path: root/gcc/tree-ssa-dse.c

/* Dead store elimination
   Copyright (C) 2004 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, 59 Temple Place - Suite 330,
Boston, MA 02111-1307, USA.  */

#include "config.h"
#include "system.h"
#include "coretypes.h"
#include "tm.h"
#include "errors.h"
#include "ggc.h"
#include "tree.h"
#include "rtl.h"
#include "tm_p.h"
#include "basic-block.h"
#include "timevar.h"
#include "diagnostic.h"
#include "tree-flow.h"
#include "tree-pass.h"
#include "tree-dump.h"
#include "domwalk.h"
#include "flags.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 defintion 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.  */
   

struct dse_global_data
{
  /* This is the global bitmap for store statements.

     Each statement has a unique ID.  When we encounter a store statement
     that we want to record, set the bit corresponding to the statement's
     unique ID in this bitmap.  */
  bitmap stores;
};

/* We allocate a bitmap-per-block for stores which are encountered
   during the scan of that block.  This allows us to restore the 
   global bitmap of stores when we finish processing a block.  */
struct dse_block_local_data
{
  bitmap stores;
};

static bool gate_dse (void);
static void tree_ssa_dse (void);
static void dse_initialize_block_local_data (struct dom_walk_data *,
					     basic_block,
					     bool);
static void dse_optimize_stmt (struct dom_walk_data *,
			       basic_block,
			       block_stmt_iterator);
static void dse_record_phis (struct dom_walk_data *, basic_block);
static void dse_finalize_block (struct dom_walk_data *, basic_block);
static void fix_phi_uses (tree, tree);
static void fix_stmt_vdefs (tree, tree);
static void record_voperand_set (bitmap, bitmap *, unsigned int);

/* Function indicating whether we ought to include information for 'var'
   when calculating immediate uses.  For this pass we only want use
   information for virtual variables.  */

static bool
need_imm_uses_for (tree var)
{
  return !is_gimple_reg (var);
}


/* Replace uses in PHI which match VDEF_RESULTs in STMT with the 
   corresponding VDEF_OP in STMT.  */

static void
fix_phi_uses (tree phi, tree stmt)
{
  stmt_ann_t ann = stmt_ann (stmt);
  vdef_optype vdefs;
  unsigned int i;
  int j;

  get_stmt_operands (stmt);
  vdefs = VDEF_OPS (ann);

  /* Walk each VDEF in STMT.  */
  for (i = 0; i < NUM_VDEFS (vdefs); i++)
    {
      tree vdef = VDEF_RESULT (vdefs, i);

      /* Find any uses in the PHI which match VDEF and replace
	 them with the appropriate VDEF_OP.  */
      for (j = 0; j < PHI_NUM_ARGS (phi); j++)
	if (vdef == PHI_ARG_DEF (phi, j))
	  PHI_ARG_DEF (phi, j) = VDEF_OP (vdefs, i);
    }
}

/* Replace the VDEF_OPs in STMT1 which match VDEF_RESULTs in STMT2 with
   the appropriate VDEF_OPs from STMT2.  */

static void
fix_stmt_vdefs (tree stmt1, tree stmt2)
{
  stmt_ann_t ann1 = stmt_ann (stmt1);
  stmt_ann_t ann2 = stmt_ann (stmt2);
  vdef_optype vdefs1;
  vdef_optype vdefs2;
  unsigned int i, j;

  get_stmt_operands (stmt1);
  get_stmt_operands (stmt2);
  vdefs1 = VDEF_OPS (ann1);
  vdefs2 = VDEF_OPS (ann2);

  /* Walk each VDEF_OP in stmt1.  */
  for (i = 0; i < NUM_VDEFS (vdefs1); i++)
    {
      tree vdef1 = VDEF_OP (vdefs1, i);

      /* Find the appropriate VDEF_RESULT in STMT2.  */
      for (j = 0; j < NUM_VDEFS (vdefs2); j++)
	{
	  if (vdef1 == VDEF_RESULT (vdefs2, j))
	    {
	      /* Update.  */
	      *VDEF_OP_PTR (vdefs1, i) = VDEF_OP (vdefs2, j);
	      break;
	    }
	}

#ifdef ENABLE_CHECKING
      /* If we did not find a corresponding VDEF_RESULT, then something
	 has gone terribly wrong.  */
      if (j == NUM_VDEFS (vdefs2))
	abort ();
#endif

    }
}


/* Set bit UID in bitmaps GLOBAL and *LOCAL, creating *LOCAL as needed.  */
static void
record_voperand_set (bitmap global, bitmap *local, unsigned int uid)
{
  /* Lazily allocate the bitmap.  Note that we do not get a notification
     when the block local data structures die, so we allocate the local
     bitmap backed by the GC system.  */
  if (*local == NULL)
    *local = BITMAP_GGC_ALLOC ();

  /* Set the bit in the local and global bitmaps.  */
  bitmap_set_bit (*local, uid);
  bitmap_set_bit (global, uid);
}
/* Initialize block local data structures.  */

static void
dse_initialize_block_local_data (struct dom_walk_data *walk_data,
				 basic_block bb ATTRIBUTE_UNUSED,
				 bool recycled)
{
  struct dse_block_local_data *bd
    = VARRAY_TOP_GENERIC_PTR (walk_data->block_data_stack);

  /* If we are given a recycled block local data structure, ensure any
     bitmap associated with the block is cleared.  */
  if (recycled)
    {
      if (bd->stores)
	bitmap_clear (bd->stores);
    }
}

/* 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 (struct dom_walk_data *walk_data,
		   basic_block bb ATTRIBUTE_UNUSED,
		   block_stmt_iterator bsi)
{
  struct dse_block_local_data *bd
    = VARRAY_TOP_GENERIC_PTR (walk_data->block_data_stack);
  struct dse_global_data *dse_gd = walk_data->global_data;
  tree stmt = bsi_stmt (bsi);
  stmt_ann_t ann = stmt_ann (stmt);
  vdef_optype vdefs;

  get_stmt_operands (stmt);
  vdefs = VDEF_OPS (ann);

  /* If this statement has no virtual uses, then there is nothing
     to do.  */
  if (NUM_VDEFS (vdefs) == 0)
    return;

  /* We know we have virtual definitions.  If this is a MODIFY_EXPR, then
     record it into our table.  */
  if (TREE_CODE (stmt) == MODIFY_EXPR
      && TREE_CODE (TREE_OPERAND (stmt, 1)) != CALL_EXPR)
    {
      dataflow_t df = get_immediate_uses (stmt);
      unsigned int num_uses = num_immediate_uses (df);
      tree use;
      tree skipped_phi;


      /* If there are no uses then there is nothing left to do.  */
      if (num_uses == 0)
	{
	  record_voperand_set (dse_gd->stores, &bd->stores, ann->uid);
	  return;
	}

      use = immediate_use (df, 0);
      skipped_phi = NULL;

      /* Skip through any PHI nodes we have already seen if the PHI
	 represents the only use of this store.

	 Note this does not handle the case where the store has
	 multiple VDEFs which all reach a set of PHI nodes in the
	 same block.  */
      while (num_uses == 1
	     && TREE_CODE (use) == PHI_NODE
	     && bitmap_bit_p (dse_gd->stores, stmt_ann (use)->uid))
	{
	  /* Record the first PHI we skip so that we can fix its
	     uses if we find that STMT is a dead store.  */
	  if (!skipped_phi)
	    skipped_phi = use;

	  /* Skip past this PHI and loop again in case we had a PHI
	     chain.  */
	  df = get_immediate_uses (use);
	  num_uses = num_immediate_uses (df);
	  use = immediate_use (df, 0);
	}

      /* If we have precisely one immediate use at this point, then we may
	 have found redundant store.  */
      if (num_uses == 1
	  && bitmap_bit_p (dse_gd->stores, stmt_ann (use)->uid)
	  && operand_equal_p (TREE_OPERAND (stmt, 0),
			      TREE_OPERAND (use, 0), 0))
	{
	  /* We need to fix the operands if either the first PHI we
	     skipped, or the store which we are not deleting if we did
	     not skip any PHIs.  */
	  if (skipped_phi)
	    fix_phi_uses (skipped_phi, stmt);
	  else
	    fix_stmt_vdefs (use, stmt);

	  if (dump_file && (dump_flags & TDF_DETAILS))
            {
              fprintf (dump_file, "  Deleted dead store '");
              print_generic_expr (dump_file, bsi_stmt (bsi), dump_flags);
              fprintf (dump_file, "'\n");
            }

	  /* Any immediate uses which reference STMT need to instead
	     reference the new consumer, either SKIPPED_PHI or USE.  
	     This allows us to cascade dead stores.  */
	  redirect_immediate_uses (stmt, skipped_phi ? skipped_phi : use);

	  /* Finally remove the dead store.  */
	  bsi_remove (&bsi);
	}

      record_voperand_set (dse_gd->stores, &bd->stores, ann->uid);
    }
}

/* Record that we have seen the PHIs at the start of BB which correspond
   to virtual operands.  */
static void
dse_record_phis (struct dom_walk_data *walk_data, basic_block bb)
{
  struct dse_block_local_data *bd
    = VARRAY_TOP_GENERIC_PTR (walk_data->block_data_stack);
  struct dse_global_data *dse_gd = walk_data->global_data;
  tree phi;

  for (phi = phi_nodes (bb); phi; phi = TREE_CHAIN (phi))
    if (need_imm_uses_for (PHI_RESULT (phi)))
      record_voperand_set (dse_gd->stores,
			   &bd->stores,
			   get_stmt_ann (phi)->uid);
}

static void
dse_finalize_block (struct dom_walk_data *walk_data,
		    basic_block bb ATTRIBUTE_UNUSED)
{
  struct dse_block_local_data *bd
    = VARRAY_TOP_GENERIC_PTR (walk_data->block_data_stack);
  struct dse_global_data *dse_gd = walk_data->global_data;
  bitmap stores = dse_gd->stores;
  unsigned int i;

  /* Unwind the stores noted in this basic block.  */
  if (bd->stores)
    EXECUTE_IF_SET_IN_BITMAP (bd->stores, 0, i, bitmap_clear_bit (stores, i););
}

static void
tree_ssa_dse (void)
{
  struct dom_walk_data walk_data;
  struct dse_global_data dse_gd;
  unsigned int uid = 0;
  basic_block bb;

  /* Create a UID for each statement in the function.  Ordering of the
     UIDs is not important for this pass.  */
  FOR_EACH_BB (bb)
    {
      block_stmt_iterator bsi;
      tree phi;

      for (bsi = bsi_start (bb); !bsi_end_p (bsi); bsi_next (&bsi))
	stmt_ann (bsi_stmt (bsi))->uid = uid++;

      for (phi = phi_nodes (bb); phi; phi = TREE_CHAIN (phi))
	stmt_ann (phi)->uid = uid++;
    }

  /* 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);

  /* We also need immediate use information for virtual operands.  */
  compute_immediate_uses (TDFA_USE_VOPS, need_imm_uses_for);

  /* Dead store elimination is fundamentally a walk of the post-dominator
     tree and a backwards walk of statements within each block.  */
  walk_data.walk_stmts_backward = true;
  walk_data.dom_direction = CDI_POST_DOMINATORS;
  walk_data.initialize_block_local_data = dse_initialize_block_local_data;
  walk_data.before_dom_children_before_stmts = NULL;
  walk_data.before_dom_children_walk_stmts = dse_optimize_stmt;
  walk_data.before_dom_children_after_stmts = dse_record_phis;
  walk_data.after_dom_children_before_stmts = NULL;
  walk_data.after_dom_children_walk_stmts = NULL;
  walk_data.after_dom_children_after_stmts = dse_finalize_block;

  walk_data.block_local_data_size = sizeof (struct dse_block_local_data);

  /* This is the main hash table for the dead store elimination pass.  */
  dse_gd.stores = BITMAP_XMALLOC ();
  walk_data.global_data = &dse_gd;

  /* 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);

  /* Release the main bitmap.  */
  BITMAP_XFREE (dse_gd.stores);

  /* Free dataflow information.  It's probably out of date now anyway.  */
  free_df ();

  /* For now, just wipe the post-dominator information.  */
  free_dominance_info (CDI_POST_DOMINATORS);
}

static bool
gate_dse (void)
{
  return flag_tree_dse != 0;
}

struct tree_opt_pass pass_dse = {
  "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_dump_func | TODO_ggc_collect	/* todo_flags_finish */
  | TODO_verify_ssa
};