xref: /dflybsd-src/contrib/gcc-4.7/gcc/tree-ssa-threadedge.c (revision 5ce9237cb80ae957ad31704a6f6ce5b9ae2dac53)

/* SSA Jump Threading
   Copyright (C) 2005, 2006, 2007, 2008, 2009, 2010, 2011
   Free Software Foundation, Inc.
   Contributed by Jeff Law  <law@redhat.com>

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 "tm.h"
#include "tree.h"
#include "flags.h"
#include "tm_p.h"
#include "basic-block.h"
#include "cfgloop.h"
#include "output.h"
#include "function.h"
#include "timevar.h"
#include "tree-dump.h"
#include "tree-flow.h"
#include "tree-pass.h"
#include "tree-ssa-propagate.h"
#include "langhooks.h"
#include "params.h"

/* To avoid code explosion due to jump threading, we limit the
   number of statements we are going to copy.  This variable
   holds the number of statements currently seen that we'll have
   to copy as part of the jump threading process.  */
static int stmt_count;

/* Array to record value-handles per SSA_NAME.  */
VEC(tree,heap) *ssa_name_values;

/* Set the value for the SSA name NAME to VALUE.  */

void
set_ssa_name_value (tree name, tree value)
{
  if (SSA_NAME_VERSION (name) >= VEC_length (tree, ssa_name_values))
    VEC_safe_grow_cleared (tree, heap, ssa_name_values,
			   SSA_NAME_VERSION (name) + 1);
  VEC_replace (tree, ssa_name_values, SSA_NAME_VERSION (name), value);
}

/* Initialize the per SSA_NAME value-handles array.  Returns it.  */
void
threadedge_initialize_values (void)
{
  gcc_assert (ssa_name_values == NULL);
  ssa_name_values = VEC_alloc(tree, heap, num_ssa_names);
}

/* Free the per SSA_NAME value-handle array.  */
void
threadedge_finalize_values (void)
{
  VEC_free(tree, heap, ssa_name_values);
}

/* Return TRUE if we may be able to thread an incoming edge into
   BB to an outgoing edge from BB.  Return FALSE otherwise.  */

bool
potentially_threadable_block (basic_block bb)
{
  gimple_stmt_iterator gsi;

  /* If BB has a single successor or a single predecessor, then
     there is no threading opportunity.  */
  if (single_succ_p (bb) || single_pred_p (bb))
    return false;

  /* If BB does not end with a conditional, switch or computed goto,
     then there is no threading opportunity.  */
  gsi = gsi_last_bb (bb);
  if (gsi_end_p (gsi)
      || ! gsi_stmt (gsi)
      || (gimple_code (gsi_stmt (gsi)) != GIMPLE_COND
	  && gimple_code (gsi_stmt (gsi)) != GIMPLE_GOTO
	  && gimple_code (gsi_stmt (gsi)) != GIMPLE_SWITCH))
    return false;

  return true;
}

/* Return the LHS of any ASSERT_EXPR where OP appears as the first
   argument to the ASSERT_EXPR and in which the ASSERT_EXPR dominates
   BB.  If no such ASSERT_EXPR is found, return OP.  */

static tree
lhs_of_dominating_assert (tree op, basic_block bb, gimple stmt)
{
  imm_use_iterator imm_iter;
  gimple use_stmt;
  use_operand_p use_p;

  FOR_EACH_IMM_USE_FAST (use_p, imm_iter, op)
    {
      use_stmt = USE_STMT (use_p);
      if (use_stmt != stmt
          && gimple_assign_single_p (use_stmt)
          && TREE_CODE (gimple_assign_rhs1 (use_stmt)) == ASSERT_EXPR
          && TREE_OPERAND (gimple_assign_rhs1 (use_stmt), 0) == op
	  && dominated_by_p (CDI_DOMINATORS, bb, gimple_bb (use_stmt)))
	{
	  return gimple_assign_lhs (use_stmt);
	}
    }
  return op;
}

/* We record temporary equivalences created by PHI nodes or
   statements within the target block.  Doing so allows us to
   identify more jump threading opportunities, even in blocks
   with side effects.

   We keep track of those temporary equivalences in a stack
   structure so that we can unwind them when we're done processing
   a particular edge.  This routine handles unwinding the data
   structures.  */

static void
remove_temporary_equivalences (VEC(tree, heap) **stack)
{
  while (VEC_length (tree, *stack) > 0)
    {
      tree prev_value, dest;

      dest = VEC_pop (tree, *stack);

      /* A NULL value indicates we should stop unwinding, otherwise
	 pop off the next entry as they're recorded in pairs.  */
      if (dest == NULL)
	break;

      prev_value = VEC_pop (tree, *stack);
      set_ssa_name_value (dest, prev_value);
    }
}

/* Record a temporary equivalence, saving enough information so that
   we can restore the state of recorded equivalences when we're
   done processing the current edge.  */

static void
record_temporary_equivalence (tree x, tree y, VEC(tree, heap) **stack)
{
  tree prev_x = SSA_NAME_VALUE (x);

  if (TREE_CODE (y) == SSA_NAME)
    {
      tree tmp = SSA_NAME_VALUE (y);
      y = tmp ? tmp : y;
    }

  set_ssa_name_value (x, y);
  VEC_reserve (tree, heap, *stack, 2);
  VEC_quick_push (tree, *stack, prev_x);
  VEC_quick_push (tree, *stack, x);
}

/* Record temporary equivalences created by PHIs at the target of the
   edge E.  Record unwind information for the equivalences onto STACK.

   If a PHI which prevents threading is encountered, then return FALSE
   indicating we should not thread this edge, else return TRUE.  */

static bool
record_temporary_equivalences_from_phis (edge e, VEC(tree, heap) **stack)
{
  gimple_stmt_iterator gsi;

  /* Each PHI creates a temporary equivalence, record them.
     These are context sensitive equivalences and will be removed
     later.  */
  for (gsi = gsi_start_phis (e->dest); !gsi_end_p (gsi); gsi_next (&gsi))
    {
      gimple phi = gsi_stmt (gsi);
      tree src = PHI_ARG_DEF_FROM_EDGE (phi, e);
      tree dst = gimple_phi_result (phi);

      /* If the desired argument is not the same as this PHI's result
	 and it is set by a PHI in E->dest, then we can not thread
	 through E->dest.  */
      if (src != dst
	  && TREE_CODE (src) == SSA_NAME
	  && gimple_code (SSA_NAME_DEF_STMT (src)) == GIMPLE_PHI
	  && gimple_bb (SSA_NAME_DEF_STMT (src)) == e->dest)
	return false;

      /* We consider any non-virtual PHI as a statement since it
	 count result in a constant assignment or copy operation.  */
      if (is_gimple_reg (dst))
	stmt_count++;

      record_temporary_equivalence (dst, src, stack);
    }
  return true;
}

/* Fold the RHS of an assignment statement and return it as a tree.
   May return NULL_TREE if no simplification is possible.  */

static tree
fold_assignment_stmt (gimple stmt)
{
  enum tree_code subcode = gimple_assign_rhs_code (stmt);

  switch (get_gimple_rhs_class (subcode))
    {
    case GIMPLE_SINGLE_RHS:
      return fold (gimple_assign_rhs1 (stmt));

    case GIMPLE_UNARY_RHS:
      {
        tree lhs = gimple_assign_lhs (stmt);
        tree op0 = gimple_assign_rhs1 (stmt);
        return fold_unary (subcode, TREE_TYPE (lhs), op0);
      }

    case GIMPLE_BINARY_RHS:
      {
        tree lhs = gimple_assign_lhs (stmt);
        tree op0 = gimple_assign_rhs1 (stmt);
        tree op1 = gimple_assign_rhs2 (stmt);
        return fold_binary (subcode, TREE_TYPE (lhs), op0, op1);
      }

    case GIMPLE_TERNARY_RHS:
      {
        tree lhs = gimple_assign_lhs (stmt);
        tree op0 = gimple_assign_rhs1 (stmt);
        tree op1 = gimple_assign_rhs2 (stmt);
        tree op2 = gimple_assign_rhs3 (stmt);

	/* Sadly, we have to handle conditional assignments specially
	   here, because fold expects all the operands of an expression
	   to be folded before the expression itself is folded, but we
	   can't just substitute the folded condition here.  */
        if (gimple_assign_rhs_code (stmt) == COND_EXPR)
	  op0 = fold (op0);

        return fold_ternary (subcode, TREE_TYPE (lhs), op0, op1, op2);
      }

    default:
      gcc_unreachable ();
    }
}

/* Try to simplify each statement in E->dest, ultimately leading to
   a simplification of the COND_EXPR at the end of E->dest.

   Record unwind information for temporary equivalences onto STACK.

   Use SIMPLIFY (a pointer to a callback function) to further simplify
   statements using pass specific information.

   We might consider marking just those statements which ultimately
   feed the COND_EXPR.  It's not clear if the overhead of bookkeeping
   would be recovered by trying to simplify fewer statements.

   If we are able to simplify a statement into the form
   SSA_NAME = (SSA_NAME | gimple invariant), then we can record
   a context sensitive equivalence which may help us simplify
   later statements in E->dest.  */

static gimple
record_temporary_equivalences_from_stmts_at_dest (edge e,
						  VEC(tree, heap) **stack,
						  tree (*simplify) (gimple,
								    gimple))
{
  gimple stmt = NULL;
  gimple_stmt_iterator gsi;
  int max_stmt_count;

  max_stmt_count = PARAM_VALUE (PARAM_MAX_JUMP_THREAD_DUPLICATION_STMTS);

  /* Walk through each statement in the block recording equivalences
     we discover.  Note any equivalences we discover are context
     sensitive (ie, are dependent on traversing E) and must be unwound
     when we're finished processing E.  */
  for (gsi = gsi_start_bb (e->dest); !gsi_end_p (gsi); gsi_next (&gsi))
    {
      tree cached_lhs = NULL;

      stmt = gsi_stmt (gsi);

      /* Ignore empty statements and labels.  */
      if (gimple_code (stmt) == GIMPLE_NOP
	  || gimple_code (stmt) == GIMPLE_LABEL
	  || is_gimple_debug (stmt))
	continue;

      /* If the statement has volatile operands, then we assume we
	 can not thread through this block.  This is overly
	 conservative in some ways.  */
      if (gimple_code (stmt) == GIMPLE_ASM && gimple_asm_volatile_p (stmt))
	return NULL;

      /* If duplicating this block is going to cause too much code
	 expansion, then do not thread through this block.  */
      stmt_count++;
      if (stmt_count > max_stmt_count)
	return NULL;

      /* If this is not a statement that sets an SSA_NAME to a new
	 value, then do not try to simplify this statement as it will
	 not simplify in any way that is helpful for jump threading.  */
      if ((gimple_code (stmt) != GIMPLE_ASSIGN
           || TREE_CODE (gimple_assign_lhs (stmt)) != SSA_NAME)
          && (gimple_code (stmt) != GIMPLE_CALL
              || gimple_call_lhs (stmt) == NULL_TREE
              || TREE_CODE (gimple_call_lhs (stmt)) != SSA_NAME))
	continue;

      /* The result of __builtin_object_size depends on all the arguments
	 of a phi node. Temporarily using only one edge produces invalid
	 results. For example

	 if (x < 6)
	   goto l;
	 else
	   goto l;

	 l:
	 r = PHI <&w[2].a[1](2), &a.a[6](3)>
	 __builtin_object_size (r, 0)

	 The result of __builtin_object_size is defined to be the maximum of
	 remaining bytes. If we use only one edge on the phi, the result will
	 change to be the remaining bytes for the corresponding phi argument.

	 Similarly for __builtin_constant_p:

	 r = PHI <1(2), 2(3)>
	 __builtin_constant_p (r)

	 Both PHI arguments are constant, but x ? 1 : 2 is still not
	 constant.  */

      if (is_gimple_call (stmt))
	{
	  tree fndecl = gimple_call_fndecl (stmt);
	  if (fndecl
	      && (DECL_FUNCTION_CODE (fndecl) == BUILT_IN_OBJECT_SIZE
		  || DECL_FUNCTION_CODE (fndecl) == BUILT_IN_CONSTANT_P))
	    continue;
	}

      /* At this point we have a statement which assigns an RHS to an
	 SSA_VAR on the LHS.  We want to try and simplify this statement
	 to expose more context sensitive equivalences which in turn may
	 allow us to simplify the condition at the end of the loop.

	 Handle simple copy operations as well as implied copies from
	 ASSERT_EXPRs.  */
      if (gimple_assign_single_p (stmt)
          && TREE_CODE (gimple_assign_rhs1 (stmt)) == SSA_NAME)
	cached_lhs = gimple_assign_rhs1 (stmt);
      else if (gimple_assign_single_p (stmt)
               && TREE_CODE (gimple_assign_rhs1 (stmt)) == ASSERT_EXPR)
	cached_lhs = TREE_OPERAND (gimple_assign_rhs1 (stmt), 0);
      else
	{
	  /* A statement that is not a trivial copy or ASSERT_EXPR.
	     We're going to temporarily copy propagate the operands
	     and see if that allows us to simplify this statement.  */
	  tree *copy;
	  ssa_op_iter iter;
	  use_operand_p use_p;
	  unsigned int num, i = 0;

	  num = NUM_SSA_OPERANDS (stmt, (SSA_OP_USE | SSA_OP_VUSE));
	  copy = XCNEWVEC (tree, num);

	  /* Make a copy of the uses & vuses into USES_COPY, then cprop into
	     the operands.  */
	  FOR_EACH_SSA_USE_OPERAND (use_p, stmt, iter, SSA_OP_USE | SSA_OP_VUSE)
	    {
	      tree tmp = NULL;
	      tree use = USE_FROM_PTR (use_p);

	      copy[i++] = use;
	      if (TREE_CODE (use) == SSA_NAME)
		tmp = SSA_NAME_VALUE (use);
	      if (tmp)
		SET_USE (use_p, tmp);
	    }

	  /* Try to fold/lookup the new expression.  Inserting the
	     expression into the hash table is unlikely to help.  */
          if (is_gimple_call (stmt))
            cached_lhs = fold_call_stmt (stmt, false);
	  else
            cached_lhs = fold_assignment_stmt (stmt);

          if (!cached_lhs
              || (TREE_CODE (cached_lhs) != SSA_NAME
                  && !is_gimple_min_invariant (cached_lhs)))
            cached_lhs = (*simplify) (stmt, stmt);

	  /* Restore the statement's original uses/defs.  */
	  i = 0;
	  FOR_EACH_SSA_USE_OPERAND (use_p, stmt, iter, SSA_OP_USE | SSA_OP_VUSE)
	    SET_USE (use_p, copy[i++]);

	  free (copy);
	}

      /* Record the context sensitive equivalence if we were able
	 to simplify this statement.  */
      if (cached_lhs
	  && (TREE_CODE (cached_lhs) == SSA_NAME
	      || is_gimple_min_invariant (cached_lhs)))
	record_temporary_equivalence (gimple_get_lhs (stmt), cached_lhs, stack);
    }
  return stmt;
}

/* Simplify the control statement at the end of the block E->dest.

   To avoid allocating memory unnecessarily, a scratch GIMPLE_COND
   is available to use/clobber in DUMMY_COND.

   Use SIMPLIFY (a pointer to a callback function) to further simplify
   a condition using pass specific information.

   Return the simplified condition or NULL if simplification could
   not be performed.  */

static tree
simplify_control_stmt_condition (edge e,
				 gimple stmt,
				 gimple dummy_cond,
				 tree (*simplify) (gimple, gimple),
				 bool handle_dominating_asserts)
{
  tree cond, cached_lhs;
  enum gimple_code code = gimple_code (stmt);

  /* For comparisons, we have to update both operands, then try
     to simplify the comparison.  */
  if (code == GIMPLE_COND)
    {
      tree op0, op1;
      enum tree_code cond_code;

      op0 = gimple_cond_lhs (stmt);
      op1 = gimple_cond_rhs (stmt);
      cond_code = gimple_cond_code (stmt);

      /* Get the current value of both operands.  */
      if (TREE_CODE (op0) == SSA_NAME)
	{
          tree tmp = SSA_NAME_VALUE (op0);
	  if (tmp)
	    op0 = tmp;
	}

      if (TREE_CODE (op1) == SSA_NAME)
	{
	  tree tmp = SSA_NAME_VALUE (op1);
	  if (tmp)
	    op1 = tmp;
	}

      if (handle_dominating_asserts)
	{
	  /* Now see if the operand was consumed by an ASSERT_EXPR
	     which dominates E->src.  If so, we want to replace the
	     operand with the LHS of the ASSERT_EXPR.  */
	  if (TREE_CODE (op0) == SSA_NAME)
	    op0 = lhs_of_dominating_assert (op0, e->src, stmt);

	  if (TREE_CODE (op1) == SSA_NAME)
	    op1 = lhs_of_dominating_assert (op1, e->src, stmt);
	}

      /* We may need to canonicalize the comparison.  For
	 example, op0 might be a constant while op1 is an
	 SSA_NAME.  Failure to canonicalize will cause us to
	 miss threading opportunities.  */
      if (tree_swap_operands_p (op0, op1, false))
	{
	  tree tmp;
	  cond_code = swap_tree_comparison (cond_code);
	  tmp = op0;
	  op0 = op1;
	  op1 = tmp;
	}

      /* Stuff the operator and operands into our dummy conditional
	 expression.  */
      gimple_cond_set_code (dummy_cond, cond_code);
      gimple_cond_set_lhs (dummy_cond, op0);
      gimple_cond_set_rhs (dummy_cond, op1);

      /* We absolutely do not care about any type conversions
         we only care about a zero/nonzero value.  */
      fold_defer_overflow_warnings ();

      cached_lhs = fold_binary (cond_code, boolean_type_node, op0, op1);
      if (cached_lhs)
	while (CONVERT_EXPR_P (cached_lhs))
          cached_lhs = TREE_OPERAND (cached_lhs, 0);

      fold_undefer_overflow_warnings ((cached_lhs
                                       && is_gimple_min_invariant (cached_lhs)),
				      stmt, WARN_STRICT_OVERFLOW_CONDITIONAL);

      /* If we have not simplified the condition down to an invariant,
	 then use the pass specific callback to simplify the condition.  */
      if (!cached_lhs
          || !is_gimple_min_invariant (cached_lhs))
        cached_lhs = (*simplify) (dummy_cond, stmt);

      return cached_lhs;
    }

  if (code == GIMPLE_SWITCH)
    cond = gimple_switch_index (stmt);
  else if (code == GIMPLE_GOTO)
    cond = gimple_goto_dest (stmt);
  else
    gcc_unreachable ();

  /* We can have conditionals which just test the state of a variable
     rather than use a relational operator.  These are simpler to handle.  */
  if (TREE_CODE (cond) == SSA_NAME)
    {
      cached_lhs = cond;

      /* Get the variable's current value from the equivalence chains.

	 It is possible to get loops in the SSA_NAME_VALUE chains
	 (consider threading the backedge of a loop where we have
	 a loop invariant SSA_NAME used in the condition.  */
      if (cached_lhs
	  && TREE_CODE (cached_lhs) == SSA_NAME
	  && SSA_NAME_VALUE (cached_lhs))
	cached_lhs = SSA_NAME_VALUE (cached_lhs);

      /* If we're dominated by a suitable ASSERT_EXPR, then
	 update CACHED_LHS appropriately.  */
      if (handle_dominating_asserts && TREE_CODE (cached_lhs) == SSA_NAME)
	cached_lhs = lhs_of_dominating_assert (cached_lhs, e->src, stmt);

      /* If we haven't simplified to an invariant yet, then use the
	 pass specific callback to try and simplify it further.  */
      if (cached_lhs && ! is_gimple_min_invariant (cached_lhs))
        cached_lhs = (*simplify) (stmt, stmt);
    }
  else
    cached_lhs = NULL;

  return cached_lhs;
}

/* Return TRUE if the statement at the end of e->dest depends on
   the output of any statement in BB.   Otherwise return FALSE.

   This is used when we are threading a backedge and need to ensure
   that temporary equivalences from BB do not affect the condition
   in e->dest.  */

static bool
cond_arg_set_in_bb (edge e, basic_block bb)
{
  ssa_op_iter iter;
  use_operand_p use_p;
  gimple last = last_stmt (e->dest);

  /* E->dest does not have to end with a control transferring
     instruction.  This can occurr when we try to extend a jump
     threading opportunity deeper into the CFG.  In that case
     it is safe for this check to return false.  */
  if (!last)
    return false;

  if (gimple_code (last) != GIMPLE_COND
      && gimple_code (last) != GIMPLE_GOTO
      && gimple_code (last) != GIMPLE_SWITCH)
    return false;

  FOR_EACH_SSA_USE_OPERAND (use_p, last, iter, SSA_OP_USE | SSA_OP_VUSE)
    {
      tree use = USE_FROM_PTR (use_p);

      if (TREE_CODE (use) == SSA_NAME
	  && gimple_code (SSA_NAME_DEF_STMT (use)) != GIMPLE_PHI
	  && gimple_bb (SSA_NAME_DEF_STMT (use)) == bb)
	return true;
    }
  return false;
}

/* TAKEN_EDGE represents the an edge taken as a result of jump threading.
   See if we can thread around TAKEN_EDGE->dest as well.  If so, return
   the edge out of TAKEN_EDGE->dest that we can statically compute will be
   traversed.

   We are much more restrictive as to the contents of TAKEN_EDGE->dest
   as the path isolation code in tree-ssa-threadupdate.c isn't prepared
   to handle copying intermediate blocks on a threaded path.

   Long term a more consistent and structured approach to path isolation
   would be a huge help.   */
static edge
thread_around_empty_block (edge taken_edge,
			   gimple dummy_cond,
			   bool handle_dominating_asserts,
			   tree (*simplify) (gimple, gimple),
			   bitmap visited)
{
  basic_block bb = taken_edge->dest;
  gimple_stmt_iterator gsi;
  gimple stmt;
  tree cond;

  /* This block must have a single predecessor (E->dest).  */
  if (!single_pred_p (bb))
    return NULL;

  /* This block must have more than one successor.  */
  if (single_succ_p (bb))
    return NULL;

  /* This block can have no PHI nodes.  This is overly conservative.  */
  if (!gsi_end_p (gsi_start_phis (bb)))
    return NULL;

  /* Skip over DEBUG statements at the start of the block.  */
  gsi = gsi_start_nondebug_bb (bb);

  if (gsi_end_p (gsi))
    return NULL;

  /* This block can have no statements other than its control altering
     statement.  This is overly conservative.  */
  stmt = gsi_stmt (gsi);
  if (gimple_code (stmt) != GIMPLE_COND
      && gimple_code (stmt) != GIMPLE_GOTO
      && gimple_code (stmt) != GIMPLE_SWITCH)
    return NULL;

  /* Extract and simplify the condition.  */
  cond = simplify_control_stmt_condition (taken_edge, stmt, dummy_cond,
					  simplify, handle_dominating_asserts);

  /* If the condition can be statically computed and we have not already
     visited the destination edge, then add the taken edge to our thread
     path.  */
  if (cond && is_gimple_min_invariant (cond))
    {
      edge taken_edge = find_taken_edge (bb, cond);

      if (bitmap_bit_p (visited, taken_edge->dest->index))
	return NULL;
      bitmap_set_bit (visited, taken_edge->dest->index);
      return taken_edge;
    }

  return NULL;
}

/* E1 and E2 are edges into the same basic block.  Return TRUE if the
   PHI arguments associated with those edges are equal or there are no
   PHI arguments, otherwise return FALSE.  */

static bool
phi_args_equal_on_edges (edge e1, edge e2)
{
  gimple_stmt_iterator gsi;
  int indx1 = e1->dest_idx;
  int indx2 = e2->dest_idx;

  for (gsi = gsi_start_phis (e1->dest); !gsi_end_p (gsi); gsi_next (&gsi))
    {
      gimple phi = gsi_stmt (gsi);

      if (!operand_equal_p (gimple_phi_arg_def (phi, indx1),
			    gimple_phi_arg_def (phi, indx2), 0))
	return false;
    }
  return true;
}

/* We are exiting E->src, see if E->dest ends with a conditional
   jump which has a known value when reached via E.

   Special care is necessary if E is a back edge in the CFG as we
   may have already recorded equivalences for E->dest into our
   various tables, including the result of the conditional at
   the end of E->dest.  Threading opportunities are severely
   limited in that case to avoid short-circuiting the loop
   incorrectly.

   Note it is quite common for the first block inside a loop to
   end with a conditional which is either always true or always
   false when reached via the loop backedge.  Thus we do not want
   to blindly disable threading across a loop backedge.

   DUMMY_COND is a shared cond_expr used by condition simplification as scratch,
   to avoid allocating memory.

   HANDLE_DOMINATING_ASSERTS is true if we should try to replace operands of
   the simplified condition with left-hand sides of ASSERT_EXPRs they are
   used in.

   STACK is used to undo temporary equivalences created during the walk of
   E->dest.

   SIMPLIFY is a pass-specific function used to simplify statements.  */

void
thread_across_edge (gimple dummy_cond,
		    edge e,
		    bool handle_dominating_asserts,
		    VEC(tree, heap) **stack,
		    tree (*simplify) (gimple, gimple))
{
  gimple stmt;

  /* If E is a backedge, then we want to verify that the COND_EXPR,
     SWITCH_EXPR or GOTO_EXPR at the end of e->dest is not affected
     by any statements in e->dest.  If it is affected, then it is not
     safe to thread this edge.  */
  if (e->flags & EDGE_DFS_BACK)
    {
      if (cond_arg_set_in_bb (e, e->dest))
	goto fail;
    }

  stmt_count = 0;

  /* PHIs create temporary equivalences.  */
  if (!record_temporary_equivalences_from_phis (e, stack))
    goto fail;

  /* Now walk each statement recording any context sensitive
     temporary equivalences we can detect.  */
  stmt = record_temporary_equivalences_from_stmts_at_dest (e, stack, simplify);
  if (!stmt)
    goto fail;

  /* If we stopped at a COND_EXPR or SWITCH_EXPR, see if we know which arm
     will be taken.  */
  if (gimple_code (stmt) == GIMPLE_COND
      || gimple_code (stmt) == GIMPLE_GOTO
      || gimple_code (stmt) == GIMPLE_SWITCH)
    {
      tree cond;

      /* Extract and simplify the condition.  */
      cond = simplify_control_stmt_condition (e, stmt, dummy_cond, simplify,
					      handle_dominating_asserts);

      if (cond && is_gimple_min_invariant (cond))
	{
	  edge taken_edge = find_taken_edge (e->dest, cond);
	  basic_block dest = (taken_edge ? taken_edge->dest : NULL);
	  bitmap visited;
	  edge e2;

	  if (dest == e->dest)
	    goto fail;

	  /* DEST could be null for a computed jump to an absolute
	     address.  If DEST is not null, then see if we can thread
	     through it as well, this helps capture secondary effects
	     of threading without having to re-run DOM or VRP.  */
	  if (dest
	      && ((e->flags & EDGE_DFS_BACK) == 0
		  || ! cond_arg_set_in_bb (taken_edge, e->dest)))
	    {
	      /* We don't want to thread back to a block we have already
 		 visited.  This may be overly conservative.  */
	      visited = BITMAP_ALLOC (NULL);
	      bitmap_set_bit (visited, dest->index);
	      bitmap_set_bit (visited, e->dest->index);
	      do
		{
		  e2 = thread_around_empty_block (taken_edge,
						  dummy_cond,
						  handle_dominating_asserts,
						  simplify,
						  visited);
		  if (e2)
		    taken_edge = e2;
		}
	      while (e2);
	      BITMAP_FREE (visited);
	    }

	  remove_temporary_equivalences (stack);
	  register_jump_thread (e, taken_edge, NULL);
	  return;
	}
    }

 /* We were unable to determine what out edge from E->dest is taken.  However,
    we might still be able to thread through successors of E->dest.  This
    often occurs when E->dest is a joiner block which then fans back out
    based on redundant tests.

    If so, we'll copy E->dest and redirect the appropriate predecessor to
    the copy.  Within the copy of E->dest, we'll thread one or more edges
    to points deeper in the CFG.

    This is a stopgap until we have a more structured approach to path
    isolation.  */
  {
    edge e2, e3, taken_edge;
    edge_iterator ei;
    bool found = false;
    bitmap visited = BITMAP_ALLOC (NULL);

    /* Look at each successor of E->dest to see if we can thread through it.  */
    FOR_EACH_EDGE (taken_edge, ei, e->dest->succs)
      {
	/* Avoid threading to any block we have already visited.  */
	bitmap_clear (visited);
	bitmap_set_bit (visited, taken_edge->dest->index);
	bitmap_set_bit (visited, e->dest->index);

	/* Record whether or not we were able to thread through a successor
	   of E->dest.  */
	found = false;
	e3 = taken_edge;
	do
	  {
	    if ((e->flags & EDGE_DFS_BACK) == 0
		|| ! cond_arg_set_in_bb (e3, e->dest))
	      e2 = thread_around_empty_block (e3,
					      dummy_cond,
					      handle_dominating_asserts,
					      simplify,
					      visited);
	    else
	      e2 = NULL;

	    if (e2)
	      {
	        e3 = e2;
		found = true;
	      }
	  }
        while (e2);

	/* If we were able to thread through a successor of E->dest, then
	   record the jump threading opportunity.  */
	if (found)
	  {
	    edge tmp;
	    /* If there is already an edge from the block to be duplicated
	       (E2->src) to the final target (E3->dest), then make sure that
	       the PHI args associated with the edges E2 and E3 are the
	       same.  */
	    tmp = find_edge (taken_edge->src, e3->dest);
	    if (!tmp || phi_args_equal_on_edges (tmp, e3))
	      register_jump_thread (e, taken_edge, e3);
	  }

      }
    BITMAP_FREE (visited);
  }

 fail:
  remove_temporary_equivalences (stack);
}