text: /* Tail call optimization on trees.
     Copyright (C) 2003, 2004, 2005, 2006, 2007, 2008, 2009, 2010, 2011, 2012
     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 "tm.h"
  #include "tree.h"
  #include "tm_p.h"
  #include "basic-block.h"
  #include "function.h"
  #include "tree-flow.h"
  #include "tree-dump.h"
  #include "gimple-pretty-print.h"
  #include "except.h"
  #include "tree-pass.h"
  #include "flags.h"
  #include "langhooks.h"
  #include "dbgcnt.h"
  #include "target.h"
  #include "common/common-target.h"
  
  /* The file implements the tail recursion elimination.  It is also used to
     analyze the tail calls in general, passing the results to the rtl level
     where they are used for sibcall optimization.
  
     In addition to the standard tail recursion elimination, we handle the most
     trivial cases of making the call tail recursive by creating accumulators.
     For example the following function
  
     int sum (int n)
     {
       if (n > 0)
         return n + sum (n - 1);
       else
         return 0;
     }
  
     is transformed into
  
     int sum (int n)
     {
       int acc = 0;
  
       while (n > 0)
         acc += n--;
  
       return acc;
     }
  
     To do this, we maintain two accumulators (a_acc and m_acc) that indicate
     when we reach the return x statement, we should return a_acc + x * m_acc
     instead.  They are initially initialized to 0 and 1, respectively,
     so the semantics of the function is obviously preserved.  If we are
     guaranteed that the value of the accumulator never change, we
     omit the accumulator.
  
     There are three cases how the function may exit.  The first one is
     handled in adjust_return_value, the other two in adjust_accumulator_values
     (the second case is actually a special case of the third one and we
     present it separately just for clarity):
  
     1) Just return x, where x is not in any of the remaining special shapes.
        We rewrite this to a gimple equivalent of return m_acc * x + a_acc.
  
     2) return f (...), where f is the current function, is rewritten in a
        classical tail-recursion elimination way, into assignment of arguments
        and jump to the start of the function.  Values of the accumulators
        are unchanged.
  
     3) return a + m * f(...), where a and m do not depend on call to f.
        To preserve the semantics described before we want this to be rewritten
        in such a way that we finally return
  
        a_acc + (a + m * f(...)) * m_acc = (a_acc + a * m_acc) + (m * m_acc) * f(...).
  
        I.e. we increase a_acc by a * m_acc, multiply m_acc by m and
        eliminate the tail call to f.  Special cases when the value is just
        added or just multiplied are obtained by setting a = 0 or m = 1.
  
     TODO -- it is possible to do similar tricks for other operations.  */
  
  /* A structure that describes the tailcall.  */
  
  struct tailcall
  {
    /* The iterator pointing to the call statement.  */
    gimple_stmt_iterator call_gsi;
  
    /* True if it is a call to the current function.  */
    bool tail_recursion;
  
    /* The return value of the caller is mult * f + add, where f is the return
       value of the call.  */
    tree mult, add;
  
    /* Next tailcall in the chain.  */
    struct tailcall *next;
  };
  
  /* The variables holding the value of multiplicative and additive
     accumulator.  */
  static tree m_acc, a_acc;
  
  static bool suitable_for_tail_opt_p (void);
  static bool optimize_tail_call (struct tailcall *, bool);
  static void eliminate_tail_call (struct tailcall *);
  static void find_tail_calls (basic_block, struct tailcall **);
  
  /* Returns false when the function is not suitable for tail call optimization
     from some reason (e.g. if it takes variable number of arguments).  */
  
  static bool
  suitable_for_tail_opt_p (void)
  {
    if (cfun->stdarg)
      return false;
  
    return true;
  }
  /* Returns false when the function is not suitable for tail call optimization
     from some reason (e.g. if it takes variable number of arguments).
     This test must pass in addition to suitable_for_tail_opt_p in order to make
     tail call discovery happen.  */
  
  static bool
  suitable_for_tail_call_opt_p (void)
  {
    tree param;
  
    /* alloca (until we have stack slot life analysis) inhibits
       sibling call optimizations, but not tail recursion.  */
    if (cfun->calls_alloca)
      return false;
  
    /* If we are using sjlj exceptions, we may need to add a call to
       _Unwind_SjLj_Unregister at exit of the function.  Which means
       that we cannot do any sibcall transformations.  */
    if (targetm_common.except_unwind_info (&global_options) == UI_SJLJ
        && current_function_has_exception_handlers ())
      return false;
  
    /* Any function that calls setjmp might have longjmp called from
       any called function.  ??? We really should represent this
       properly in the CFG so that this needn't be special cased.  */
    if (cfun->calls_setjmp)
      return false;
  
    /* ??? It is OK if the argument of a function is taken in some cases,
       but not in all cases.  See PR15387 and PR19616.  Revisit for 4.1.  */
    for (param = DECL_ARGUMENTS (current_function_decl);
         param;
         param = DECL_CHAIN (param))
      if (TREE_ADDRESSABLE (param))
        return false;
  
    return true;
  }
  
  /* Checks whether the expression EXPR in stmt AT is independent of the
     statement pointed to by GSI (in a sense that we already know EXPR's value
     at GSI).  We use the fact that we are only called from the chain of
     basic blocks that have only single successor.  Returns the expression
     containing the value of EXPR at GSI.  */
  
  static tree
  independent_of_stmt_p (tree expr, gimple at, gimple_stmt_iterator gsi)
  {
    basic_block bb, call_bb, at_bb;
    edge e;
    edge_iterator ei;
  
    if (is_gimple_min_invariant (expr))
      return expr;
  
    if (TREE_CODE (expr) != SSA_NAME)
      return NULL_TREE;
  
    /* Mark the blocks in the chain leading to the end.  */
    at_bb = gimple_bb (at);
    call_bb = gimple_bb (gsi_stmt (gsi));
    for (bb = call_bb; bb != at_bb; bb = single_succ (bb))
      bb->aux = &bb->aux;
    bb->aux = &bb->aux;
  
    while (1)
      {
        at = SSA_NAME_DEF_STMT (expr);
        bb = gimple_bb (at);
  
        /* The default definition or defined before the chain.  */
        if (!bb || !bb->aux)
  	break;
  
        if (bb == call_bb)
  	{
  	  for (; !gsi_end_p (gsi); gsi_next (&gsi))
  	    if (gsi_stmt (gsi) == at)
  	      break;
  
  	  if (!gsi_end_p (gsi))
  	    expr = NULL_TREE;
  	  break;
  	}
  
        if (gimple_code (at) != GIMPLE_PHI)
  	{
  	  expr = NULL_TREE;
  	  break;
  	}
  
        FOR_EACH_EDGE (e, ei, bb->preds)
  	if (e->src->aux)
  	  break;
        gcc_assert (e);
  
        expr = PHI_ARG_DEF_FROM_EDGE (at, e);
        if (TREE_CODE (expr) != SSA_NAME)
  	{
  	  /* The value is a constant.  */
  	  break;
  	}
      }
  
    /* Unmark the blocks.  */
    for (bb = call_bb; bb != at_bb; bb = single_succ (bb))
      bb->aux = NULL;
    bb->aux = NULL;
  
    return expr;
  }
  
  /* Simulates the effect of an assignment STMT on the return value of the tail
     recursive CALL passed in ASS_VAR.  M and A are the multiplicative and the
     additive factor for the real return value.  */
  
  static bool
  process_assignment (gimple stmt, gimple_stmt_iterator call, tree *m,
  		    tree *a, tree *ass_var)
  {
    tree op0, op1 = NULL_TREE, non_ass_var = NULL_TREE;
    tree dest = gimple_assign_lhs (stmt);
    enum tree_code code = gimple_assign_rhs_code (stmt);
    enum gimple_rhs_class rhs_class = get_gimple_rhs_class (code);
    tree src_var = gimple_assign_rhs1 (stmt);
  
    /* See if this is a simple copy operation of an SSA name to the function
       result.  In that case we may have a simple tail call.  Ignore type
       conversions that can never produce extra code between the function
       call and the function return.  */
    if ((rhs_class == GIMPLE_SINGLE_RHS || gimple_assign_cast_p (stmt))
        && (TREE_CODE (src_var) == SSA_NAME))
      {
        /* Reject a tailcall if the type conversion might need
  	 additional code.  */
        if (gimple_assign_cast_p (stmt)
  	  && TYPE_MODE (TREE_TYPE (dest)) != TYPE_MODE (TREE_TYPE (src_var)))
  	return false;
  
        if (src_var != *ass_var)
  	return false;
  
        *ass_var = dest;
        return true;
      }
  
    switch (rhs_class)
      {
      case GIMPLE_BINARY_RHS:
        op1 = gimple_assign_rhs2 (stmt);
  
        /* Fall through.  */
  
      case GIMPLE_UNARY_RHS:
        op0 = gimple_assign_rhs1 (stmt);
        break;
  
      default:
        return false;
      }
  
    /* Accumulator optimizations will reverse the order of operations.
       We can only do that for floating-point types if we're assuming
       that addition and multiplication are associative.  */
    if (!flag_associative_math)
      if (FLOAT_TYPE_P (TREE_TYPE (DECL_RESULT (current_function_decl))))
        return false;
  
    if (rhs_class == GIMPLE_UNARY_RHS)
      ;
    else if (op0 == *ass_var
        && (non_ass_var = independent_of_stmt_p (op1, stmt, call)))
      ;
    else if (op1 == *ass_var
  	   && (non_ass_var = independent_of_stmt_p (op0, stmt, call)))
      ;
    else
      return false;
  
    switch (code)
      {
      case PLUS_EXPR:
        *a = non_ass_var;
        *ass_var = dest;
        return true;
  
      case MULT_EXPR:
        *m = non_ass_var;
        *ass_var = dest;
        return true;
  
      case NEGATE_EXPR:
        if (FLOAT_TYPE_P (TREE_TYPE (op0)))
          *m = build_real (TREE_TYPE (op0), dconstm1);
        else
          *m = build_int_cst (TREE_TYPE (op0), -1);
  
        *ass_var = dest;
        return true;
  
      case MINUS_EXPR:
        if (*ass_var == op0)
          *a = fold_build1 (NEGATE_EXPR, TREE_TYPE (non_ass_var), non_ass_var);
        else
          {
            if (FLOAT_TYPE_P (TREE_TYPE (non_ass_var)))
              *m = build_real (TREE_TYPE (non_ass_var), dconstm1);
            else
              *m = build_int_cst (TREE_TYPE (non_ass_var), -1);
  
            *a = fold_build1 (NEGATE_EXPR, TREE_TYPE (non_ass_var), non_ass_var);
          }
  
        *ass_var = dest;
        return true;
  
        /* TODO -- Handle POINTER_PLUS_EXPR.  */
  
      default:
        return false;
      }
  }
  
  /* Propagate VAR through phis on edge E.  */
  
  static tree
  propagate_through_phis (tree var, edge e)
  {
    basic_block dest = e->dest;
    gimple_stmt_iterator gsi;
  
    for (gsi = gsi_start_phis (dest); !gsi_end_p (gsi); gsi_next (&gsi))
      {
        gimple phi = gsi_stmt (gsi);
        if (PHI_ARG_DEF_FROM_EDGE (phi, e) == var)
          return PHI_RESULT (phi);
      }
    return var;
  }
  
  /* Finds tailcalls falling into basic block BB. The list of found tailcalls is
     added to the start of RET.  */
  
  static void
  find_tail_calls (basic_block bb, struct tailcall **ret)
  {
    tree ass_var = NULL_TREE, ret_var, func, param;
    gimple stmt, call = NULL;
    gimple_stmt_iterator gsi, agsi;
    bool tail_recursion;
    struct tailcall *nw;
    edge e;
    tree m, a;
    basic_block abb;
    size_t idx;
    tree var;
    referenced_var_iterator rvi;
  
    if (!single_succ_p (bb))
      return;
  
    for (gsi = gsi_last_bb (bb); !gsi_end_p (gsi); gsi_prev (&gsi))
      {
        stmt = gsi_stmt (gsi);
  
        /* Ignore labels, returns, clobbers and debug stmts.  */
        if (gimple_code (stmt) == GIMPLE_LABEL
  	  || gimple_code (stmt) == GIMPLE_RETURN
  	  || gimple_clobber_p (stmt)
  	  || is_gimple_debug (stmt))
  	continue;
  
        /* Check for a call.  */
        if (is_gimple_call (stmt))
  	{
  	  call = stmt;
  	  ass_var = gimple_call_lhs (stmt);
  	  break;
  	}
  
        /* If the statement references memory or volatile operands, fail.  */
        if (gimple_references_memory_p (stmt)
  	  || gimple_has_volatile_ops (stmt))
  	return;
      }
  
    if (gsi_end_p (gsi))
      {
        edge_iterator ei;
        /* Recurse to the predecessors.  */
        FOR_EACH_EDGE (e, ei, bb->preds)
  	find_tail_calls (e->src, ret);
  
        return;
      }
  
    /* If the LHS of our call is not just a simple register, we can't
       transform this into a tail or sibling call.  This situation happens,
       in (e.g.) "*p = foo()" where foo returns a struct.  In this case
       we won't have a temporary here, but we need to carry out the side
       effect anyway, so tailcall is impossible.
  
       ??? In some situations (when the struct is returned in memory via
       invisible argument) we could deal with this, e.g. by passing 'p'
       itself as that argument to foo, but it's too early to do this here,
       and expand_call() will not handle it anyway.  If it ever can, then
       we need to revisit this here, to allow that situation.  */
    if (ass_var && !is_gimple_reg (ass_var))
      return;
  
    /* We found the call, check whether it is suitable.  */
    tail_recursion = false;
    func = gimple_call_fndecl (call);
    if (func == current_function_decl)
      {
        tree arg;
  
        for (param = DECL_ARGUMENTS (func), idx = 0;
  	   param && idx < gimple_call_num_args (call);
  	   param = DECL_CHAIN (param), idx ++)
  	{
  	  arg = gimple_call_arg (call, idx);
  	  if (param != arg)
  	    {
  	      /* Make sure there are no problems with copying.  The parameter
  	         have a copyable type and the two arguments must have reasonably
  	         equivalent types.  The latter requirement could be relaxed if
  	         we emitted a suitable type conversion statement.  */
  	      if (!is_gimple_reg_type (TREE_TYPE (param))
  		  || !useless_type_conversion_p (TREE_TYPE (param),
  					         TREE_TYPE (arg)))
  		break;
  
  	      /* The parameter should be a real operand, so that phi node
  		 created for it at the start of the function has the meaning
  		 of copying the value.  This test implies is_gimple_reg_type
  		 from the previous condition, however this one could be
  		 relaxed by being more careful with copying the new value
  		 of the parameter (emitting appropriate GIMPLE_ASSIGN and
  		 updating the virtual operands).  */
  	      if (!is_gimple_reg (param))
  		break;
  	    }
  	}
        if (idx == gimple_call_num_args (call) && !param)
  	tail_recursion = true;
      }
  
    /* Make sure the tail invocation of this function does not refer
       to local variables.  */
    FOR_EACH_REFERENCED_VAR (cfun, var, rvi)
      {
        if (TREE_CODE (var) != PARM_DECL
  	  && auto_var_in_fn_p (var, cfun->decl)
  	  && (ref_maybe_used_by_stmt_p (call, var)
  	      || call_may_clobber_ref_p (call, var)))
  	return;
      }
  
    /* Now check the statements after the call.  None of them has virtual
       operands, so they may only depend on the call through its return
       value.  The return value should also be dependent on each of them,
       since we are running after dce.  */
    m = NULL_TREE;
    a = NULL_TREE;
  
    abb = bb;
    agsi = gsi;
    while (1)
      {
        tree tmp_a = NULL_TREE;
        tree tmp_m = NULL_TREE;
        gsi_next (&agsi);
  
        while (gsi_end_p (agsi))
  	{
  	  ass_var = propagate_through_phis (ass_var, single_succ_edge (abb));
  	  abb = single_succ (abb);
  	  agsi = gsi_start_bb (abb);
  	}
  
        stmt = gsi_stmt (agsi);
  
        if (gimple_code (stmt) == GIMPLE_LABEL)
  	continue;
  
        if (gimple_code (stmt) == GIMPLE_RETURN)
  	break;
  
        if (gimple_clobber_p (stmt))
  	continue;
  
        if (is_gimple_debug (stmt))
  	continue;
  
        if (gimple_code (stmt) != GIMPLE_ASSIGN)
  	return;
  
        /* This is a gimple assign. */
        if (! process_assignment (stmt, gsi, &tmp_m, &tmp_a, &ass_var))
  	return;
  
        if (tmp_a)
  	{
  	  tree type = TREE_TYPE (tmp_a);
  	  if (a)
  	    a = fold_build2 (PLUS_EXPR, type, fold_convert (type, a), tmp_a);
  	  else
  	    a = tmp_a;
  	}
        if (tmp_m)
  	{
  	  tree type = TREE_TYPE (tmp_m);
  	  if (m)
  	    m = fold_build2 (MULT_EXPR, type, fold_convert (type, m), tmp_m);
  	  else
  	    m = tmp_m;
  
  	  if (a)
  	    a = fold_build2 (MULT_EXPR, type, fold_convert (type, a), tmp_m);
  	}
      }
  
    /* See if this is a tail call we can handle.  */
    ret_var = gimple_return_retval (stmt);
  
    /* We may proceed if there either is no return value, or the return value
       is identical to the call's return.  */
    if (ret_var
        && (ret_var != ass_var))
      return;
  
    /* If this is not a tail recursive call, we cannot handle addends or
       multiplicands.  */
    if (!tail_recursion && (m || a))
      return;
  
    nw = XNEW (struct tailcall);
  
    nw->call_gsi = gsi;
  
    nw->tail_recursion = tail_recursion;
  
    nw->mult = m;
    nw->add = a;
  
    nw->next = *ret;
    *ret = nw;
  }
  
  /* Helper to insert PHI_ARGH to the phi of VAR in the destination of edge E.  */
  
  static void
  add_successor_phi_arg (edge e, tree var, tree phi_arg)
  {
    gimple_stmt_iterator gsi;
  
    for (gsi = gsi_start_phis (e->dest); !gsi_end_p (gsi); gsi_next (&gsi))
      if (PHI_RESULT (gsi_stmt (gsi)) == var)
        break;
  
    gcc_assert (!gsi_end_p (gsi));
    add_phi_arg (gsi_stmt (gsi), phi_arg, e, UNKNOWN_LOCATION);
  }
  
  /* Creates a GIMPLE statement which computes the operation specified by
     CODE, ACC and OP1 to a new variable with name LABEL and inserts the
     statement in the position specified by GSI.  Returns the
     tree node of the statement's result.  */
  
  static tree
  adjust_return_value_with_ops (enum tree_code code, const char *label,
  			      tree acc, tree op1, gimple_stmt_iterator gsi)
  {
  
    tree ret_type = TREE_TYPE (DECL_RESULT (current_function_decl));
    tree tmp = create_tmp_reg (ret_type, label);
    gimple stmt;
    tree result;
  
    add_referenced_var (tmp);
  
    if (types_compatible_p (TREE_TYPE (acc), TREE_TYPE (op1)))
      stmt = gimple_build_assign_with_ops (code, tmp, acc, op1);
    else
      {
        tree rhs = fold_convert (TREE_TYPE (acc),
  			       fold_build2 (code,
  					    TREE_TYPE (op1),
  					    fold_convert (TREE_TYPE (op1), acc),
  					    op1));
        rhs = force_gimple_operand_gsi (&gsi, rhs,
  				      false, NULL, true, GSI_SAME_STMT);
        stmt = gimple_build_assign (NULL_TREE, rhs);
      }
  
    result = make_ssa_name (tmp, stmt);
    gimple_assign_set_lhs (stmt, result);
    update_stmt (stmt);
    gsi_insert_before (&gsi, stmt, GSI_NEW_STMT);
    return result;
  }
  
  /* Creates a new GIMPLE statement that adjusts the value of accumulator ACC by
     the computation specified by CODE and OP1 and insert the statement
     at the position specified by GSI as a new statement.  Returns new SSA name
     of updated accumulator.  */
  
  static tree
  update_accumulator_with_ops (enum tree_code code, tree acc, tree op1,
  			     gimple_stmt_iterator gsi)
  {
    gimple stmt;
    tree var;
    if (types_compatible_p (TREE_TYPE (acc), TREE_TYPE (op1)))
      stmt = gimple_build_assign_with_ops (code, SSA_NAME_VAR (acc), acc, op1);
    else
      {
        tree rhs = fold_convert (TREE_TYPE (acc),
  			       fold_build2 (code,
  					    TREE_TYPE (op1),
  					    fold_convert (TREE_TYPE (op1), acc),
  					    op1));
        rhs = force_gimple_operand_gsi (&gsi, rhs,
  				      false, NULL, false, GSI_CONTINUE_LINKING);
        stmt = gimple_build_assign (NULL_TREE, rhs);
      }
    var = make_ssa_name (SSA_NAME_VAR (acc), stmt);
    gimple_assign_set_lhs (stmt, var);
    update_stmt (stmt);
    gsi_insert_after (&gsi, stmt, GSI_NEW_STMT);
    return var;
  }
  
  /* Adjust the accumulator values according to A and M after GSI, and update
     the phi nodes on edge BACK.  */
  
  static void
  adjust_accumulator_values (gimple_stmt_iterator gsi, tree m, tree a, edge back)
  {
    tree var, a_acc_arg, m_acc_arg;
  
    if (m)
      m = force_gimple_operand_gsi (&gsi, m, true, NULL, true, GSI_SAME_STMT);
    if (a)
      a = force_gimple_operand_gsi (&gsi, a, true, NULL, true, GSI_SAME_STMT);
  
    a_acc_arg = a_acc;
    m_acc_arg = m_acc;
    if (a)
      {
        if (m_acc)
  	{
  	  if (integer_onep (a))
  	    var = m_acc;
  	  else
  	    var = adjust_return_value_with_ops (MULT_EXPR, "acc_tmp", m_acc,
  						a, gsi);
  	}
        else
  	var = a;
  
        a_acc_arg = update_accumulator_with_ops (PLUS_EXPR, a_acc, var, gsi);
      }
  
    if (m)
      m_acc_arg = update_accumulator_with_ops (MULT_EXPR, m_acc, m, gsi);
  
    if (a_acc)
      add_successor_phi_arg (back, a_acc, a_acc_arg);
  
    if (m_acc)
      add_successor_phi_arg (back, m_acc, m_acc_arg);
  }
  
  /* Adjust value of the return at the end of BB according to M and A
     accumulators.  */
  
  static void
  adjust_return_value (basic_block bb, tree m, tree a)
  {
    tree retval;
    gimple ret_stmt = gimple_seq_last_stmt (bb_seq (bb));
    gimple_stmt_iterator gsi = gsi_last_bb (bb);
  
    gcc_assert (gimple_code (ret_stmt) == GIMPLE_RETURN);
  
    retval = gimple_return_retval (ret_stmt);
    if (!retval || retval == error_mark_node)
      return;
  
    if (m)
      retval = adjust_return_value_with_ops (MULT_EXPR, "mul_tmp", m_acc, retval,
  					   gsi);
    if (a)
      retval = adjust_return_value_with_ops (PLUS_EXPR, "acc_tmp", a_acc, retval,
  					   gsi);
    gimple_return_set_retval (ret_stmt, retval);
    update_stmt (ret_stmt);
  }
  
  /* Subtract COUNT and FREQUENCY from the basic block and it's
     outgoing edge.  */
  static void
  decrease_profile (basic_block bb, gcov_type count, int frequency)
  {
    edge e;
    bb->count -= count;
    if (bb->count < 0)
      bb->count = 0;
    bb->frequency -= frequency;
    if (bb->frequency < 0)
      bb->frequency = 0;
    if (!single_succ_p (bb))
      {
        gcc_assert (!EDGE_COUNT (bb->succs));
        return;
      }
    e = single_succ_edge (bb);
    e->count -= count;
    if (e->count < 0)
      e->count = 0;
  }
  
  /* Returns true if argument PARAM of the tail recursive call needs to be copied
     when the call is eliminated.  */
  
  static bool
  arg_needs_copy_p (tree param)
  {
    tree def;
  
    if (!is_gimple_reg (param) || !var_ann (param))
      return false;
  
    /* Parameters that are only defined but never used need not be copied.  */
    def = gimple_default_def (cfun, param);
    if (!def)
      return false;
  
    return true;
  }
  
  /* Eliminates tail call described by T.  TMP_VARS is a list of
     temporary variables used to copy the function arguments.  */
  
  static void
  eliminate_tail_call (struct tailcall *t)
  {
    tree param, rslt;
    gimple stmt, call;
    tree arg;
    size_t idx;
    basic_block bb, first;
    edge e;
    gimple phi;
    gimple_stmt_iterator gsi;
    gimple orig_stmt;
  
    stmt = orig_stmt = gsi_stmt (t->call_gsi);
    bb = gsi_bb (t->call_gsi);
  
    if (dump_file && (dump_flags & TDF_DETAILS))
      {
        fprintf (dump_file, "Eliminated tail recursion in bb %d : ",
  	       bb->index);
        print_gimple_stmt (dump_file, stmt, 0, TDF_SLIM);
        fprintf (dump_file, "\n");
      }
  
    gcc_assert (is_gimple_call (stmt));
  
    first = single_succ (ENTRY_BLOCK_PTR);
  
    /* Remove the code after call_gsi that will become unreachable.  The
       possibly unreachable code in other blocks is removed later in
       cfg cleanup.  */
    gsi = t->call_gsi;
    gsi_next (&gsi);
    while (!gsi_end_p (gsi))
      {
        gimple t = gsi_stmt (gsi);
        /* Do not remove the return statement, so that redirect_edge_and_branch
  	 sees how the block ends.  */
        if (gimple_code (t) == GIMPLE_RETURN)
  	break;
  
        gsi_remove (&gsi, true);
        release_defs (t);
      }
  
    /* Number of executions of function has reduced by the tailcall.  */
    e = single_succ_edge (gsi_bb (t->call_gsi));
    decrease_profile (EXIT_BLOCK_PTR, e->count, EDGE_FREQUENCY (e));
    decrease_profile (ENTRY_BLOCK_PTR, e->count, EDGE_FREQUENCY (e));
    if (e->dest != EXIT_BLOCK_PTR)
      decrease_profile (e->dest, e->count, EDGE_FREQUENCY (e));
  
    /* Replace the call by a jump to the start of function.  */
    e = redirect_edge_and_branch (single_succ_edge (gsi_bb (t->call_gsi)),
  				first);
    gcc_assert (e);
    PENDING_STMT (e) = NULL;
  
    /* Add phi node entries for arguments.  The ordering of the phi nodes should
       be the same as the ordering of the arguments.  */
    for (param = DECL_ARGUMENTS (current_function_decl),
  	 idx = 0, gsi = gsi_start_phis (first);
         param;
         param = DECL_CHAIN (param), idx++)
      {
        if (!arg_needs_copy_p (param))
  	continue;
  
        arg = gimple_call_arg (stmt, idx);
        phi = gsi_stmt (gsi);
        gcc_assert (param == SSA_NAME_VAR (PHI_RESULT (phi)));
  
        add_phi_arg (phi, arg, e, gimple_location (stmt));
        gsi_next (&gsi);
      }
  
    /* Update the values of accumulators.  */
    adjust_accumulator_values (t->call_gsi, t->mult, t->add, e);
  
    call = gsi_stmt (t->call_gsi);
    rslt = gimple_call_lhs (call);
    if (rslt != NULL_TREE)
      {
        /* Result of the call will no longer be defined.  So adjust the
  	 SSA_NAME_DEF_STMT accordingly.  */
        SSA_NAME_DEF_STMT (rslt) = gimple_build_nop ();
      }
  
    gsi_remove (&t->call_gsi, true);
    release_defs (call);
  }
  
  /* Add phi nodes for the virtual operands defined in the function to the
     header of the loop created by tail recursion elimination.
  
     Originally, we used to add phi nodes only for call clobbered variables,
     as the value of the non-call clobbered ones obviously cannot be used
     or changed within the recursive call.  However, the local variables
     from multiple calls now share the same location, so the virtual ssa form
     requires us to say that the location dies on further iterations of the loop,
     which requires adding phi nodes.
  */
  static void
  add_virtual_phis (void)
  {
    referenced_var_iterator rvi;
    tree var;
  
    /* The problematic part is that there is no way how to know what
       to put into phi nodes (there in fact does not have to be such
       ssa name available).  A solution would be to have an artificial
       use/kill for all virtual operands in EXIT node.  Unless we have
       this, we cannot do much better than to rebuild the ssa form for
       possibly affected virtual ssa names from scratch.  */
  
    FOR_EACH_REFERENCED_VAR (cfun, var, rvi)
      {
        if (!is_gimple_reg (var) && gimple_default_def (cfun, var) != NULL_TREE)
  	mark_sym_for_renaming (var);
      }
  }
  
  /* Optimizes the tailcall described by T.  If OPT_TAILCALLS is true, also
     mark the tailcalls for the sibcall optimization.  */
  
  static bool
  optimize_tail_call (struct tailcall *t, bool opt_tailcalls)
  {
    if (t->tail_recursion)
      {
        eliminate_tail_call (t);
        return true;
      }
  
    if (opt_tailcalls)
      {
        gimple stmt = gsi_stmt (t->call_gsi);
  
        gimple_call_set_tail (stmt, true);
        if (dump_file && (dump_flags & TDF_DETAILS))
          {
  	  fprintf (dump_file, "Found tail call ");
  	  print_gimple_stmt (dump_file, stmt, 0, dump_flags);
  	  fprintf (dump_file, " in bb %i\n", (gsi_bb (t->call_gsi))->index);
  	}
      }
  
    return false;
  }
  
  /* Creates a tail-call accumulator of the same type as the return type of the
     current function.  LABEL is the name used to creating the temporary
     variable for the accumulator.  The accumulator will be inserted in the
     phis of a basic block BB with single predecessor with an initial value
     INIT converted to the current function return type.  */
  
  static tree
  create_tailcall_accumulator (const char *label, basic_block bb, tree init)
  {
    tree ret_type = TREE_TYPE (DECL_RESULT (current_function_decl));
    tree tmp = create_tmp_reg (ret_type, label);
    gimple phi;
  
    add_referenced_var (tmp);
    phi = create_phi_node (tmp, bb);
    /* RET_TYPE can be a float when -ffast-maths is enabled.  */
    add_phi_arg (phi, fold_convert (ret_type, init), single_pred_edge (bb),
  	       UNKNOWN_LOCATION);
    return PHI_RESULT (phi);
  }
  
  /* Optimizes tail calls in the function, turning the tail recursion
     into iteration.  */
  
  static unsigned int
  tree_optimize_tail_calls_1 (bool opt_tailcalls)
  {
    edge e;
    bool phis_constructed = false;
    struct tailcall *tailcalls = NULL, *act, *next;
    bool changed = false;
    basic_block first = single_succ (ENTRY_BLOCK_PTR);
    tree param;
    gimple stmt;
    edge_iterator ei;
  
    if (!suitable_for_tail_opt_p ())
      return 0;
    if (opt_tailcalls)
      opt_tailcalls = suitable_for_tail_call_opt_p ();
  
    FOR_EACH_EDGE (e, ei, EXIT_BLOCK_PTR->preds)
      {
        /* Only traverse the normal exits, i.e. those that end with return
  	 statement.  */
        stmt = last_stmt (e->src);
  
        if (stmt
  	  && gimple_code (stmt) == GIMPLE_RETURN)
  	find_tail_calls (e->src, &tailcalls);
      }
  
    /* Construct the phi nodes and accumulators if necessary.  */
    a_acc = m_acc = NULL_TREE;
    for (act = tailcalls; act; act = act->next)
      {
        if (!act->tail_recursion)
  	continue;
  
        if (!phis_constructed)
  	{
  	  /* Ensure that there is only one predecessor of the block
  	     or if there are existing degenerate PHI nodes.  */
  	  if (!single_pred_p (first)
  	      || !gimple_seq_empty_p (phi_nodes (first)))
  	    first = split_edge (single_succ_edge (ENTRY_BLOCK_PTR));
  
  	  /* Copy the args if needed.  */
  	  for (param = DECL_ARGUMENTS (current_function_decl);
  	       param;
  	       param = DECL_CHAIN (param))
  	    if (arg_needs_copy_p (param))
  	      {
  		tree name = gimple_default_def (cfun, param);
  		tree new_name = make_ssa_name (param, SSA_NAME_DEF_STMT (name));
  		gimple phi;
  
  		set_default_def (param, new_name);
  		phi = create_phi_node (name, first);
  		SSA_NAME_DEF_STMT (name) = phi;
  		add_phi_arg (phi, new_name, single_pred_edge (first),
  			     EXPR_LOCATION (param));
  	      }
  	  phis_constructed = true;
  	}
  
        if (act->add && !a_acc)
  	a_acc = create_tailcall_accumulator ("add_acc", first,
  					     integer_zero_node);
  
        if (act->mult && !m_acc)
  	m_acc = create_tailcall_accumulator ("mult_acc", first,
  					     integer_one_node);
      }
  
    if (a_acc || m_acc)
      {
        /* When the tail call elimination using accumulators is performed,
  	 statements adding the accumulated value are inserted at all exits.
  	 This turns all other tail calls to non-tail ones.  */
        opt_tailcalls = false;
      }
  
    for (; tailcalls; tailcalls = next)
      {
        next = tailcalls->next;
        changed |= optimize_tail_call (tailcalls, opt_tailcalls);
        free (tailcalls);
      }
  
    if (a_acc || m_acc)
      {
        /* Modify the remaining return statements.  */
        FOR_EACH_EDGE (e, ei, EXIT_BLOCK_PTR->preds)
  	{
  	  stmt = last_stmt (e->src);
  
  	  if (stmt
  	      && gimple_code (stmt) == GIMPLE_RETURN)
  	    adjust_return_value (e->src, m_acc, a_acc);
  	}
      }
  
    if (changed)
      free_dominance_info (CDI_DOMINATORS);
  
    if (phis_constructed)
      add_virtual_phis ();
    if (changed)
      return TODO_cleanup_cfg | TODO_update_ssa_only_virtuals;
    return 0;
  }
  
  static unsigned int
  execute_tail_recursion (void)
  {
    return tree_optimize_tail_calls_1 (false);
  }
  
  static bool
  gate_tail_calls (void)
  {
    return flag_optimize_sibling_calls != 0 && dbg_cnt (tail_call);
  }
  
  static unsigned int
  execute_tail_calls (void)
  {
    return tree_optimize_tail_calls_1 (true);
  }
  
  struct gimple_opt_pass pass_tail_recursion =
  {
   {
    GIMPLE_PASS,
    "tailr",				/* name */
    gate_tail_calls,			/* gate */
    execute_tail_recursion,		/* execute */
    NULL,					/* sub */
    NULL,					/* next */
    0,					/* static_pass_number */
    TV_NONE,				/* tv_id */
    PROP_cfg | PROP_ssa,			/* properties_required */
    0,					/* properties_provided */
    0,					/* properties_destroyed */
    0,					/* todo_flags_start */
    TODO_verify_ssa	                /* todo_flags_finish */
   }
  };
  
  struct gimple_opt_pass pass_tail_calls =
  {
   {
    GIMPLE_PASS,
    "tailc",				/* name */
    gate_tail_calls,			/* gate */
    execute_tail_calls,			/* execute */
    NULL,					/* sub */
    NULL,					/* next */
    0,					/* static_pass_number */
    TV_NONE,				/* tv_id */
    PROP_cfg | PROP_ssa,			/* properties_required */
    0,					/* properties_provided */
    0,					/* properties_destroyed */
    0,					/* todo_flags_start */
    TODO_verify_ssa	                /* todo_flags_finish */
   }
  };
  