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/* Dead store elimination

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

   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 "ggc.h"

#include "tree.h"

#include "tm_p.h"

#include "basic-block.h"

#include "timevar.h"

#include "gimple-pretty-print.h"

#include "tree-flow.h"

#include "tree-pass.h"

#include "tree-dump.h"

#include "domwalk.h"

#include "flags.h"

#include "langhooks.h"

/* This file implements dead store elimination.

   A dead store is a store into a memory location which will later be

   overwritten by another store without any intervening loads.  In this

   case the earlier store can be deleted.

   In our SSA + virtual operand world we use immediate uses of virtual

   operands to detect dead stores.  If a store's virtual definition

   is used precisely once by a later store to the same location which

   post dominates the first store, then the first store is dead.

   The single use of the store's virtual definition ensures that

   there are no intervening aliased loads and the requirement that

   the second load post dominate the first ensures that if the earlier

   store executes, then the later stores will execute before the function

   exits.

   It may help to think of this as first moving the earlier store to

   the point immediately before the later store.  Again, the single

   use of the virtual definition and the post-dominance relationship

   ensure that such movement would be safe.  Clearly if there are

   back to back stores, then the second is redundant.

   Reviewing section 10.7.2 in Morgan's "Building an Optimizing Compiler"

   may also help in understanding this code since it discusses the

   relationship between dead store and redundant load elimination.  In

   fact, they are the same transformation applied to different views of

   the CFG.  */

/* Bitmap of blocks that have had EH statements cleaned.  We should

   remove their dead edges eventually.  */

static bitmap need_eh_cleanup;

static bool gate_dse (void);

static unsigned int tree_ssa_dse (void);

static void dse_enter_block (struct dom_walk_data *, basic_block);

/* A helper of dse_optimize_stmt.

   Given a GIMPLE_ASSIGN in STMT, find a candidate statement *USE_STMT that

   may prove STMT to be dead.

   Return TRUE if the above conditions are met, otherwise FALSE.  */

static bool

dse_possible_dead_store_p (gimple stmt, gimple *use_stmt)

{

  gimple temp;

  unsigned cnt = 0;

  *use_stmt = NULL;

  /* Find the first dominated statement that clobbers (part of) the

     memory stmt stores to with no intermediate statement that may use

     part of the memory stmt stores.  That is, find a store that may

     prove stmt to be a dead store.  */

  temp = stmt;

  do

    {

      gimple use_stmt;

      imm_use_iterator ui;

      bool fail = false;

      tree defvar;

      /* Limit stmt walking to be linear in the number of possibly

         dead stores.  */

      if (++cnt > 256)

        return false;

      if (gimple_code (temp) == GIMPLE_PHI)

        defvar = PHI_RESULT (temp);

      else

        defvar = gimple_vdef (temp);

      temp = NULL;

      FOR_EACH_IMM_USE_STMT (use_stmt, ui, defvar)

        {

          cnt++;

          /* If we ever reach our DSE candidate stmt again fail.  We

             cannot handle dead stores in loops.  */

          if (use_stmt == stmt)

            {

              fail = true;

              BREAK_FROM_IMM_USE_STMT (ui);

            }

          /* In simple cases we can look through PHI nodes, but we

             have to be careful with loops and with memory references

             containing operands that are also operands of PHI nodes.

             See gcc.c-torture/execute/20051110-*.c.  */

          else if (gimple_code (use_stmt) == GIMPLE_PHI)

            {

              if (temp

                  /* Make sure we are not in a loop latch block.  */

                  || gimple_bb (stmt) == gimple_bb (use_stmt)

                  || dominated_by_p (CDI_DOMINATORS,

                                     gimple_bb (stmt), gimple_bb (use_stmt))

                  /* We can look through PHIs to regions post-dominating

                     the DSE candidate stmt.  */

                  || !dominated_by_p (CDI_POST_DOMINATORS,

                                      gimple_bb (stmt), gimple_bb (use_stmt)))

                {

                  fail = true;

                  BREAK_FROM_IMM_USE_STMT (ui);

                }

              temp = use_stmt;

            }

          /* If the statement is a use the store is not dead.  */

          else if (ref_maybe_used_by_stmt_p (use_stmt,

                                             gimple_assign_lhs (stmt)))

            {

              fail = true;

              BREAK_FROM_IMM_USE_STMT (ui);

            }

          /* If this is a store, remember it or bail out if we have

             multiple ones (the will be in different CFG parts then).  */

          else if (gimple_vdef (use_stmt))

            {

              if (temp)

                {

                  fail = true;

                  BREAK_FROM_IMM_USE_STMT (ui);

                }

              temp = use_stmt;

            }

        }

      if (fail)

        return false;

      /* If we didn't find any definition this means the store is dead

         if it isn't a store to global reachable memory.  In this case

         just pretend the stmt makes itself dead.  Otherwise fail.  */

      if (!temp)

        {

          if (is_hidden_global_store (stmt))

            return false;

          temp = stmt;

          break;

        }

    }

  /* We deliberately stop on clobbering statements and not only on

     killing ones to make walking cheaper.  Otherwise we can just

     continue walking until both stores have equal reference trees.  */

  while (!stmt_may_clobber_ref_p (temp, gimple_assign_lhs (stmt)));

  *use_stmt = temp;

  return true;

}

/* Attempt to eliminate dead stores in the statement referenced by BSI.

   A dead store is a store into a memory location which will later be

   overwritten by another store without any intervening loads.  In this

   case the earlier store can be deleted.

   In our SSA + virtual operand world we use immediate uses of virtual

   operands to detect dead stores.  If a store's virtual definition

   is used precisely once by a later store to the same location which

   post dominates the first store, then the first store is dead.  */

static void

dse_optimize_stmt (gimple_stmt_iterator gsi)

{

  gimple stmt = gsi_stmt (gsi);

  /* If this statement has no virtual defs, then there is nothing

     to do.  */

  if (!gimple_vdef (stmt))

    return;

  /* We know we have virtual definitions.  If this is a GIMPLE_ASSIGN

     that's not also a function call, then record it into our table.  */

  if (is_gimple_call (stmt) && gimple_call_fndecl (stmt))

    return;

  if (gimple_has_volatile_ops (stmt))

    return;

  if (is_gimple_assign (stmt))

    {

      gimple use_stmt;

      if (!dse_possible_dead_store_p (stmt, &use_stmt))

        return;

      /* If we have precisely one immediate use at this point and the

         stores are to the same memory location or there is a chain of

         virtual uses from stmt and the stmt which stores to that same

         memory location, then we may have found redundant store.  */

      if ((gimple_has_lhs (use_stmt)

           && (operand_equal_p (gimple_assign_lhs (stmt),

                                gimple_get_lhs (use_stmt), 0)))

          || stmt_kills_ref_p (use_stmt, gimple_assign_lhs (stmt)))

        {

          /* If use_stmt is or might be a nop assignment, e.g. for

             struct { ... } S a, b, *p; ...

             b = a; b = b;

             or

             b = a; b = *p; where p might be &b,

             or

             *p = a; *p = b; where p might be &b,

             or

             *p = *u; *p = *v; where p might be v, then USE_STMT

             acts as a use as well as definition, so store in STMT

             is not dead.  */

          if (stmt != use_stmt

              && ref_maybe_used_by_stmt_p (use_stmt, gimple_assign_lhs (stmt)))

            return;

          if (dump_file && (dump_flags & TDF_DETAILS))

            {

              fprintf (dump_file, "  Deleted dead store '");

              print_gimple_stmt (dump_file, gsi_stmt (gsi), dump_flags, 0);

              fprintf (dump_file, "'\n");

            }

          /* Then we need to fix the operand of the consuming stmt.  */

          unlink_stmt_vdef (stmt);

          bitmap_set_bit (need_eh_cleanup, gimple_bb (stmt)->index);

          /* Remove the dead store.  */

          gsi_remove (&gsi, true);

          /* And release any SSA_NAMEs set in this statement back to the

             SSA_NAME manager.  */

          release_defs (stmt);

        }

    }

}

static void

dse_enter_block (struct dom_walk_data *walk_data ATTRIBUTE_UNUSED,

                 basic_block bb)

{

  gimple_stmt_iterator gsi;

  for (gsi = gsi_last (bb_seq (bb)); !gsi_end_p (gsi); gsi_prev (&gsi))

    dse_optimize_stmt (gsi);

}

/* Main entry point.  */

static unsigned int

tree_ssa_dse (void)

{

  struct dom_walk_data walk_data;

  need_eh_cleanup = BITMAP_ALLOC (NULL);

  renumber_gimple_stmt_uids ();

  /* We might consider making this a property of each pass so that it

     can be [re]computed on an as-needed basis.  Particularly since

     this pass could be seen as an extension of DCE which needs post

     dominators.  */

  calculate_dominance_info (CDI_POST_DOMINATORS);

  calculate_dominance_info (CDI_DOMINATORS);

  /* Dead store elimination is fundamentally a walk of the post-dominator

     tree and a backwards walk of statements within each block.  */

  walk_data.dom_direction = CDI_POST_DOMINATORS;

  walk_data.initialize_block_local_data = NULL;

  walk_data.before_dom_children = dse_enter_block;

  walk_data.after_dom_children = NULL;

  walk_data.block_local_data_size = 0;

  walk_data.global_data = NULL;

  /* Initialize the dominator walker.  */

  init_walk_dominator_tree (&walk_data);

  /* Recursively walk the dominator tree.  */

  walk_dominator_tree (&walk_data, EXIT_BLOCK_PTR);

  /* Finalize the dominator walker.  */

  fini_walk_dominator_tree (&walk_data);

  /* Removal of stores may make some EH edges dead.  Purge such edges from

     the CFG as needed.  */

  if (!bitmap_empty_p (need_eh_cleanup))

    {

      gimple_purge_all_dead_eh_edges (need_eh_cleanup);

      cleanup_tree_cfg ();

    }

  BITMAP_FREE (need_eh_cleanup);

  /* For now, just wipe the post-dominator information.  */

  free_dominance_info (CDI_POST_DOMINATORS);

  return 0;

}

static bool

gate_dse (void)

{

  return flag_tree_dse != 0;

}

struct gimple_opt_pass pass_dse =

{

 {

  GIMPLE_PASS,

  "dse",                        /* name */

  gate_dse,                     /* gate */

  tree_ssa_dse,                 /* execute */

  NULL,                         /* sub */

  NULL,                         /* next */

  0,                             /* static_pass_number */

  TV_TREE_DSE,                  /* tv_id */

  PROP_cfg | PROP_ssa,          /* properties_required */

  0,                             /* properties_provided */

  0,                             /* properties_destroyed */

  0,                             /* todo_flags_start */

  TODO_ggc_collect

    | TODO_verify_ssa           /* todo_flags_finish */

 }

};