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/* Dead code elimination pass for the GNU compiler.

   Copyright (C) 2002, 2003, 2004, 2005, 2006, 2007, 2008, 2009, 2010

   Free Software Foundation, Inc.

   Contributed by Ben Elliston <bje@redhat.com>

   and Andrew MacLeod <amacleod@redhat.com>

   Adapted to use control dependence by Steven Bosscher, SUSE Labs.

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/>.  */

/* Dead code elimination.

   References:

     Building an Optimizing Compiler,

     Robert Morgan, Butterworth-Heinemann, 1998, Section 8.9.

     Advanced Compiler Design and Implementation,

     Steven Muchnick, Morgan Kaufmann, 1997, Section 18.10.

   Dead-code elimination is the removal of statements which have no

   impact on the program's output.  "Dead statements" have no impact

   on the program's output, while "necessary statements" may have

   impact on the output.

   The algorithm consists of three phases:

   1. Marking as necessary all statements known to be necessary,

      e.g. most function calls, writing a value to memory, etc;

   2. Propagating necessary statements, e.g., the statements

      giving values to operands in necessary statements; and

   3. Removing dead statements.  */

#include "config.h"

#include "system.h"

#include "coretypes.h"

#include "tm.h"

#include "ggc.h"

/* These RTL headers are needed for basic-block.h.  */

#include "rtl.h"

#include "tm_p.h"

#include "hard-reg-set.h"

#include "obstack.h"

#include "basic-block.h"

#include "tree.h"

#include "diagnostic.h"

#include "tree-flow.h"

#include "gimple.h"

#include "tree-dump.h"

#include "tree-pass.h"

#include "timevar.h"

#include "flags.h"

#include "cfgloop.h"

#include "tree-scalar-evolution.h"

static struct stmt_stats

{

  int total;

  int total_phis;

  int removed;

  int removed_phis;

} stats;

#define STMT_NECESSARY GF_PLF_1

static VEC(gimple,heap) *worklist;

/* Vector indicating an SSA name has already been processed and marked

   as necessary.  */

static sbitmap processed;

/* Vector indicating that last_stmt if a basic block has already been

   marked as necessary.  */

static sbitmap last_stmt_necessary;

/* Vector indicating that BB contains statements that are live.  */

static sbitmap bb_contains_live_stmts;

/* Before we can determine whether a control branch is dead, we need to

   compute which blocks are control dependent on which edges.

   We expect each block to be control dependent on very few edges so we

   use a bitmap for each block recording its edges.  An array holds the

   bitmap.  The Ith bit in the bitmap is set if that block is dependent

   on the Ith edge.  */

static bitmap *control_dependence_map;

/* Vector indicating that a basic block has already had all the edges

   processed that it is control dependent on.  */

static sbitmap visited_control_parents;

/* TRUE if this pass alters the CFG (by removing control statements).

   FALSE otherwise.

   If this pass alters the CFG, then it will arrange for the dominators

   to be recomputed.  */

static bool cfg_altered;

/* Execute code that follows the macro for each edge (given number

   EDGE_NUMBER within the CODE) for which the block with index N is

   control dependent.  */

#define EXECUTE_IF_CONTROL_DEPENDENT(BI, N, EDGE_NUMBER)        \

  EXECUTE_IF_SET_IN_BITMAP (control_dependence_map[(N)], 0,      \

                            (EDGE_NUMBER), (BI))

/* Indicate block BB is control dependent on an edge with index EDGE_INDEX.  */

static inline void

set_control_dependence_map_bit (basic_block bb, int edge_index)

{

  if (bb == ENTRY_BLOCK_PTR)

    return;

  gcc_assert (bb != EXIT_BLOCK_PTR);

  bitmap_set_bit (control_dependence_map[bb->index], edge_index);

}

/* Clear all control dependences for block BB.  */

static inline void

clear_control_dependence_bitmap (basic_block bb)

{

  bitmap_clear (control_dependence_map[bb->index]);

}

/* Find the immediate postdominator PDOM of the specified basic block BLOCK.

   This function is necessary because some blocks have negative numbers.  */

static inline basic_block

find_pdom (basic_block block)

{

  gcc_assert (block != ENTRY_BLOCK_PTR);

  if (block == EXIT_BLOCK_PTR)

    return EXIT_BLOCK_PTR;

  else

    {

      basic_block bb = get_immediate_dominator (CDI_POST_DOMINATORS, block);

      if (! bb)

        return EXIT_BLOCK_PTR;

      return bb;

    }

}

/* Determine all blocks' control dependences on the given edge with edge_list

   EL index EDGE_INDEX, ala Morgan, Section 3.6.  */

static void

find_control_dependence (struct edge_list *el, int edge_index)

{

  basic_block current_block;

  basic_block ending_block;

  gcc_assert (INDEX_EDGE_PRED_BB (el, edge_index) != EXIT_BLOCK_PTR);

  if (INDEX_EDGE_PRED_BB (el, edge_index) == ENTRY_BLOCK_PTR)

    ending_block = single_succ (ENTRY_BLOCK_PTR);

  else

    ending_block = find_pdom (INDEX_EDGE_PRED_BB (el, edge_index));

  for (current_block = INDEX_EDGE_SUCC_BB (el, edge_index);

       current_block != ending_block && current_block != EXIT_BLOCK_PTR;

       current_block = find_pdom (current_block))

    {

      edge e = INDEX_EDGE (el, edge_index);

      /* For abnormal edges, we don't make current_block control

         dependent because instructions that throw are always necessary

         anyway.  */

      if (e->flags & EDGE_ABNORMAL)

        continue;

      set_control_dependence_map_bit (current_block, edge_index);

    }

}

/* Record all blocks' control dependences on all edges in the edge

   list EL, ala Morgan, Section 3.6.  */

static void

find_all_control_dependences (struct edge_list *el)

{

  int i;

  for (i = 0; i < NUM_EDGES (el); ++i)

    find_control_dependence (el, i);

}

/* If STMT is not already marked necessary, mark it, and add it to the

   worklist if ADD_TO_WORKLIST is true.  */

static inline void

mark_stmt_necessary (gimple stmt, bool add_to_worklist)

{

  gcc_assert (stmt);

  if (gimple_plf (stmt, STMT_NECESSARY))

    return;

  if (dump_file && (dump_flags & TDF_DETAILS))

    {

      fprintf (dump_file, "Marking useful stmt: ");

      print_gimple_stmt (dump_file, stmt, 0, TDF_SLIM);

      fprintf (dump_file, "\n");

    }

  gimple_set_plf (stmt, STMT_NECESSARY, true);

  if (add_to_worklist)

    VEC_safe_push (gimple, heap, worklist, stmt);

  if (bb_contains_live_stmts && !is_gimple_debug (stmt))

    SET_BIT (bb_contains_live_stmts, gimple_bb (stmt)->index);

}

/* Mark the statement defining operand OP as necessary.  */

static inline void

mark_operand_necessary (tree op)

{

  gimple stmt;

  int ver;

  gcc_assert (op);

  ver = SSA_NAME_VERSION (op);

  if (TEST_BIT (processed, ver))

    {

      stmt = SSA_NAME_DEF_STMT (op);

      gcc_assert (gimple_nop_p (stmt)

                  || gimple_plf (stmt, STMT_NECESSARY));

      return;

    }

  SET_BIT (processed, ver);

  stmt = SSA_NAME_DEF_STMT (op);

  gcc_assert (stmt);

  if (gimple_plf (stmt, STMT_NECESSARY) || gimple_nop_p (stmt))

    return;

  if (dump_file && (dump_flags & TDF_DETAILS))

    {

      fprintf (dump_file, "marking necessary through ");

      print_generic_expr (dump_file, op, 0);

      fprintf (dump_file, " stmt ");

      print_gimple_stmt (dump_file, stmt, 0, 0);

    }

  gimple_set_plf (stmt, STMT_NECESSARY, true);

  if (bb_contains_live_stmts)

    SET_BIT (bb_contains_live_stmts, gimple_bb (stmt)->index);

  VEC_safe_push (gimple, heap, worklist, stmt);

}

/* Mark STMT as necessary if it obviously is.  Add it to the worklist if

   it can make other statements necessary.

   If AGGRESSIVE is false, control statements are conservatively marked as

   necessary.  */

static void

mark_stmt_if_obviously_necessary (gimple stmt, bool aggressive)

{

  tree lhs = NULL_TREE;

  /* With non-call exceptions, we have to assume that all statements could

     throw.  If a statement may throw, it is inherently necessary.  */

  if (flag_non_call_exceptions

      && stmt_could_throw_p (stmt))

    {

      mark_stmt_necessary (stmt, true);

      return;

    }

  /* Statements that are implicitly live.  Most function calls, asm

     and return statements are required.  Labels and GIMPLE_BIND nodes

     are kept because they are control flow, and we have no way of

     knowing whether they can be removed.  DCE can eliminate all the

     other statements in a block, and CFG can then remove the block

     and labels.  */

  switch (gimple_code (stmt))

    {

    case GIMPLE_PREDICT:

    case GIMPLE_LABEL:

      mark_stmt_necessary (stmt, false);

      return;

    case GIMPLE_ASM:

    case GIMPLE_RESX:

    case GIMPLE_RETURN:

      mark_stmt_necessary (stmt, true);

      return;

    case GIMPLE_CALL:

      /* Most, but not all function calls are required.  Function calls that

         produce no result and have no side effects (i.e. const pure

         functions) are unnecessary.  */

      if (gimple_has_side_effects (stmt))

        {

          mark_stmt_necessary (stmt, true);

          return;

        }

      if (!gimple_call_lhs (stmt))

        return;

      lhs = gimple_call_lhs (stmt);

      /* Fall through */

    case GIMPLE_ASSIGN:

      if (!lhs)

        lhs = gimple_assign_lhs (stmt);

      break;

    case GIMPLE_DEBUG:

      /* Debug temps without a value are not useful.  ??? If we could

         easily locate the debug temp bind stmt for a use thereof,

         would could refrain from marking all debug temps here, and

         mark them only if they're used.  */

      if (gimple_debug_bind_has_value_p (stmt)

          || TREE_CODE (gimple_debug_bind_get_var (stmt)) != DEBUG_EXPR_DECL)

        mark_stmt_necessary (stmt, false);

      return;

    case GIMPLE_GOTO:

      gcc_assert (!simple_goto_p (stmt));

      mark_stmt_necessary (stmt, true);

      return;

    case GIMPLE_COND:

      gcc_assert (EDGE_COUNT (gimple_bb (stmt)->succs) == 2);

      /* Fall through.  */

    case GIMPLE_SWITCH:

      if (! aggressive)

        mark_stmt_necessary (stmt, true);

      break;

    default:

      break;

    }

  /* If the statement has volatile operands, it needs to be preserved.

     Same for statements that can alter control flow in unpredictable

     ways.  */

  if (gimple_has_volatile_ops (stmt) || is_ctrl_altering_stmt (stmt))

    {

      mark_stmt_necessary (stmt, true);

      return;

    }

  if (is_hidden_global_store (stmt))

    {

      mark_stmt_necessary (stmt, true);

      return;

    }

  return;

}

/* Make corresponding control dependent edges necessary.  We only

   have to do this once for each basic block, so we clear the bitmap

   after we're done.  */

static void

mark_control_dependent_edges_necessary (basic_block bb, struct edge_list *el)

{

  bitmap_iterator bi;

  unsigned edge_number;

  gcc_assert (bb != EXIT_BLOCK_PTR);

  if (bb == ENTRY_BLOCK_PTR)

    return;

  EXECUTE_IF_CONTROL_DEPENDENT (bi, bb->index, edge_number)

    {

      gimple stmt;

      basic_block cd_bb = INDEX_EDGE_PRED_BB (el, edge_number);

      if (TEST_BIT (last_stmt_necessary, cd_bb->index))

        continue;

      SET_BIT (last_stmt_necessary, cd_bb->index);

      SET_BIT (bb_contains_live_stmts, cd_bb->index);

      stmt = last_stmt (cd_bb);

      if (stmt && is_ctrl_stmt (stmt))

        mark_stmt_necessary (stmt, true);

    }

}

/* Find obviously necessary statements.  These are things like most function

   calls, and stores to file level variables.

   If EL is NULL, control statements are conservatively marked as

   necessary.  Otherwise it contains the list of edges used by control

   dependence analysis.  */

static void

find_obviously_necessary_stmts (struct edge_list *el)

{

  basic_block bb;

  gimple_stmt_iterator gsi;

  edge e;

  gimple phi, stmt;

  FOR_EACH_BB (bb)

    {

      /* PHI nodes are never inherently necessary.  */

      for (gsi = gsi_start_phis (bb); !gsi_end_p (gsi); gsi_next (&gsi))

        {

          phi = gsi_stmt (gsi);

          gimple_set_plf (phi, STMT_NECESSARY, false);

        }

      /* Check all statements in the block.  */

      for (gsi = gsi_start_bb (bb); !gsi_end_p (gsi); gsi_next (&gsi))

        {

          stmt = gsi_stmt (gsi);

          gimple_set_plf (stmt, STMT_NECESSARY, false);

          mark_stmt_if_obviously_necessary (stmt, el != NULL);

        }

    }

  /* Pure and const functions are finite and thus have no infinite loops in

     them.  */

  if ((TREE_READONLY (current_function_decl)

       || DECL_PURE_P (current_function_decl))

      && !DECL_LOOPING_CONST_OR_PURE_P (current_function_decl))

    return;

  /* Prevent the empty possibly infinite loops from being removed.  */

  if (el)

    {

      loop_iterator li;

      struct loop *loop;

      scev_initialize ();

      if (mark_irreducible_loops ())

        FOR_EACH_BB (bb)

          {

            edge_iterator ei;

            FOR_EACH_EDGE (e, ei, bb->succs)

              if ((e->flags & EDGE_DFS_BACK)

                  && (e->flags & EDGE_IRREDUCIBLE_LOOP))

                {

                  if (dump_file)

                    fprintf (dump_file, "Marking back edge of irreducible loop %i->%i\n",

                             e->src->index, e->dest->index);

                  mark_control_dependent_edges_necessary (e->dest, el);

                }

          }

      FOR_EACH_LOOP (li, loop, 0)

        if (!finite_loop_p (loop))

          {

            if (dump_file)

              fprintf (dump_file, "can not prove finiteness of loop %i\n", loop->num);

            mark_control_dependent_edges_necessary (loop->latch, el);

          }

      scev_finalize ();

    }

}

/* Return true if REF is based on an aliased base, otherwise false.  */

static bool

ref_may_be_aliased (tree ref)

{

  while (handled_component_p (ref))

    ref = TREE_OPERAND (ref, 0);

  return !(DECL_P (ref)

           && !may_be_aliased (ref));

}

static bitmap visited = NULL;

static unsigned int longest_chain = 0;

static unsigned int total_chain = 0;

static unsigned int nr_walks = 0;

static bool chain_ovfl = false;

/* Worker for the walker that marks reaching definitions of REF,

   which is based on a non-aliased decl, necessary.  It returns

   true whenever the defining statement of the current VDEF is

   a kill for REF, as no dominating may-defs are necessary for REF

   anymore.  DATA points to the basic-block that contains the

   stmt that refers to REF.  */

static bool

mark_aliased_reaching_defs_necessary_1 (ao_ref *ref, tree vdef, void *data)

{

  gimple def_stmt = SSA_NAME_DEF_STMT (vdef);

  /* All stmts we visit are necessary.  */

  mark_operand_necessary (vdef);

  /* If the stmt lhs kills ref, then we can stop walking.  */

  if (gimple_has_lhs (def_stmt)

      && TREE_CODE (gimple_get_lhs (def_stmt)) != SSA_NAME)

    {

      tree base, lhs = gimple_get_lhs (def_stmt);

      HOST_WIDE_INT size, offset, max_size;

      ao_ref_base (ref);

      base = get_ref_base_and_extent (lhs, &offset, &size, &max_size);

      /* We can get MEM[symbol: sZ, index: D.8862_1] here,

         so base == refd->base does not always hold.  */

      if (base == ref->base)

        {

          /* For a must-alias check we need to be able to constrain

             the accesses properly.  */

          if (size != -1 && size == max_size

              && ref->max_size != -1)

            {

              if (offset <= ref->offset

                  && offset + size >= ref->offset + ref->max_size)

                return true;

            }

          /* Or they need to be exactly the same.  */

          else if (ref->ref

                   /* Make sure there is no induction variable involved

                      in the references (gcc.c-torture/execute/pr42142.c).

                      The simplest way is to check if the kill dominates

                      the use.  */

                   && dominated_by_p (CDI_DOMINATORS, (basic_block) data,

                                      gimple_bb (def_stmt))

                   && operand_equal_p (ref->ref, lhs, 0))

            return true;

        }

    }

  /* Otherwise keep walking.  */

  return false;

}

static void

mark_aliased_reaching_defs_necessary (gimple stmt, tree ref)

{

  unsigned int chain;

  ao_ref refd;

  gcc_assert (!chain_ovfl);

  ao_ref_init (&refd, ref);

  chain = walk_aliased_vdefs (&refd, gimple_vuse (stmt),

                              mark_aliased_reaching_defs_necessary_1,

                              gimple_bb (stmt), NULL);

  if (chain > longest_chain)

    longest_chain = chain;

  total_chain += chain;

  nr_walks++;

}

/* Worker for the walker that marks reaching definitions of REF, which

   is not based on a non-aliased decl.  For simplicity we need to end

   up marking all may-defs necessary that are not based on a non-aliased

   decl.  The only job of this walker is to skip may-defs based on

   a non-aliased decl.  */

static bool

mark_all_reaching_defs_necessary_1 (ao_ref *ref ATTRIBUTE_UNUSED,

                                    tree vdef, void *data ATTRIBUTE_UNUSED)

{

  gimple def_stmt = SSA_NAME_DEF_STMT (vdef);

  /* We have to skip already visited (and thus necessary) statements

     to make the chaining work after we dropped back to simple mode.  */

  if (chain_ovfl

      && TEST_BIT (processed, SSA_NAME_VERSION (vdef)))

    {

      gcc_assert (gimple_nop_p (def_stmt)

                  || gimple_plf (def_stmt, STMT_NECESSARY));

      return false;

    }

  /* We want to skip stores to non-aliased variables.  */

  if (!chain_ovfl

      && gimple_assign_single_p (def_stmt))

    {

      tree lhs = gimple_assign_lhs (def_stmt);

      if (!ref_may_be_aliased (lhs))

        return false;

    }

  mark_operand_necessary (vdef);

  return false;

}

static void

mark_all_reaching_defs_necessary (gimple stmt)

{

  walk_aliased_vdefs (NULL, gimple_vuse (stmt),

                      mark_all_reaching_defs_necessary_1, NULL, &visited);

}

/* Return true for PHI nodes with one or identical arguments

   can be removed.  */

static bool

degenerate_phi_p (gimple phi)

{

  unsigned int i;

  tree op = gimple_phi_arg_def (phi, 0);

  for (i = 1; i < gimple_phi_num_args (phi); i++)

    if (gimple_phi_arg_def (phi, i) != op)

      return false;

  return true;

}

/* Propagate necessity using the operands of necessary statements.

   Process the uses on each statement in the worklist, and add all

   feeding statements which contribute to the calculation of this

   value to the worklist.

   In conservative mode, EL is NULL.  */

static void

propagate_necessity (struct edge_list *el)

{

  gimple stmt;

  bool aggressive = (el ? true : false);

  if (dump_file && (dump_flags & TDF_DETAILS))

    fprintf (dump_file, "\nProcessing worklist:\n");

  while (VEC_length (gimple, worklist) > 0)

    {

      /* Take STMT from worklist.  */

      stmt = VEC_pop (gimple, worklist);

      if (dump_file && (dump_flags & TDF_DETAILS))

        {

          fprintf (dump_file, "processing: ");

          print_gimple_stmt (dump_file, stmt, 0, TDF_SLIM);

          fprintf (dump_file, "\n");

        }

      if (aggressive)

        {

          /* Mark the last statements of the basic blocks that the block

             containing STMT is control dependent on, but only if we haven't

             already done so.  */

          basic_block bb = gimple_bb (stmt);

          if (bb != ENTRY_BLOCK_PTR

              && ! TEST_BIT (visited_control_parents, bb->index))

            {

              SET_BIT (visited_control_parents, bb->index);

              mark_control_dependent_edges_necessary (bb, el);

            }

        }

      if (gimple_code (stmt) == GIMPLE_PHI

          /* We do not process virtual PHI nodes nor do we track their

             necessity.  */

          && is_gimple_reg (gimple_phi_result (stmt)))

        {

          /* PHI nodes are somewhat special in that each PHI alternative has

             data and control dependencies.  All the statements feeding the

             PHI node's arguments are always necessary.  In aggressive mode,

             we also consider the control dependent edges leading to the

             predecessor block associated with each PHI alternative as

             necessary.  */

          size_t k;

          for (k = 0; k < gimple_phi_num_args (stmt); k++)

            {

              tree arg = PHI_ARG_DEF (stmt, k);

              if (TREE_CODE (arg) == SSA_NAME)

                mark_operand_necessary (arg);

            }

          if (aggressive && !degenerate_phi_p (stmt))

            {

              for (k = 0; k < gimple_phi_num_args (stmt); k++)

                {

                  basic_block arg_bb = gimple_phi_arg_edge (stmt, k)->src;

                  if (arg_bb != ENTRY_BLOCK_PTR

                      && ! TEST_BIT (visited_control_parents, arg_bb->index))

                    {

                      SET_BIT (visited_control_parents, arg_bb->index);

                      mark_control_dependent_edges_necessary (arg_bb, el);

                    }

                }

            }

        }

      else

        {

          /* Propagate through the operands.  Examine all the USE, VUSE and

             VDEF operands in this statement.  Mark all the statements

             which feed this statement's uses as necessary.  */

          ssa_op_iter iter;

          tree use;

          FOR_EACH_SSA_TREE_OPERAND (use, stmt, iter, SSA_OP_USE)

            mark_operand_necessary (use);

          use = gimple_vuse (stmt);

          if (!use)

            continue;

          /* If we dropped to simple mode make all immediately

             reachable definitions necessary.  */

          if (chain_ovfl)

            {

              mark_all_reaching_defs_necessary (stmt);

              continue;

            }

          /* For statements that may load from memory (have a VUSE) we

             have to mark all reaching (may-)definitions as necessary.

             We partition this task into two cases:

              1) explicit loads based on decls that are not aliased

              2) implicit loads (like calls) and explicit loads not

                 based on decls that are not aliased (like indirect

                 references or loads from globals)

             For 1) we mark all reaching may-defs as necessary, stopping

             at dominating kills.  For 2) we want to mark all dominating