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/* Tail call optimization on trees.

   Copyright (C) 2003, 2004, 2005 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 2, 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 COPYING.  If not, write to

the Free Software Foundation, 51 Franklin Street, Fifth Floor,

Boston, MA 02110-1301, USA.  */

#include "config.h"

#include "system.h"

#include "coretypes.h"

#include "tm.h"

#include "tree.h"

#include "rtl.h"

#include "tm_p.h"

#include "hard-reg-set.h"

#include "basic-block.h"

#include "function.h"

#include "tree-flow.h"

#include "tree-dump.h"

#include "diagnostic.h"

#include "except.h"

#include "tree-pass.h"

#include "flags.h"

#include "langhooks.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 block in that the call occur.  */

  basic_block call_block;

  /* The iterator pointing to the call statement.  */

  block_stmt_iterator call_bsi;

  /* 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)

{

  referenced_var_iterator rvi;

  tree var;

  if (current_function_stdarg)

    return false;

  /* No local variable nor structure field should be call-clobbered.  We

     ignore any kind of memory tag, as these are not real variables.  */

  FOR_EACH_REFERENCED_VAR (var, rvi)

    {

      if (!(TREE_STATIC (var) || DECL_EXTERNAL (var))

          && (var_ann (var)->mem_tag_kind == NOT_A_TAG

              || var_ann (var)->mem_tag_kind == STRUCT_FIELD)

          && is_call_clobbered (var))

        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 (current_function_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 (USING_SJLJ_EXCEPTIONS && 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 (current_function_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 = TREE_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 BSI (in a sense that we already know EXPR's value

   at BSI).  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 BSI.  */

static tree

independent_of_stmt_p (tree expr, tree at, block_stmt_iterator bsi)

{

  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 = bb_for_stmt (at);

  call_bb = bb_for_stmt (bsi_stmt (bsi));

  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 = bb_for_stmt (at);

      /* The default definition or defined before the chain.  */

      if (!bb || !bb->aux)

        break;

      if (bb == call_bb)

        {

          for (; !bsi_end_p (bsi); bsi_next (&bsi))

            if (bsi_stmt (bsi) == at)

              break;

          if (!bsi_end_p (bsi))

            expr = NULL_TREE;

          break;

        }

      if (TREE_CODE (at) != PHI_NODE)

        {

          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 of ASS in 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 (tree ass, tree stmt, block_stmt_iterator call, tree *m,

                    tree *a, tree *ass_var)

{

  tree op0, op1, non_ass_var;

  tree dest = TREE_OPERAND (ass, 0);

  tree src = TREE_OPERAND (ass, 1);

  enum tree_code code = TREE_CODE (src);

  tree src_var = src;

  /* 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.  */

  STRIP_NOPS (src_var);

  if (TREE_CODE (src_var) == SSA_NAME)

    {

      if (src_var != *ass_var)

        return false;

      *ass_var = dest;

      return true;

    }

  if (TREE_CODE_CLASS (code) != tcc_binary)

    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_unsafe_math_optimizations)

    if (FLOAT_TYPE_P (TREE_TYPE (DECL_RESULT (current_function_decl))))

      return false;

  /* We only handle the code like

     x = call ();

     y = m * x;

     z = y + a;

     return z;

     TODO -- Extend it for cases where the linear transformation of the output

     is expressed in a more complicated way.  */

  op0 = TREE_OPERAND (src, 0);

  op1 = TREE_OPERAND (src, 1);

  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:

      /* There should be no previous addition.  TODO -- it should be fairly

         straightforward to lift this restriction -- just allow storing

         more complicated expressions in *A, and gimplify it in

         adjust_accumulator_values.  */

      if (*a)

        return false;

      *a = non_ass_var;

      *ass_var = dest;

      return true;

    case MULT_EXPR:

      /* Similar remark applies here.  Handling multiplication after addition

         is just slightly more complicated -- we need to multiply both *A and

         *M.  */

      if (*a || *m)

        return false;

      *m = non_ass_var;

      *ass_var = dest;

      return true;

      /* TODO -- Handle other codes (NEGATE_EXPR, MINUS_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;

  tree phi;

  for (phi = phi_nodes (dest); phi; phi = PHI_CHAIN (phi))

    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, ret_var, stmt, func, param, args, call = NULL_TREE;

  block_stmt_iterator bsi, absi;

  bool tail_recursion;

  struct tailcall *nw;

  edge e;

  tree m, a;

  basic_block abb;

  stmt_ann_t ann;

  if (!single_succ_p (bb))

    return;

  for (bsi = bsi_last (bb); !bsi_end_p (bsi); bsi_prev (&bsi))

    {

      stmt = bsi_stmt (bsi);

      /* Ignore labels.  */

      if (TREE_CODE (stmt) == LABEL_EXPR)

        continue;

      /* Check for a call.  */

      if (TREE_CODE (stmt) == MODIFY_EXPR)

        {

          ass_var = TREE_OPERAND (stmt, 0);

          call = TREE_OPERAND (stmt, 1);

          if (TREE_CODE (call) == WITH_SIZE_EXPR)

            call = TREE_OPERAND (call, 0);

        }

      else

        {

          ass_var = NULL_TREE;

          call = stmt;

        }

      if (TREE_CODE (call) == CALL_EXPR)

        break;

      /* If the statement has virtual or volatile operands, fail.  */

      ann = stmt_ann (stmt);

      if (!ZERO_SSA_OPERANDS (stmt, (SSA_OP_VUSE | SSA_OP_VIRTUAL_DEFS))

          || ann->has_volatile_ops)

        return;

    }

  if (bsi_end_p (bsi))

    {

      edge_iterator ei;

      /* Recurse to the predecessors.  */

      FOR_EACH_EDGE (e, ei, bb->preds)

        find_tail_calls (e->src, ret);

      return;

    }

  /* We found the call, check whether it is suitable.  */

  tail_recursion = false;

  func = get_callee_fndecl (call);

  if (func == current_function_decl)

    {

      for (param = DECL_ARGUMENTS (func), args = TREE_OPERAND (call, 1);

           param && args;

           param = TREE_CHAIN (param), args = TREE_CHAIN (args))

        {

          tree arg = TREE_VALUE (args);

          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))

                  || !lang_hooks.types_compatible_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 MODIFY_EXPR and

                 updating the virtual operands).  */

              if (!is_gimple_reg (param))

                break;

            }

        }

      if (!args && !param)

        tail_recursion = true;

    }

  /* 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;

  absi = bsi;

  while (1)

    {

      bsi_next (&absi);

      while (bsi_end_p (absi))

        {

          ass_var = propagate_through_phis (ass_var, single_succ_edge (abb));

          abb = single_succ (abb);

          absi = bsi_start (abb);

        }

      stmt = bsi_stmt (absi);

      if (TREE_CODE (stmt) == LABEL_EXPR)

        continue;

      if (TREE_CODE (stmt) == RETURN_EXPR)

        break;

      if (TREE_CODE (stmt) != MODIFY_EXPR)

        return;

      if (!process_assignment (stmt, stmt, bsi, &m, &a, &ass_var))

        return;

    }

  /* See if this is a tail call we can handle.  */

  ret_var = TREE_OPERAND (stmt, 0);

  if (ret_var

      && TREE_CODE (ret_var) == MODIFY_EXPR)

    {

      tree ret_op = TREE_OPERAND (ret_var, 1);

      STRIP_NOPS (ret_op);

      if (!tail_recursion

          && TREE_CODE (ret_op) != SSA_NAME)

        return;

      if (!process_assignment (ret_var, stmt, bsi, &m, &a, &ass_var))

        return;

      ret_var = TREE_OPERAND (ret_var, 0);

    }

  /* 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 = xmalloc (sizeof (struct tailcall));

  nw->call_block = bb;

  nw->call_bsi = bsi;

  nw->tail_recursion = tail_recursion;

  nw->mult = m;

  nw->add = a;

  nw->next = *ret;

  *ret = nw;

}

/* Adjust the accumulator values according to A and M after BSI, and update

   the phi nodes on edge BACK.  */

static void

adjust_accumulator_values (block_stmt_iterator bsi, tree m, tree a, edge back)

{

  tree stmt, var, phi, tmp;

  tree ret_type = TREE_TYPE (DECL_RESULT (current_function_decl));

  tree a_acc_arg = a_acc, m_acc_arg = m_acc;

  if (a)

    {

      if (m_acc)

        {

          if (integer_onep (a))

            var = m_acc;

          else

            {

              stmt = build (MODIFY_EXPR, ret_type, NULL_TREE,

                            build (MULT_EXPR, ret_type, m_acc, a));

              tmp = create_tmp_var (ret_type, "acc_tmp");

              add_referenced_tmp_var (tmp);

              var = make_ssa_name (tmp, stmt);

              TREE_OPERAND (stmt, 0) = var;

              bsi_insert_after (&bsi, stmt, BSI_NEW_STMT);

            }

        }

      else

        var = a;

      stmt = build (MODIFY_EXPR, ret_type, NULL_TREE,

                    build (PLUS_EXPR, ret_type, a_acc, var));

      var = make_ssa_name (SSA_NAME_VAR (a_acc), stmt);

      TREE_OPERAND (stmt, 0) = var;

      bsi_insert_after (&bsi, stmt, BSI_NEW_STMT);

      a_acc_arg = var;

    }

  if (m)

    {

      stmt = build (MODIFY_EXPR, ret_type, NULL_TREE,

                    build (MULT_EXPR, ret_type, m_acc, m));

      var = make_ssa_name (SSA_NAME_VAR (m_acc), stmt);

      TREE_OPERAND (stmt, 0) = var;

      bsi_insert_after (&bsi, stmt, BSI_NEW_STMT);

      m_acc_arg = var;

    }

  if (a_acc)

    {

      for (phi = phi_nodes (back->dest); phi; phi = PHI_CHAIN (phi))

        if (PHI_RESULT (phi) == a_acc)

          break;

      add_phi_arg (phi, a_acc_arg, back);

    }

  if (m_acc)

    {

      for (phi = phi_nodes (back->dest); phi; phi = PHI_CHAIN (phi))

        if (PHI_RESULT (phi) == m_acc)

          break;

      add_phi_arg (phi, m_acc_arg, back);

    }

}

/* 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 ret_stmt = last_stmt (bb), ret_var, var, stmt, tmp;

  tree ret_type = TREE_TYPE (DECL_RESULT (current_function_decl));

  block_stmt_iterator bsi = bsi_last (bb);

  gcc_assert (TREE_CODE (ret_stmt) == RETURN_EXPR);

  ret_var = TREE_OPERAND (ret_stmt, 0);

  if (!ret_var)

    return;

  if (TREE_CODE (ret_var) == MODIFY_EXPR)

    {

      ret_var->common.ann = (tree_ann_t) stmt_ann (ret_stmt);

      bsi_replace (&bsi, ret_var, true);

      SSA_NAME_DEF_STMT (TREE_OPERAND (ret_var, 0)) = ret_var;

      ret_var = TREE_OPERAND (ret_var, 0);

      ret_stmt = build1 (RETURN_EXPR, TREE_TYPE (ret_stmt), ret_var);

      bsi_insert_after (&bsi, ret_stmt, BSI_NEW_STMT);

    }

  if (m)

    {

      stmt = build (MODIFY_EXPR, ret_type, NULL_TREE,

                    build (MULT_EXPR, ret_type, m_acc, ret_var));

      tmp = create_tmp_var (ret_type, "acc_tmp");

      add_referenced_tmp_var (tmp);

      var = make_ssa_name (tmp, stmt);

      TREE_OPERAND (stmt, 0) = var;

      bsi_insert_before (&bsi, stmt, BSI_SAME_STMT);

    }

  else

    var = ret_var;

  if (a)

    {

      stmt = build (MODIFY_EXPR, ret_type, NULL_TREE,

                    build (PLUS_EXPR, ret_type, a_acc, var));

      tmp = create_tmp_var (ret_type, "acc_tmp");

      add_referenced_tmp_var (tmp);

      var = make_ssa_name (tmp, stmt);

      TREE_OPERAND (stmt, 0) = var;

      bsi_insert_before (&bsi, stmt, BSI_SAME_STMT);

    }

  TREE_OPERAND (ret_stmt, 0) = var;

  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;

}

/* 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, stmt, args, rslt, call;

  basic_block bb, first;

  edge e;

  tree phi;

  block_stmt_iterator bsi;

  use_operand_p mayuse;

  def_operand_p maydef;

  ssa_op_iter iter;

  tree orig_stmt;

  stmt = orig_stmt = bsi_stmt (t->call_bsi);

  bb = t->call_block;

  if (dump_file && (dump_flags & TDF_DETAILS))

    {

      fprintf (dump_file, "Eliminated tail recursion in bb %d : ",

               bb->index);

      print_generic_stmt (dump_file, stmt, TDF_SLIM);

      fprintf (dump_file, "\n");