc.dg/tree-ssa/isolate-2.c
new file mode 100644
index 0000000..290b44c
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/tree-ssa/isolate-2.c
@@ -0,0 +1,43 @@
+/* { dg-do compile } */ 
+/* { dg-options "-O2 -fdump-tree-isolate-paths -fdump-tree-phicprop1" } */
+
+
+int z;
+int y;
+
+int * foo(int a) __attribute__((returns_nonnull));
+int * bar(void) __attribute__((returns_nonnull));
+
+int *
+foo(int a)
+
+{
+  switch (a)
+    {
+      case 0:
+        return &z;
+      default:
+        return (int *)0;
+    }
+}
+
+
+int *
+bar (void)
+{
+  return 0;
+}
+
+/* We testing that the path isolation code can take advantage of the
+   returns non-null attribute to isolate a path where NULL flows into
+   a return statement.  We test this twice, once where the NULL flows
+   from a PHI, the second with an explicit return 0 in the IL.
+
+   We also verify that after isolation phi-cprop simplifies the
+   return statement so that it returns &z directly.
+/* { dg-final { scan-tree-dump-times "__builtin_trap" 2 "isolate-paths"} } */
+/* { dg-final { scan-tree-dump-times "return &z;" 1 "phicprop1"} } */
+/* { dg-final { cleanup-tree-dump "isolate-paths" } } */
+/* { dg-final { cleanup-tree-dump "phicprop1" } } */
+
+
diff --git a/gcc/testsuite/gcc.dg/tree-ssa/isolate-3.c b/gcc/testsuite/gcc.dg/tree-ssa/isolate-3.c
new file mode 100644
index 0000000..7dddd80
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/tree-ssa/isolate-3.c
@@ -0,0 +1,65 @@
+/* { dg-do compile } */ 
+/* { dg-options "-O2 -fdump-tree-isolate-paths" } */
+
+
+typedef long unsigned int size_t;
+extern void *memset (void *__s, int __c, size_t __n)
+  __attribute__ ((__nothrow__, __leaf__)) __attribute__ ((__nonnull__ (1)));
+struct rtx_def;
+typedef struct rtx_def *rtx;
+typedef struct VEC_rtx_base
+
+{
+  unsigned num;
+  unsigned alloc;
+  rtx vec[1];
+} VEC_rtx_base;
+static __inline__ rtx *
+VEC_rtx_base_address (VEC_rtx_base * vec_)
+{
+  return vec_ ? vec_->vec : 0;
+}
+typedef struct VEC_rtx_gc
+{
+  VEC_rtx_base base;
+} VEC_rtx_gc;
+
+static __inline__ void
+VEC_rtx_gc_safe_grow (VEC_rtx_gc ** vec_, int size_, const char *file_,
+                      unsigned line_, const char *function_)
+{
+  ((*vec_) ? &(*vec_)->base : 0)->num = size_;
+} 
+
+static __inline__ void
+VEC_rtx_gc_safe_grow_cleared (VEC_rtx_gc ** vec_, int size_,
+                              const char *file_, unsigned line_,
+                              const char *function_, int oldsize)
+{
+  VEC_rtx_gc_safe_grow (vec_, size_, file_, line_, function_);
+  memset (&(VEC_rtx_base_address ((*vec_) ? &(*vec_)->base : 0))[oldsize], 0,
+          sizeof (rtx) * (size_ - oldsize));
+}
+
+static VEC_rtx_gc *reg_base_value;
+void
+init_alias_analysis (void)
+{
+  unsigned int maxreg = max_reg_num ();
+  (VEC_rtx_gc_safe_grow_cleared
+   (&(reg_base_value), maxreg, "../../../gcc-4.6.0/gcc/alias.c", 2755,
+    __FUNCTION__, arf ()));
+}
+
+
+
+/* This is an example of how a NULL pointer dereference can show up
+   without a PHI.  Note VEC_rtx_gcc_safe_grow.  If an earlier pass
+   (such as VRP) isolates the NULL path for some reason or another
+   we end up with an explicit NULL dereference in the IL.  Yes, it
+   started with a PHI, but by the time the path isolation code runs
+   its explicit in the IL.  */
+/* { dg-final { scan-tree-dump-times "__builtin_trap" 1 "isolate-paths"} } */
+/* { dg-final { cleanup-tree-dump "isolate-paths" } } */
+
+
diff --git a/gcc/testsuite/gcc.dg/tree-ssa/isolate-4.c b/gcc/testsuite/gcc.dg/tree-ssa/isolate-4.c
new file mode 100644
index 0000000..6937d25
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/tree-ssa/isolate-4.c
@@ -0,0 +1,32 @@
+/* { dg-do compile } */ 
+/* { dg-options "-O2 -fdump-tree-isolate-paths -fdump-tree-phicprop1" } */
+
+
+extern void foo(void *) __attribute__ ((__nonnull__ (1)));
+
+int z;
+
+void
+com (int a)
+{
+    foo (a == 42 ? &z  : (void *) 0);
+}
+
+void
+bar (void)
+{
+  foo ((void *)0);
+}
+
+/* We testing that the path isolation code can take advantage of the
+   returns non-null attribute to isolate a path where NULL flows into
+   a return statement.
+
+   We also verify that after isolation phi-cprop simplifies the
+   return statement so that it returns &z directly.
+/* { dg-final { scan-tree-dump-times "__builtin_trap" 2 "isolate-paths"} } */
+/* { dg-final { scan-tree-dump-times "foo .&z.;" 1 "phicprop1"} } */
+/* { dg-final { cleanup-tree-dump "isolate-paths" } } */
+/* { dg-final { cleanup-tree-dump "phicprop1" } } */
+
+
diff --git a/gcc/timevar.def b/gcc/timevar.def
index 66d61ae..afdadb8 100644
--- a/gcc/timevar.def
+++ b/gcc/timevar.def
@@ -144,6 +144,7 @@ DEFTIMEVAR (TV_TREE_SSA_INCREMENTAL  , "tree SSA incremental")
 DEFTIMEVAR (TV_TREE_OPS	             , "tree operand scan")
 DEFTIMEVAR (TV_TREE_SSA_DOMINATOR_OPTS   , "dominator optimization")
 DEFTIMEVAR (TV_TREE_SRA              , "tree SRA")
+DEFTIMEVAR (TV_ISOLATE_ERRONEOUS_PATHS    , "isolate eroneous paths")
 DEFTIMEVAR (TV_TREE_CCP		     , "tree CCP")
 DEFTIMEVAR (TV_TREE_PHI_CPROP	     , "tree PHI const/copy prop")
 DEFTIMEVAR (TV_TREE_SPLIT_EDGES      , "tree split crit edges")
diff --git a/gcc/tree-pass.h b/gcc/tree-pass.h
index c4d09fe..3aeaeeb 100644
--- a/gcc/tree-pass.h
+++ b/gcc/tree-pass.h
@@ -425,6 +425,7 @@ extern gimple_opt_pass *make_pass_sink_code (gcc::context *ctxt);
 extern gimple_opt_pass *make_pass_fre (gcc::context *ctxt);
 extern gimple_opt_pass *make_pass_check_data_deps (gcc::context *ctxt);
 extern gimple_opt_pass *make_pass_copy_prop (gcc::context *ctxt);
+extern gimple_opt_pass *make_pass_isolate_erroneous_paths (gcc::context *ctxt);
 extern gimple_opt_pass *make_pass_vrp (gcc::context *ctxt);
 extern gimple_opt_pass *make_pass_uncprop (gcc::context *ctxt);
 extern gimple_opt_pass *make_pass_return_slot (gcc::context *ctxt);
diff --git a/gcc/tree-ssa.c b/gcc/tree-ssa.c
index 0b743d1..ba8045d 100644
--- a/gcc/tree-ssa.c
+++ b/gcc/tree-ssa.c
@@ -236,100 +236,6 @@ flush_pending_stmts (edge e)
   redirect_edge_var_map_clear (e);
 }
 
-
-/* Data structure used to count the number of dereferences to PTR
-   inside an expression.  */
-struct count_ptr_d
-{
-  tree ptr;
-  unsigned num_stores;
-  unsigned num_loads;
-};
-
-
-/* Helper for count_uses_and_derefs.  Called by walk_tree to look for
-   (ALIGN/MISALIGNED_)INDIRECT_REF nodes for the pointer passed in DATA.  */
-
-static tree
-count_ptr_derefs (tree *tp, int *walk_subtrees, void *data)
-{
-  struct walk_stmt_info *wi_p = (struct walk_stmt_info *) data;
-  struct count_ptr_d *count_p = (struct count_ptr_d *) wi_p->info;
-
-  /* Do not walk inside ADDR_EXPR nodes.  In the expression &ptr->fld,
-     pointer 'ptr' is *not* dereferenced, it is simply used to compute
-     the address of 'fld' as 'ptr + offsetof(fld)'.  */
-  if (TREE_CODE (*tp) == ADDR_EXPR)
-    {
-      *walk_subtrees = 0;
-      return NULL_TREE;
-    }
-
-  if (TREE_CODE (*tp) == MEM_REF && TREE_OPERAND (*tp, 0) == count_p->ptr)
-    {
-      if (wi_p->is_lhs)
-	count_p->num_stores++;
-      else
-	count_p->num_loads++;
-    }
-
-  return NULL_TREE;
-}
-
-
-/* Count the number of direct and indirect uses for pointer PTR in
-   statement STMT.  The number of direct uses is stored in
-   *NUM_USES_P.  Indirect references are counted separately depending
-   on whether they are store or load operations.  The counts are
-   stored in *NUM_STORES_P and *NUM_LOADS_P.  */
-
-void
-count_uses_and_derefs (tree ptr, gimple stmt, unsigned *num_uses_p,
-		       unsigned *num_loads_p, unsigned *num_stores_p)
-{
-  ssa_op_iter i;
-  tree use;
-
-  *num_uses_p = 0;
-  *num_loads_p = 0;
-  *num_stores_p = 0;
-
-  /* Find out the total number of uses of PTR in STMT.  */
-  FOR_EACH_SSA_TREE_OPERAND (use, stmt, i, SSA_OP_USE)
-    if (use == ptr)
-      (*num_uses_p)++;
-
-  /* Now count the number of indirect references to PTR.  This is
-     truly awful, but we don't have much choice.  There are no parent
-     pointers inside INDIRECT_REFs, so an expression like
-     '*x_1 = foo (x_1, *x_1)' needs to be traversed piece by piece to
-     find all the indirect and direct uses of x_1 inside.  The only
-     shortcut we can take is the fact that GIMPLE only allows
-     INDIRECT_REFs inside the expressions below.  */
-  if (is_gimple_assign (stmt)
-      || gimple_code (stmt) == GIMPLE_RETURN
-      || gimple_code (stmt) == GIMPLE_ASM
-      || is_gimple_call (stmt))
-    {
-      struct walk_stmt_info wi;
-      struct count_ptr_d count;
-
-      count.ptr = ptr;
-      count.num_stores = 0;
-      count.num_loads = 0;
-
-      memset (&wi, 0, sizeof (wi));
-      wi.info = &count;
-      walk_gimple_op (stmt, count_ptr_derefs, &wi);
-
-      *num_stores_p = count.num_stores;
-      *num_loads_p = count.num_loads;
-    }
-
-  gcc_assert (*num_uses_p >= *num_loads_p + *num_stores_p);
-}
-
-
 /* Replace the LHS of STMT, an assignment, either a GIMPLE_ASSIGN or a
    GIMPLE_CALL, with NLHS, in preparation for modifying the RHS to an
    expression with a different value.
diff --git a/gcc/tree-ssa.h b/gcc/tree-ssa.h
index ab1c920..89ea5c6 100644
--- a/gcc/tree-ssa.h
+++ b/gcc/tree-ssa.h
@@ -39,8 +39,6 @@ extern edge_var_map_vector *redirect_edge_var_map_vector (edge);
 extern void redirect_edge_var_map_destroy (void);
 extern edge ssa_redirect_edge (edge, basic_block);
 extern void flush_pending_stmts (edge);
-extern void count_uses_and_derefs (tree, gimple, unsigned *, unsigned *,
-				   unsigned *);
 extern void gimple_replace_ssa_lhs (gimple, tree);
 extern tree target_for_debug_bind (tree);
 extern void insert_debug_temp_for_var_def (gimple_stmt_iterator *, tree);
diff --git a/gcc/tree-vrp.c b/gcc/tree-vrp.c
index b74bed3..2a90430 100644
--- a/gcc/tree-vrp.c
+++ b/gcc/tree-vrp.c
@@ -4476,57 +4476,6 @@ fp_predicate (gimple stmt)
   return FLOAT_TYPE_P (TREE_TYPE (gimple_cond_lhs (stmt)));
 }
 
-
-/* If OP can be inferred to be non-zero after STMT executes, return true.  */
-
-static bool
-infer_nonnull_range (gimple stmt, tree op)
-{
-  /* We can only assume that a pointer dereference will yield
-     non-NULL if -fdelete-null-pointer-checks is enabled.  */
-  if (!flag_delete_null_pointer_checks
-      || !POINTER_TYPE_P (TREE_TYPE (op))
-      || gimple_code (stmt) == GIMPLE_ASM)
-    return false;
-
-  unsigned num_uses, num_loads, num_stores;
-
-  count_uses_and_derefs (op, stmt, &num_uses, &num_loads, &num_stores);
-  if (num_loads + num_stores > 0)
-    return true;
-
-  if (is_gimple_call (stmt) && !gimple_call_internal_p (stmt))
-    {
-      tree fntype = gimple_call_fntype (stmt);
-      tree attrs = TYPE_ATTRIBUTES (fntype);
-      for (; attrs; attrs = TREE_CHAIN (attrs))
-	{
-	  attrs = lookup_attribute ("nonnull", attrs);
-
-	  /* If "nonnull" wasn't specified, we know nothing about
-	     the argument.  */
-	  if (attrs == NULL_TREE)
-	    return false;
-
-	  /* If "nonnull" applies to all the arguments, then ARG
-	     is non-null.  */
-	  if (TREE_VALUE (attrs) == NULL_TREE)
-	    return true;
-
-	  /* Now see if op appears in the nonnull list.  */
-	  for (tree t = TREE_VALUE (attrs); t; t = TREE_CHAIN (t))
-	    {
-	      int idx = TREE_INT_CST_LOW (TREE_VALUE (t)) - 1;
-	      tree arg = gimple_call_arg (stmt, idx);
-	      if (op == arg)
-		return true;
-	    }
-	}
-    }
-
-  return false;
-}
-
 /* If the range of values taken by OP can be inferred after STMT executes,
    return the comparison code (COMP_CODE_P) and value (VAL_P) that
    describes the inferred range.  Return true if a range could be

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