re PR tree-optimization/33562 (aggregate DSE disabled)

[thirdparty/gcc.git] / gcc / tree-ssa-dse.c

/* Dead store elimination
   Copyright (C) 2004-2017 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 "backend.h"
#include "rtl.h"
#include "tree.h"
#include "gimple.h"
#include "tree-pass.h"
#include "ssa.h"
#include "gimple-pretty-print.h"
#include "fold-const.h"
#include "gimple-iterator.h"
#include "tree-cfg.h"
#include "tree-dfa.h"
#include "domwalk.h"
#include "tree-cfgcleanup.h"
#include "params.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;

/* Return value from dse_classify_store */
enum dse_store_status
{
  DSE_STORE_LIVE,
  DSE_STORE_MAYBE_PARTIAL_DEAD,
  DSE_STORE_DEAD
};

/* STMT is a statement that may write into memory.  Analyze it and
   initialize WRITE to describe how STMT affects memory.

   Return TRUE if the the statement was analyzed, FALSE otherwise.

   It is always safe to return FALSE.  But typically better optimziation
   can be achieved by analyzing more statements.  */

static bool
initialize_ao_ref_for_dse (gimple *stmt, ao_ref *write)
{
  /* It's advantageous to handle certain mem* functions.  */
  if (gimple_call_builtin_p (stmt, BUILT_IN_NORMAL))
    {
      switch (DECL_FUNCTION_CODE (gimple_call_fndecl (stmt)))
        {
          case BUILT_IN_MEMCPY:
          case BUILT_IN_MEMMOVE:
          case BUILT_IN_MEMSET:
            {
              tree size = NULL_TREE;
              if (gimple_call_num_args (stmt) == 3)
                size = gimple_call_arg (stmt, 2);
              tree ptr = gimple_call_arg (stmt, 0);
              ao_ref_init_from_ptr_and_size (write, ptr, size);
              return true;
            }
          default:
            break;
        }
    }
  else if (is_gimple_assign (stmt))
    {
      ao_ref_init (write, gimple_assign_lhs (stmt));
      return true;
    }
  return false;
}

/* Given REF from the the alias oracle, return TRUE if it is a valid
   memory reference for dead store elimination, false otherwise.

   In particular, the reference must have a known base, known maximum
   size, start at a byte offset and have a size that is one or more
   bytes.  */

static bool
valid_ao_ref_for_dse (ao_ref *ref)
{
  return (ao_ref_base (ref)
          && ref->max_size != -1
          && (ref->offset % BITS_PER_UNIT) == 0
          && (ref->size % BITS_PER_UNIT) == 0
          && (ref->size != -1));
}

/* Normalize COPY (an ao_ref) relative to REF.  Essentially when we are
   done COPY will only refer bytes found within REF.

   We have already verified that COPY intersects at least one
   byte with REF.  */

static void
normalize_ref (ao_ref *copy, ao_ref *ref)
{
  /* If COPY starts before REF, then reset the beginning of
     COPY to match REF and decrease the size of COPY by the
     number of bytes removed from COPY.  */
  if (copy->offset < ref->offset)
    {
      copy->size -= (ref->offset - copy->offset);
      copy->offset = ref->offset;
    }

  /* If COPY extends beyond REF, chop off its size appropriately.  */
  if (copy->offset + copy->size > ref->offset + ref->size)
    copy->size -= (copy->offset + copy->size - (ref->offset + ref->size));
}

/* Clear any bytes written by STMT from the bitmap LIVE_BYTES.  The base
   address written by STMT must match the one found in REF, which must
   have its base address previously initialized.

   This routine must be conservative.  If we don't know the offset or
   actual size written, assume nothing was written.  */

static void
clear_bytes_written_by (sbitmap live_bytes, gimple *stmt, ao_ref *ref)
{
  ao_ref write;
  if (!initialize_ao_ref_for_dse (stmt, &write))
    return;

  /* Verify we have the same base memory address, the write
     has a known size and overlaps with REF.  */
  if (valid_ao_ref_for_dse (&write)
      && write.base == ref->base
      && write.size == write.max_size
      && ((write.offset < ref->offset
           && write.offset + write.size > ref->offset)
          || (write.offset >= ref->offset
              && write.offset < ref->offset + ref->size)))
    {
      normalize_ref (&write, ref);
      bitmap_clear_range (live_bytes,
                          (write.offset - ref->offset) / BITS_PER_UNIT,
                          write.size / BITS_PER_UNIT);
    }
}

/* REF is a memory write.  Extract relevant information from it and
   initialize the LIVE_BYTES bitmap.  If successful, return TRUE.
   Otherwise return FALSE.  */

static bool
setup_live_bytes_from_ref (ao_ref *ref, sbitmap live_bytes)
{
  if (valid_ao_ref_for_dse (ref)
      && (ref->size / BITS_PER_UNIT
          <= PARAM_VALUE (PARAM_DSE_MAX_OBJECT_SIZE)))
    {
      bitmap_clear (live_bytes);
      bitmap_set_range (live_bytes, 0, ref->size / BITS_PER_UNIT);
      return true;
    }
  return false;
}

/* Compute the number of elements that we can trim from the head and
   tail of ORIG resulting in a bitmap that is a superset of LIVE.

   Store the number of elements trimmed from the head and tail in
   TRIM_HEAD and TRIM_TAIL.  */

static void
compute_trims (ao_ref *ref, sbitmap live, int *trim_head, int *trim_tail)
{
  /* We use sbitmaps biased such that ref->offset is bit zero and the bitmap
     extends through ref->size.  So we know that in the original bitmap
     bits 0..ref->size were true.  We don't actually need the bitmap, just
     the REF to compute the trims.  */

  /* Now identify how much, if any of the tail we can chop off.  */
  *trim_tail = 0;
  int last_orig = (ref->size / BITS_PER_UNIT) - 1;
  int last_live = bitmap_last_set_bit (live);
  *trim_tail = (last_orig - last_live) & ~0x1;

  /* Identify how much, if any of the head we can chop off.  */
  int first_orig = 0;
  int first_live = bitmap_first_set_bit (live);
  *trim_head = (first_live - first_orig) & ~0x1;
}

/* STMT initializes an object from COMPLEX_CST where one or more of the
   bytes written may be dead stores.  REF is a representation of the
   memory written.  LIVE is the bitmap of stores that are actually live.

   Attempt to rewrite STMT so that only the real or imaginary part of
   the object is actually stored.  */

static void
maybe_trim_complex_store (ao_ref *ref, sbitmap live, gimple *stmt)
{
  int trim_head, trim_tail;
  compute_trims (ref, live, &trim_head, &trim_tail);

  /* The amount of data trimmed from the head or tail must be at
     least half the size of the object to ensure we're trimming
     the entire real or imaginary half.  By writing things this
     way we avoid more O(n) bitmap operations.  */
  if (trim_tail * 2 >= ref->size / BITS_PER_UNIT)
    {
      /* TREE_REALPART is live */
      tree x = TREE_REALPART (gimple_assign_rhs1 (stmt));
      tree y = gimple_assign_lhs (stmt);
      y = build1 (REALPART_EXPR, TREE_TYPE (x), y);
      gimple_assign_set_lhs (stmt, y);
      gimple_assign_set_rhs1 (stmt, x);
    }
  else if (trim_head * 2 >= ref->size / BITS_PER_UNIT)
    {
      /* TREE_IMAGPART is live */
      tree x = TREE_IMAGPART (gimple_assign_rhs1 (stmt));
      tree y = gimple_assign_lhs (stmt);
      y = build1 (IMAGPART_EXPR, TREE_TYPE (x), y);
      gimple_assign_set_lhs (stmt, y);
      gimple_assign_set_rhs1 (stmt, x);
    }

  /* Other cases indicate parts of both the real and imag subobjects
     are live.  We do not try to optimize those cases.  */
}

/* STMT is a memory write where one or more bytes written are dead
   stores.  ORIG is the bitmap of bytes stored by STMT.  LIVE is the
   bitmap of stores that are actually live.

   Attempt to rewrite STMT so that it writes fewer memory locations.  Right
   now we only support trimming at the start or end of the memory region.
   It's not clear how much there is to be gained by trimming from the middle
   of the region.  */

static void
maybe_trim_partially_dead_store (ao_ref *ref, sbitmap live, gimple *stmt)
{
  if (is_gimple_assign (stmt))
    {
      switch (gimple_assign_rhs_code (stmt))
        {
        case COMPLEX_CST:
          maybe_trim_complex_store (ref, live, stmt);
          break;
        default:
          break;
        }
    }
}

/* A helper of dse_optimize_stmt.
   Given a GIMPLE_ASSIGN in STMT that writes to REF, find a candidate
   statement *USE_STMT that may prove STMT to be dead.
   Return TRUE if the above conditions are met, otherwise FALSE.  */

static dse_store_status
dse_classify_store (ao_ref *ref, gimple *stmt, gimple **use_stmt,
                    bool byte_tracking_enabled, sbitmap live_bytes)
{
  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, *defvar_def;
      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 DSE_STORE_LIVE;

      if (gimple_code (temp) == GIMPLE_PHI)
        defvar = PHI_RESULT (temp);
      else
        defvar = gimple_vdef (temp);
      defvar_def = 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);
                }
              /* Do not consider the PHI as use if it dominates the
                 stmt defining the virtual operand we are processing,
                 we have processed it already in this case.  */
              if (gimple_bb (defvar_def) != gimple_bb (use_stmt)
                  && !dominated_by_p (CDI_DOMINATORS,
                                      gimple_bb (defvar_def),
                                      gimple_bb (use_stmt)))
                temp = use_stmt;
            }
          /* If the statement is a use the store is not dead.  */
          else if (ref_maybe_used_by_stmt_p (use_stmt, ref))
            {
              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)
        {
          /* STMT might be partially dead and we may be able to reduce
             how many memory locations it stores into.  */
          if (byte_tracking_enabled && !gimple_clobber_p (stmt))
            return DSE_STORE_MAYBE_PARTIAL_DEAD;
          return DSE_STORE_LIVE;
        }

      /* 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 (ref_may_alias_global_p (ref))
            return DSE_STORE_LIVE;

          temp = stmt;
          break;
        }

      if (byte_tracking_enabled && temp)
        clear_bytes_written_by (live_bytes, temp, ref);
    }
  /* Continue walking until we reach a full kill as a single statement
     or there are no more live bytes.  */
  while (!stmt_kills_ref_p (temp, ref)
         && !(byte_tracking_enabled && bitmap_empty_p (live_bytes)));

  *use_stmt = temp;
  return DSE_STORE_DEAD;
}


class dse_dom_walker : public dom_walker
{
public:
  dse_dom_walker (cdi_direction direction)
    : dom_walker (direction), m_byte_tracking_enabled (false)

  { m_live_bytes = sbitmap_alloc (PARAM_VALUE (PARAM_DSE_MAX_OBJECT_SIZE)); }

  ~dse_dom_walker () { sbitmap_free (m_live_bytes); }

  virtual edge before_dom_children (basic_block);

private:
  sbitmap m_live_bytes;
  bool m_byte_tracking_enabled;
  void dse_optimize_stmt (gimple_stmt_iterator *);
};

/* Delete a dead call STMT, which is mem* call of some kind.  */
static void
delete_dead_call (gimple *stmt)
{
  if (dump_file && (dump_flags & TDF_DETAILS))
    {
      fprintf (dump_file, "  Deleted dead call: ");
      print_gimple_stmt (dump_file, stmt, dump_flags, 0);
      fprintf (dump_file, "\n");
    }

  tree lhs = gimple_call_lhs (stmt);
  gimple_stmt_iterator gsi = gsi_for_stmt (stmt);
  if (lhs)
    {
      tree ptr = gimple_call_arg (stmt, 0);
      gimple *new_stmt = gimple_build_assign (lhs, ptr);
      unlink_stmt_vdef (stmt);
      if (gsi_replace (&gsi, new_stmt, true))
        bitmap_set_bit (need_eh_cleanup, gimple_bb (stmt)->index);
    }
  else
    {
      /* Then we need to fix the operand of the consuming stmt.  */
      unlink_stmt_vdef (stmt);

      /* Remove the dead store.  */
      if (gsi_remove (&gsi, true))
        bitmap_set_bit (need_eh_cleanup, gimple_bb (stmt)->index);
      release_defs (stmt);
    }
}

/* Delete a dead store STMT, which is a gimple assignment. */

static void
delete_dead_assignment (gimple *stmt)
{
  if (dump_file && (dump_flags & TDF_DETAILS))
    {
      fprintf (dump_file, "  Deleted dead store: ");
      print_gimple_stmt (dump_file, stmt, dump_flags, 0);
      fprintf (dump_file, "\n");
    }

  /* Then we need to fix the operand of the consuming stmt.  */
  unlink_stmt_vdef (stmt);

  /* Remove the dead store.  */
  gimple_stmt_iterator gsi = gsi_for_stmt (stmt);
  basic_block bb = gimple_bb (stmt);
  if (gsi_remove (&gsi, true))
    bitmap_set_bit (need_eh_cleanup, bb->index);

  /* And release any SSA_NAMEs set in this statement back to the
     SSA_NAME manager.  */
  release_defs (stmt);
}

/* 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.  */

void
dse_dom_walker::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;

  /* Don't return early on *this_2(D) ={v} {CLOBBER}.  */
  if (gimple_has_volatile_ops (stmt)
      && (!gimple_clobber_p (stmt)
          || TREE_CODE (gimple_assign_lhs (stmt)) != MEM_REF))
    return;

  ao_ref ref;
  if (!initialize_ao_ref_for_dse (stmt, &ref))
    return;

  /* We know we have virtual definitions.  We can handle assignments and
     some builtin calls.  */
  if (gimple_call_builtin_p (stmt, BUILT_IN_NORMAL))
    {
      switch (DECL_FUNCTION_CODE (gimple_call_fndecl (stmt)))
        {
          case BUILT_IN_MEMCPY:
          case BUILT_IN_MEMMOVE:
          case BUILT_IN_MEMSET:
            {
              gimple *use_stmt;
              enum dse_store_status store_status;
              m_byte_tracking_enabled
                = setup_live_bytes_from_ref (&ref, m_live_bytes);
              store_status = dse_classify_store (&ref, stmt, &use_stmt,
                                                 m_byte_tracking_enabled,
                                                 m_live_bytes);
              if (store_status == DSE_STORE_LIVE)
                return;

              if (store_status == DSE_STORE_MAYBE_PARTIAL_DEAD)
                {
                  maybe_trim_partially_dead_store (&ref, m_live_bytes, stmt);
                  return;
                }

              if (store_status == DSE_STORE_DEAD)
                delete_dead_call (stmt);
              return;
            }

          default:
            return;
        }
    }

  if (is_gimple_assign (stmt))
    {
      gimple *use_stmt;

      /* Self-assignments are zombies.  */
      if (operand_equal_p (gimple_assign_rhs1 (stmt),
                           gimple_assign_lhs (stmt), 0))
        use_stmt = stmt;
      else
        {
          m_byte_tracking_enabled
            = setup_live_bytes_from_ref (&ref, m_live_bytes);
          enum dse_store_status store_status;
          store_status = dse_classify_store (&ref, stmt, &use_stmt,
                                             m_byte_tracking_enabled,
                                             m_live_bytes);
          if (store_status == DSE_STORE_LIVE)
            return;

          if (store_status == DSE_STORE_MAYBE_PARTIAL_DEAD)
            {
              maybe_trim_partially_dead_store (&ref, m_live_bytes, stmt);
              return;
            }
        }

      /* Now we know that use_stmt kills the LHS of stmt.  */

      /* But only remove *this_2(D) ={v} {CLOBBER} if killed by
         another clobber stmt.  */
      if (gimple_clobber_p (stmt)
          && !gimple_clobber_p (use_stmt))
        return;

      delete_dead_assignment (stmt);
    }
}

edge
dse_dom_walker::before_dom_children (basic_block bb)
{
  gimple_stmt_iterator gsi;

  for (gsi = gsi_last_bb (bb); !gsi_end_p (gsi);)
    {
      dse_optimize_stmt (&gsi);
      if (gsi_end_p (gsi))
        gsi = gsi_last_bb (bb);
      else
        gsi_prev (&gsi);
    }
  return NULL;
}

namespace {

const pass_data pass_data_dse =
{
  GIMPLE_PASS, /* type */
  "dse", /* name */
  OPTGROUP_NONE, /* optinfo_flags */
  TV_TREE_DSE, /* tv_id */
  ( PROP_cfg | PROP_ssa ), /* properties_required */
  0, /* properties_provided */
  0, /* properties_destroyed */
  0, /* todo_flags_start */
  0, /* todo_flags_finish */
};

class pass_dse : public gimple_opt_pass
{
public:
  pass_dse (gcc::context *ctxt)
    : gimple_opt_pass (pass_data_dse, ctxt)
  {}

  /* opt_pass methods: */
  opt_pass * clone () { return new pass_dse (m_ctxt); }
  virtual bool gate (function *) { return flag_tree_dse != 0; }
  virtual unsigned int execute (function *);

}; // class pass_dse

unsigned int
pass_dse::execute (function *fun)
{
  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.  */
  dse_dom_walker (CDI_POST_DOMINATORS).walk (fun->cfg->x_exit_block_ptr);

  /* 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;
}

} // anon namespace

gimple_opt_pass *
make_pass_dse (gcc::context *ctxt)
{
  return new pass_dse (ctxt);
}