path: root/gcc/dce.c

/* RTL dead code elimination.
   Copyright (C) 2005, 2006, 2007 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 "hashtab.h"
#include "tm.h"
#include "rtl.h"
#include "tree.h"
#include "regs.h"
#include "hard-reg-set.h"
#include "flags.h"
#include "df.h"
#include "cselib.h"
#include "dce.h"
#include "timevar.h"
#include "tree-pass.h"
#include "dbgcnt.h"

DEF_VEC_I(int);
DEF_VEC_ALLOC_I(int,heap);


/* -------------------------------------------------------------------------
   Core mark/delete routines
   ------------------------------------------------------------------------- */

/* The data-flow information needed by this pass.  */
static bool df_in_progress = false;

/* True if we deleted at least one instruction.  */
static bool something_changed;

/* Instructions that have been marked but whose dependencies have not
   yet been processed.  */
static VEC(rtx,heap) *worklist;

static bitmap_obstack dce_blocks_bitmap_obstack;
static bitmap_obstack dce_tmp_bitmap_obstack;

static sbitmap marked = NULL;

/* A subroutine for which BODY is part of the instruction being tested;
   either the top-level pattern, or an element of a PARALLEL.  The
   instruction is known not to be a bare USE or CLOBBER.  */

static bool
deletable_insn_p_1 (rtx body)
{
  switch (GET_CODE (body))
    {
    case PREFETCH:
    case TRAP_IF:
      /* The UNSPEC case was added here because the ia-64 claims that
	 USEs do not work after reload and generates UNSPECS rather
	 than USEs.  Since dce is run after reload we need to avoid
	 deleting these even if they are dead.  If it turns out that
	 USEs really do work after reload, the ia-64 should be
	 changed, and the UNSPEC case can be removed.  */
    case UNSPEC:
      return false;

    default:
      if (volatile_insn_p (body))
	return false;

      if (flag_non_call_exceptions && may_trap_p (body))
	return false;

      return true;
    }
}

/* Return true if INSN is a normal instruction that can be deleted by
   the DCE pass.  */

static bool
deletable_insn_p (rtx insn, bool fast)
{
  rtx body, x;
  int i;

  if (!NONJUMP_INSN_P (insn))
    return false;

  body = PATTERN (insn);
  switch (GET_CODE (body))
    {
    case USE:
      return false;

    case CLOBBER:
      if (fast)
	{
	  /* A CLOBBER of a dead pseudo register serves no purpose.
	     That is not necessarily true for hard registers until
	     after reload.  */
	  x = XEXP (body, 0);
	  return REG_P (x) && (!HARD_REGISTER_P (x) || reload_completed);
	}
      else
	/* Because of the way that use-def chains are built, it is not
	   possible to tell if the clobber is dead because it can
	   never be the target of a use-def chain.  */
	return false;

    case PARALLEL:
      for (i = XVECLEN (body, 0) - 1; i >= 0; i--)
	if (!deletable_insn_p_1 (XVECEXP (body, 0, i)))
	  return false;
      return true;

    default:
      return deletable_insn_p_1 (body);
    }
}


/* Return true if INSN has not been marked as needed.  */

static inline int
marked_insn_p (rtx insn)
{
  if (insn)
    return TEST_BIT (marked, INSN_UID (insn));
  else 
    /* Artificial defs are always needed and they do not have an
       insn.  */
    return true;
}


/* If INSN has not yet been marked as needed, mark it now, and add it to
   the worklist.  */

static void
mark_insn (rtx insn, bool fast)
{
  if (!marked_insn_p (insn))
    {
      if (!fast)
	VEC_safe_push (rtx, heap, worklist, insn);
      SET_BIT (marked, INSN_UID (insn));
      if (dump_file)
	fprintf (dump_file, "  Adding insn %d to worklist\n", INSN_UID (insn));
    }
}


/* A note_stores callback used by mark_nonreg_stores.  DATA is the
   instruction containing DEST.  */

static void
mark_nonreg_stores_1 (rtx dest, const_rtx pattern, void *data)
{
  if (GET_CODE (pattern) != CLOBBER && !REG_P (dest))
    mark_insn ((rtx) data, true);
}


/* A note_stores callback used by mark_nonreg_stores.  DATA is the
   instruction containing DEST.  */

static void
mark_nonreg_stores_2 (rtx dest, const_rtx pattern, void *data)
{
  if (GET_CODE (pattern) != CLOBBER && !REG_P (dest))
    mark_insn ((rtx) data, false);
}


/* Mark INSN if BODY stores to a non-register destination.  */

static void
mark_nonreg_stores (rtx body, rtx insn, bool fast)
{
  if (fast)
    note_stores (body, mark_nonreg_stores_1, insn);
  else
    note_stores (body, mark_nonreg_stores_2, insn);
}


/* Initialize global variables for a new DCE pass.  */

static void
init_dce (bool fast)
{
  if (!df_in_progress)
    {
      if (!fast)
	df_chain_add_problem (DF_UD_CHAIN);
      df_analyze ();
    }

  if (dump_file)
    df_dump (dump_file);

  bitmap_obstack_initialize (&dce_blocks_bitmap_obstack);
  bitmap_obstack_initialize (&dce_tmp_bitmap_obstack);
  marked = sbitmap_alloc (get_max_uid () + 1);
  sbitmap_zero (marked);
}


/* Delete all REG_EQUAL notes of the registers INSN writes, to prevent
   bad dangling REG_EQUAL notes. */

static void
delete_corresponding_reg_eq_notes (rtx insn)
{
  struct df_ref **def_rec;
  for (def_rec = DF_INSN_DEFS (insn); *def_rec; def_rec++)
    {
      struct df_ref *def = *def_rec;
      unsigned int regno = DF_REF_REGNO (def);
      /* This loop is a little tricky.  We cannot just go down the
	 chain because it is being modified by the actions in the
	 loop.  So we just get the head.  We plan to drain the list
	 anyway.  */
      while (DF_REG_EQ_USE_CHAIN (regno))
	{
	  struct df_ref *eq_use = DF_REG_EQ_USE_CHAIN (regno);
	  rtx noted_insn = DF_REF_INSN (eq_use);
	  rtx note = find_reg_note (noted_insn, REG_EQUAL, NULL_RTX);
	  if (!note)
	    note = find_reg_note (noted_insn, REG_EQUIV, NULL_RTX);

	  /* This assert is generally triggered when someone deletes a
	     REG_EQUAL or REG_EQUIV note by hacking the list manually
	     rather than calling remove_note.  */
	  gcc_assert (note);
	  remove_note (noted_insn, note);
	}
    }
}


/* Delete every instruction that hasn't been marked.  Clear the insn
   from DCE_DF if DF_DELETE is true.  */

static void
delete_unmarked_insns (void)
{
  basic_block bb;
  rtx insn, next;

  something_changed = false;
  FOR_EACH_BB (bb)
    FOR_BB_INSNS_SAFE (bb, insn, next)
      if (INSN_P (insn))
	{
	  if (noop_move_p (insn))
	    {
	      /* Note that this code does not handle the case where
		 the last insn of libcall is deleted.  As it turns out
		 this case is excluded in the call to noop_move_p.  */
	      rtx note = find_reg_note (insn, REG_LIBCALL, NULL_RTX);
	      if (note && (XEXP (note, 0) != insn))
		{
		  rtx new_libcall_insn = next_real_insn (insn);
		  rtx retval_note = find_reg_note (XEXP (note, 0),
						   REG_RETVAL, NULL_RTX);
		  REG_NOTES (new_libcall_insn)
		    = gen_rtx_INSN_LIST (REG_LIBCALL, XEXP (note, 0),
					 REG_NOTES (new_libcall_insn));
		  XEXP (retval_note, 0) = new_libcall_insn;
		}
	    }
	  else if (marked_insn_p (insn))
	    continue;

	  /* WARNING, this debugging can itself cause problems if the
	     edge of the counter causes part of a libcall to be
	     deleted but not all of it.  */
	  if (!dbg_cnt (dce))
	    continue;

	  if (dump_file)
	    fprintf (dump_file, "DCE: Deleting insn %d\n", INSN_UID (insn));

          /* Before we delete the insn, we have to delete
             REG_EQUAL of the destination regs of the deleted insn
             to prevent dangling REG_EQUAL. */
          delete_corresponding_reg_eq_notes (insn);

	  delete_insn_and_edges (insn);
	  something_changed = true;
	}
}


/* Mark all insns using DELETE_PARM in the libcall that contains
   START_INSN.  */
static void 
mark_libcall (rtx start_insn, bool delete_parm)
{
  rtx note = find_reg_note (start_insn, REG_LIBCALL_ID, NULL_RTX);
  int id = INTVAL (XEXP (note, 0));
  rtx insn;

  mark_insn (start_insn, delete_parm);
  insn = NEXT_INSN (start_insn);

  /* There are tales, long ago and far away, of the mystical nested
     libcall.  No one alive has actually seen one, but other parts of
     the compiler support them so we will here.  */
  for (insn = NEXT_INSN (start_insn); insn; insn = NEXT_INSN (insn))
    {
      if (INSN_P (insn))
	{
	  /* Stay in the loop as long as we are in any libcall.  */
	  if ((note = find_reg_note (insn, REG_LIBCALL_ID, NULL_RTX)))
	    {
	      if (id == INTVAL (XEXP (note, 0)))
		{
		  mark_insn (insn, delete_parm);
		  if (dump_file)
		    fprintf (dump_file, "matching forward libcall %d[%d]\n",
			     INSN_UID (insn), id);
		}
	    }
	  else 
	    break;
	}
    }
  
  for (insn = PREV_INSN (start_insn); insn; insn = PREV_INSN (insn))
    {
      if (INSN_P (insn))
	{
	  /* Stay in the loop as long as we are in any libcall.  */
	  if ((note = find_reg_note (insn, REG_LIBCALL_ID, NULL_RTX)))
	    {
	      if (id == INTVAL (XEXP (note, 0)))
		{
		  mark_insn (insn, delete_parm);
		  if (dump_file)
		    fprintf (dump_file, "matching backward libcall %d[%d]\n",
			     INSN_UID (insn), id);
		}
	    }
	  else 
	    break;
	}
    }
}


/* Go through the instructions and mark those whose necessity is not
   dependent on inter-instruction information.  Make sure all other
   instructions are not marked.  */

static void
prescan_insns_for_dce (bool fast)
{
  basic_block bb;
  rtx insn;
  
  if (dump_file)
    fprintf (dump_file, "Finding needed instructions:\n");
  
  FOR_EACH_BB (bb)
    FOR_BB_INSNS (bb, insn)
    if (INSN_P (insn))
      {
        rtx note = find_reg_note (insn, REG_LIBCALL_ID, NULL_RTX);
        if (note)
          mark_libcall (insn, fast);
        else if (deletable_insn_p (insn, fast))
          mark_nonreg_stores (PATTERN (insn), insn, fast);
        else
          mark_insn (insn, fast);
      }

  if (dump_file)
    fprintf (dump_file, "Finished finding needed instructions:\n");
}


/* UD-based DSE routines. */

/* Mark instructions that define artificially-used registers, such as
   the frame pointer and the stack pointer.  */

static void
mark_artificial_uses (void)
{
  basic_block bb;
  struct df_link *defs;
  struct df_ref **use_rec;

  FOR_ALL_BB (bb)
    {
      for (use_rec = df_get_artificial_uses (bb->index); 
	   *use_rec; use_rec++)
	for (defs = DF_REF_CHAIN (*use_rec); defs; defs = defs->next)
	  mark_insn (DF_REF_INSN (defs->ref), false);
    }
}

/* Mark every instruction that defines a register value that INSN uses.  */

static void
mark_reg_dependencies (rtx insn)
{
  struct df_link *defs;
  struct df_ref **use_rec;

  /* If this is part of a libcall, mark the entire libcall.  */
  if (find_reg_note (insn, REG_LIBCALL_ID, NULL_RTX))
    mark_libcall (insn, false);

  for (use_rec = DF_INSN_USES (insn); *use_rec; use_rec++)
    {
      struct df_ref *use = *use_rec;
      if (dump_file)
	{
	  fprintf (dump_file, "Processing use of ");
	  print_simple_rtl (dump_file, DF_REF_REG (use));
	  fprintf (dump_file, " in insn %d:\n", INSN_UID (insn));
	}
      for (defs = DF_REF_CHAIN (use); defs; defs = defs->next)
	mark_insn (DF_REF_INSN (defs->ref), false);
    }
}


static void
end_ud_dce (void)
{
  sbitmap_free (marked);
  gcc_assert (VEC_empty (rtx, worklist));
}


/* UD-chain based DCE.  */

static unsigned int
rest_of_handle_ud_dce (void)
{
  rtx insn;

  df_in_progress = false;
  init_dce (false);

  prescan_insns_for_dce (false);
  mark_artificial_uses ();
  while (VEC_length (rtx, worklist) > 0)
    {
      insn = VEC_pop (rtx, worklist);
      mark_reg_dependencies (insn);
    }
  /* Before any insns are deleted, we must remove the chains since
     they are not bidirectional.  */
  df_remove_problem (df_chain);
  delete_unmarked_insns ();

  end_ud_dce ();
  return 0;
}


static bool
gate_ud_dce (void)
{
  return optimize > 1 && flag_dce;
}

struct tree_opt_pass pass_ud_rtl_dce =
{
  "dce",                                /* name */
  gate_ud_dce,                        /* gate */
  rest_of_handle_ud_dce,              /* execute */
  NULL,                                 /* sub */
  NULL,                                 /* next */
  0,                                    /* static_pass_number */
  TV_DCE,                               /* tv_id */
  0,                                    /* properties_required */
  0,                                    /* properties_provided */
  0,                                    /* properties_destroyed */
  0,                                    /* todo_flags_start */
  TODO_dump_func |
  TODO_df_finish | TODO_verify_rtl_sharing |
  TODO_ggc_collect,                     /* todo_flags_finish */
  'w'                                   /* letter */
};

/* -------------------------------------------------------------------------
   Fast DCE functions
   ------------------------------------------------------------------------- */


/* Free the data allocated by init_dce.  */

static void
fini_dce (void)
{
  sbitmap_free (marked);
  bitmap_obstack_release (&dce_blocks_bitmap_obstack);
  bitmap_obstack_release (&dce_tmp_bitmap_obstack);
  df_in_progress = false;
}


/* Process basic block BB.  Return true if the live_in set has
   changed.  */

static bool
dce_process_block (basic_block bb, bool redo_out)
{
  bitmap local_live = BITMAP_ALLOC (&dce_tmp_bitmap_obstack);
  bitmap au;
  rtx insn;
  bool block_changed;
  struct df_ref **def_rec, **use_rec;
  unsigned int bb_index = bb->index;

  if (redo_out)
    {
      /* Need to redo the live_out set of this block if when one of
	 the succs of this block has had a change in it live in
	 set.  */
      edge e;
      edge_iterator ei;
      df_confluence_function_n con_fun_n = df_lr->problem->con_fun_n;
      bitmap_clear (DF_LR_OUT (bb));
      FOR_EACH_EDGE (e, ei, bb->succs)
	(*con_fun_n) (e);
    }

  if (dump_file)
    {
      fprintf (dump_file, "processing block %d live out = ", bb->index);
      df_print_regset (dump_file, DF_LR_OUT (bb));
    }

  bitmap_copy (local_live, DF_LR_OUT (bb));

  /* Process the artificial defs and uses at the bottom of the block.  */
  for (def_rec = df_get_artificial_defs (bb_index); *def_rec; def_rec++)
    {
      struct df_ref *def = *def_rec;
      if (((DF_REF_FLAGS (def) & DF_REF_AT_TOP) == 0)
	  && (!(DF_REF_FLAGS (def) & (DF_REF_PARTIAL | DF_REF_CONDITIONAL))))
	bitmap_clear_bit (local_live, DF_REF_REGNO (def));
    }

  for (use_rec = df_get_artificial_uses (bb_index); *use_rec; use_rec++)
    {
      struct df_ref *use = *use_rec;
      if ((DF_REF_FLAGS (use) & DF_REF_AT_TOP) == 0)
	bitmap_set_bit (local_live, DF_REF_REGNO (use));
    }

  /* These regs are considered always live so if they end up dying
     because of some def, we need to bring the back again.
     Calling df_simulate_fixup_sets has the disadvantage of calling
     bb_has_eh_pred once per insn, so we cache the information here.  */
  if (bb_has_eh_pred (bb))
    au = df->eh_block_artificial_uses;
  else
    au = df->regular_block_artificial_uses;

  FOR_BB_INSNS_REVERSE (bb, insn)
    if (INSN_P (insn))
      {
	/* If this is a recursive call, the libcall will have already
	   been marked.  */
	if (!marked_insn_p (insn))
	  {	
	    bool needed = false;

	    /* The insn is needed if there is someone who uses the output.  */
	    for (def_rec = DF_INSN_DEFS (insn); *def_rec; def_rec++)
	      if (bitmap_bit_p (local_live, DF_REF_REGNO (*def_rec))
		  || bitmap_bit_p (au, DF_REF_REGNO (*def_rec)))
		{
		  needed = true;
		  break;
		}
	    
	    if (needed)
	      {
		rtx note = find_reg_note (insn, REG_LIBCALL_ID, NULL_RTX);

		/* If we need to mark an insn in the middle of a
		   libcall, we need to back up to mark the entire
		   libcall.  Given that libcalls are rare, rescanning
		   the block should be a reasonable solution to trying
		   to figure out how to back up.  */
		if (note)
		  {
		    if (dump_file)
		      fprintf (dump_file, "needed libcall %d\n", INSN_UID (insn));
		    mark_libcall (insn, true);
		    BITMAP_FREE (local_live);
		    return dce_process_block (bb, false);
		  }
		else
		  mark_insn (insn, true);
	      }
	  }
	
	/* No matter if the instruction is needed or not, we remove
	   any regno in the defs from the live set.  */
	df_simulate_defs (insn, local_live);

	/* On the other hand, we do not allow the dead uses to set
	   anything in local_live.  */
	if (marked_insn_p (insn))
	  df_simulate_uses (insn, local_live);
      }
  
  for (def_rec = df_get_artificial_defs (bb_index); *def_rec; def_rec++)
    {
      struct df_ref *def = *def_rec;
      if ((DF_REF_FLAGS (def) & DF_REF_AT_TOP)
	  && (!(DF_REF_FLAGS (def) & (DF_REF_PARTIAL | DF_REF_CONDITIONAL))))
	bitmap_clear_bit (local_live, DF_REF_REGNO (def));
    }
#ifdef EH_USES
  /* Process the uses that are live into an exception handler.  */
  for (use_rec = df_get_artificial_uses (bb_index); *use_rec; use_rec++)
    {
      /* Add use to set of uses in this BB.  */
      struct df_ref *use = *use_rec;
      if (DF_REF_FLAGS (use) & DF_REF_AT_TOP)
	bitmap_set_bit (local_live, DF_REF_REGNO (use));
    }
#endif

  block_changed = !bitmap_equal_p (local_live, DF_LR_IN (bb));
  if (block_changed)
    bitmap_copy (DF_LR_IN (bb), local_live);

  BITMAP_FREE (local_live);
  return block_changed;
}

static void
fast_dce (void)
{
  int *postorder = df_get_postorder (DF_BACKWARD);
  int n_blocks = df_get_n_blocks (DF_BACKWARD);
  int i;
  /* The set of blocks that have been seen on this iteration.  */
  bitmap processed = BITMAP_ALLOC (&dce_blocks_bitmap_obstack);
  /* The set of blocks that need to have the out vectors reset because
     the in of one of their successors has changed.  */
  bitmap redo_out = BITMAP_ALLOC (&dce_blocks_bitmap_obstack);
  bitmap all_blocks = BITMAP_ALLOC (&dce_blocks_bitmap_obstack);
  bool global_changed = true;

  int loop_count = 0;

  prescan_insns_for_dce (true);

  for (i = 0; i < n_blocks; i++)
    bitmap_set_bit (all_blocks, postorder[i]);

  while (global_changed)
    {
      global_changed = false;
      for (i = 0; i < n_blocks; i++)
	{
	  int index = postorder[i];
	  basic_block bb = BASIC_BLOCK (index);
	  bool local_changed;

	  if (index < NUM_FIXED_BLOCKS)
	    {
	      bitmap_set_bit (processed, index);
	      continue;
	    }

	  local_changed 
	    = dce_process_block (bb, bitmap_bit_p (redo_out, index));
	  bitmap_set_bit (processed, index);
	  
	  if (local_changed)
	    {
	      edge e;
	      edge_iterator ei;
	      FOR_EACH_EDGE (e, ei, bb->preds)
		if (bitmap_bit_p (processed, e->src->index))
		  /* Be tricky about when we need to iterate the
		     analysis.  We only have redo the analysis if the
		     bitmaps change at the top of a block that is the
		     entry to a loop.  */
		  global_changed = true;
		else
		  bitmap_set_bit (redo_out, e->src->index);
	    }
	}
      
      if (global_changed)
	{
	  /* Turn off the RUN_DCE flag to prevent recursive calls to
	     dce.  */
	  int old_flag = df_clear_flags (DF_LR_RUN_DCE);

	  /* So something was deleted that requires a redo.  Do it on
	     the cheap.  */
	  delete_unmarked_insns ();
	  sbitmap_zero (marked);
	  bitmap_clear (processed);
	  bitmap_clear (redo_out);
	  
	  /* We do not need to rescan any instructions.  We only need
	     to redo the dataflow equations for the blocks that had a
	     change at the top of the block.  Then we need to redo the
	     iteration.  */ 
	  df_analyze_problem (df_lr, all_blocks, postorder, n_blocks);

	  if (old_flag & DF_LR_RUN_DCE)
	    df_set_flags (DF_LR_RUN_DCE);
	  prescan_insns_for_dce (true);
	}
      loop_count++;
    }

  delete_unmarked_insns ();

  BITMAP_FREE (processed);
  BITMAP_FREE (redo_out);
  BITMAP_FREE (all_blocks);
}


/* Callback for running pass_rtl_dce.  */

static unsigned int
rest_of_handle_fast_dce (void)
{
  init_dce (true);
  fast_dce ();
  fini_dce ();
  df_in_progress = false;
  return 0;
}


/* This is an internal call that is used by the df live register
   problem to run fast dce as a side effect of creating the live
   information.  The stack is organized so that the lr problem is run,
   this pass is run, which updates the live info and the df scanning
   info, and then returns to allow the rest of the problems to be run.

   This can be called by elsewhere but it will not update the bit
   vectors for any other problems than LR.
*/

void
run_fast_df_dce (void)
{
  if (flag_dce)
    {
      /* If dce is able to delete something, it has to happen
	 immediately.  Otherwise there will be problems handling the
	 eq_notes.  */
      enum df_changeable_flags old_flags 
	= df_clear_flags (DF_DEFER_INSN_RESCAN + DF_NO_INSN_RESCAN);
      
      df_in_progress = true;
      rest_of_handle_fast_dce ();
      df_set_flags (old_flags);
    }
}

static bool
gate_fast_dce (void)
{
  return optimize > 0 && flag_dce;
}


/* Run a fast DCE pass and return true if any instructions were
   deleted.  */

bool
run_fast_dce (void)
{
  return gate_fast_dce () && (rest_of_handle_fast_dce (), something_changed);
}


struct tree_opt_pass pass_fast_rtl_dce =
{
  "dce",                                /* name */
  gate_fast_dce,                        /* gate */
  rest_of_handle_fast_dce,              /* execute */
  NULL,                                 /* sub */
  NULL,                                 /* next */
  0,                                    /* static_pass_number */
  TV_DCE,                               /* tv_id */
  0,                                    /* properties_required */
  0,                                    /* properties_provided */
  0,                                    /* properties_destroyed */
  0,                                    /* todo_flags_start */
  TODO_dump_func |
  TODO_df_finish | TODO_verify_rtl_sharing |
  TODO_ggc_collect,                     /* todo_flags_finish */
  'w'                                   /* letter */
};