gcc/tree-ssa-phiprop.c

/* Backward propagation of indirect loads through PHIs.
   Copyright (C) 2007-2021 Free Software Foundation, Inc.
   Contributed by Richard Guenther <rguenther@suse.de>

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 "tree.h"
#include "gimple.h"
#include "tree-pass.h"
#include "ssa.h"
#include "gimple-pretty-print.h"
#include "fold-const.h"
#include "tree-eh.h"
#include "gimplify.h"
#include "gimple-iterator.h"
#include "stor-layout.h"
#include "tree-ssa-loop.h"

/* This pass propagates indirect loads through the PHI node for its
   address to make the load source possibly non-addressable and to
   allow for PHI optimization to trigger.

   For example the pass changes

     # addr_1 = PHI <&a, &b>
     tmp_1 = *addr_1;

   to

     # tmp_1 = PHI <a, b>

   but also handles more complex scenarios like

     D.2077_2 = &this_1(D)->a1;
     ...

     # b_12 = PHI <&c(2), D.2077_2(3)>
     D.2114_13 = *b_12;
     ...

     # b_15 = PHI <b_12(4), &b(5)>
     D.2080_5 = &this_1(D)->a0;
     ...

     # b_18 = PHI <D.2080_5(6), &c(7)>
     ...

     # b_21 = PHI <b_15(8), b_18(9)>
     D.2076_8 = *b_21;

   where the addresses loaded are defined by PHIs itself.
   The above happens for

     std::max(std::min(a0, c), std::min(std::max(a1, c), b))

   where this pass transforms it to a form later PHI optimization
   recognizes and transforms it to the simple

     D.2109_10 = this_1(D)->a1;
     D.2110_11 = c;
     D.2114_31 = MAX_EXPR <D.2109_10, D.2110_11>;
     D.2115_14 = b;
     D.2125_17 = MIN_EXPR <D.2115_14, D.2114_31>;
     D.2119_16 = this_1(D)->a0;
     D.2124_32 = MIN_EXPR <D.2110_11, D.2119_16>;
     D.2076_33 = MAX_EXPR <D.2125_17, D.2124_32>;

   The pass does a dominator walk processing loads using a basic-block
   local analysis and stores the result for use by transformations on
   dominated basic-blocks.  */


/* Structure to keep track of the value of a dereferenced PHI result
   and the virtual operand used for that dereference.  */

struct phiprop_d
{
  tree value;
  tree vuse;
};

/* Verify if the value recorded for NAME in PHIVN is still valid at
   the start of basic block BB.  */

static bool
phivn_valid_p (struct phiprop_d *phivn, tree name, basic_block bb)
{
  tree vuse = phivn[SSA_NAME_VERSION (name)].vuse;
  gimple *use_stmt;
  imm_use_iterator ui2;
  bool ok = true;

  /* The def stmts of the virtual uses need to be dominated by bb.  */
  gcc_assert (vuse != NULL_TREE);

  FOR_EACH_IMM_USE_STMT (use_stmt, ui2, vuse)
    {
      /* If BB does not dominate a VDEF, the value is invalid.  */
      if ((gimple_vdef (use_stmt) != NULL_TREE
	   || gimple_code (use_stmt) == GIMPLE_PHI)
	  && !dominated_by_p (CDI_DOMINATORS, gimple_bb (use_stmt), bb))
	{
	  ok = false;
	  break;
	}
    }

  return ok;
}

/* Insert a new phi node for the dereference of PHI at basic_block
   BB with the virtual operands from USE_STMT.  */

static tree
phiprop_insert_phi (basic_block bb, gphi *phi, gimple *use_stmt,
		    struct phiprop_d *phivn, size_t n)
{
  tree res;
  gphi *new_phi = NULL;
  edge_iterator ei;
  edge e;

  gcc_assert (is_gimple_assign (use_stmt)
	      && gimple_assign_rhs_code (use_stmt) == MEM_REF);

  /* Build a new PHI node to replace the definition of
     the indirect reference lhs.  */
  res = gimple_assign_lhs (use_stmt);
  if (TREE_CODE (res) == SSA_NAME)
    new_phi = create_phi_node (res, bb);

  if (dump_file && (dump_flags & TDF_DETAILS))
    {
      fprintf (dump_file, "Inserting PHI for result of load ");
      print_gimple_stmt (dump_file, use_stmt, 0);
    }

  /* Add PHI arguments for each edge inserting loads of the
     addressable operands.  */
  FOR_EACH_EDGE (e, ei, bb->preds)
    {
      tree old_arg, new_var;
      gassign *tmp;
      location_t locus;

      old_arg = PHI_ARG_DEF_FROM_EDGE (phi, e);
      locus = gimple_phi_arg_location_from_edge (phi, e);
      while (TREE_CODE (old_arg) == SSA_NAME
	     && (SSA_NAME_VERSION (old_arg) >= n
	         || phivn[SSA_NAME_VERSION (old_arg)].value == NULL_TREE))
	{
	  gimple *def_stmt = SSA_NAME_DEF_STMT (old_arg);
	  old_arg = gimple_assign_rhs1 (def_stmt);
	  locus = gimple_location (def_stmt);
	}

      if (TREE_CODE (old_arg) == SSA_NAME)
	{
	  if (dump_file && (dump_flags & TDF_DETAILS))
	    {
	      fprintf (dump_file, "  for edge defining ");
	      print_generic_expr (dump_file, PHI_ARG_DEF_FROM_EDGE (phi, e));
	      fprintf (dump_file, " reusing PHI result ");
	      print_generic_expr (dump_file,
				  phivn[SSA_NAME_VERSION (old_arg)].value);
	      fprintf (dump_file, "\n");
	    }
	  /* Reuse a formerly created dereference.  */
	  new_var = phivn[SSA_NAME_VERSION (old_arg)].value;
	}
      else
	{
	  tree rhs = gimple_assign_rhs1 (use_stmt);
	  gcc_assert (TREE_CODE (old_arg) == ADDR_EXPR);
	  if (TREE_CODE (res) == SSA_NAME)
	    new_var = make_ssa_name (TREE_TYPE (rhs));
	  else
	    new_var = unshare_expr (res);
	  if (!is_gimple_min_invariant (old_arg))
	    old_arg = PHI_ARG_DEF_FROM_EDGE (phi, e);
	  else
	    old_arg = unshare_expr (old_arg);
	  tmp = gimple_build_assign (new_var,
				     fold_build2 (MEM_REF, TREE_TYPE (rhs),
						  old_arg,
						  TREE_OPERAND (rhs, 1)));
	  gimple_set_location (tmp, locus);

	  gsi_insert_on_edge (e, tmp);
	  update_stmt (tmp);

	  if (dump_file && (dump_flags & TDF_DETAILS))
	    {
	      fprintf (dump_file, "  for edge defining ");
	      print_generic_expr (dump_file, PHI_ARG_DEF_FROM_EDGE (phi, e));
	      fprintf (dump_file, " inserting load ");
	      print_gimple_stmt (dump_file, tmp, 0);
	    }
	}

      if (new_phi)
	add_phi_arg (new_phi, new_var, e, locus);
    }

  if (new_phi)
    {
      update_stmt (new_phi);

      if (dump_file && (dump_flags & TDF_DETAILS))
	print_gimple_stmt (dump_file, new_phi, 0);
    }

  return res;
}

/* Verify if *idx is available at *DATA.  */

static bool
chk_uses (tree, tree *idx, void *data)
{
  basic_block dom = (basic_block) data;
  if (TREE_CODE (*idx) == SSA_NAME)
    return (SSA_NAME_IS_DEFAULT_DEF (*idx)
	    || ! dominated_by_p (CDI_DOMINATORS,
				 gimple_bb (SSA_NAME_DEF_STMT (*idx)), dom));
  return true;
}

/* Propagate between the phi node arguments of PHI in BB and phi result
   users.  For now this matches
        # p_2 = PHI <&x, &y>
      <Lx>:;
	p_3 = p_2;
	z_2 = *p_3;
   and converts it to
	# z_2 = PHI <x, y>
      <Lx>:;
   Returns true if a transformation was done and edge insertions
   need to be committed.  Global data PHIVN and N is used to track
   past transformation results.  We need to be especially careful here
   with aliasing issues as we are moving memory reads.  */

static bool
propagate_with_phi (basic_block bb, gphi *phi, struct phiprop_d *phivn,
		    size_t n)
{
  tree ptr = PHI_RESULT (phi);
  gimple *use_stmt;
  tree res = NULL_TREE;
  gimple_stmt_iterator gsi;
  imm_use_iterator ui;
  use_operand_p arg_p, use;
  ssa_op_iter i;
  bool phi_inserted;
  bool changed;
  tree type = NULL_TREE;

  if (!POINTER_TYPE_P (TREE_TYPE (ptr))
      || (!is_gimple_reg_type (TREE_TYPE (TREE_TYPE (ptr)))
	  && TYPE_MODE (TREE_TYPE (TREE_TYPE (ptr))) == BLKmode))
    return false;

  /* Check if we can "cheaply" dereference all phi arguments.  */
  FOR_EACH_PHI_ARG (arg_p, phi, i, SSA_OP_USE)
    {
      tree arg = USE_FROM_PTR (arg_p);
      /* Walk the ssa chain until we reach a ssa name we already
	 created a value for or we reach a definition of the form
	 ssa_name_n = &var;  */
      while (TREE_CODE (arg) == SSA_NAME
	     && !SSA_NAME_IS_DEFAULT_DEF (arg)
	     && (SSA_NAME_VERSION (arg) >= n
	         || phivn[SSA_NAME_VERSION (arg)].value == NULL_TREE))
	{
	  gimple *def_stmt = SSA_NAME_DEF_STMT (arg);
	  if (!gimple_assign_single_p (def_stmt))
	    return false;
	  arg = gimple_assign_rhs1 (def_stmt);
	}
      if (TREE_CODE (arg) != ADDR_EXPR
	  && !(TREE_CODE (arg) == SSA_NAME
	       && SSA_NAME_VERSION (arg) < n
	       && phivn[SSA_NAME_VERSION (arg)].value != NULL_TREE
	       && (!type
		   || types_compatible_p
		       (type, TREE_TYPE (phivn[SSA_NAME_VERSION (arg)].value)))
	       && phivn_valid_p (phivn, arg, bb)))
	return false;
      if (!type
	  && TREE_CODE (arg) == SSA_NAME)
	type = TREE_TYPE (phivn[SSA_NAME_VERSION (arg)].value);
    }

  /* Find a dereferencing use.  First follow (single use) ssa
     copy chains for ptr.  */
  while (single_imm_use (ptr, &use, &use_stmt)
	 && gimple_assign_ssa_name_copy_p (use_stmt))
    ptr = gimple_assign_lhs (use_stmt);

  /* Replace the first dereference of *ptr if there is one and if we
     can move the loads to the place of the ptr phi node.  */
  phi_inserted = false;
  changed = false;
  FOR_EACH_IMM_USE_STMT (use_stmt, ui, ptr)
    {
      gimple *def_stmt;
      tree vuse;

      /* Only replace loads in blocks that post-dominate the PHI node.  That
         makes sure we don't end up speculating loads.  */
      if (!dominated_by_p (CDI_POST_DOMINATORS,
			   bb, gimple_bb (use_stmt)))
	continue;

      /* Check whether this is a load of *ptr.  */
      if (!(is_gimple_assign (use_stmt)
	    && gimple_assign_rhs_code (use_stmt) == MEM_REF
	    && TREE_OPERAND (gimple_assign_rhs1 (use_stmt), 0) == ptr
	    && integer_zerop (TREE_OPERAND (gimple_assign_rhs1 (use_stmt), 1))
	    && (!type
		|| types_compatible_p
		     (TREE_TYPE (gimple_assign_lhs (use_stmt)), type))
	    /* We cannot replace a load that may throw or is volatile.
	       For volatiles the transform can change the number of
	       executions if the load is inside a loop but the address
	       computations outside (PR91812).  We could relax this
	       if we guard against that appropriately.  For loads that can
	       throw we could relax things if the moved loads all are
	       known to not throw.  */
	    && !stmt_can_throw_internal (cfun, use_stmt)
	    && !gimple_has_volatile_ops (use_stmt)))
	continue;

      /* Check if we can move the loads.  The def stmt of the virtual use
	 needs to be in a different basic block dominating bb.  When the
	 def is an edge-inserted one we know it dominates us.  */
      vuse = gimple_vuse (use_stmt);
      def_stmt = SSA_NAME_DEF_STMT (vuse);
      if (!SSA_NAME_IS_DEFAULT_DEF (vuse)
	  && (gimple_bb (def_stmt) == bb
	      || (gimple_bb (def_stmt)
		  && !dominated_by_p (CDI_DOMINATORS,
				      bb, gimple_bb (def_stmt)))))
	goto next;

      /* Found a proper dereference with an aggregate copy.  Just
         insert aggregate copies on the edges instead.  */
      if (!is_gimple_reg_type (TREE_TYPE (gimple_assign_lhs (use_stmt))))
	{
	  if (!gimple_vdef (use_stmt))
	    goto next;

	  /* As we replicate the lhs on each incoming edge all
	     used SSA names have to be available there.  */
	  if (! for_each_index (gimple_assign_lhs_ptr (use_stmt),
				chk_uses,
				get_immediate_dominator (CDI_DOMINATORS,
							 gimple_bb (phi))))
	    goto next;

	  gimple *vuse_stmt;
	  imm_use_iterator vui;
	  use_operand_p vuse_p;
	  /* In order to move the aggregate copies earlier, make sure
	     there are no statements that could read from memory
	     aliasing the lhs in between the start of bb and use_stmt.
	     As we require use_stmt to have a VDEF above, loads after
	     use_stmt will use a different virtual SSA_NAME.  */
	  FOR_EACH_IMM_USE_FAST (vuse_p, vui, vuse)
	    {
	      vuse_stmt = USE_STMT (vuse_p);
	      if (vuse_stmt == use_stmt)
		continue;
	      if (!dominated_by_p (CDI_DOMINATORS,
				   gimple_bb (vuse_stmt), bb))
		continue;
	      if (ref_maybe_used_by_stmt_p (vuse_stmt,
					    gimple_assign_lhs (use_stmt)))
		goto next;
	    }

	  phiprop_insert_phi (bb, phi, use_stmt, phivn, n);

	  /* Remove old stmt.  The phi is taken care of by DCE.  */
	  gsi = gsi_for_stmt (use_stmt);
	  /* Unlinking the VDEF here is fine as we are sure that we process
	     stmts in execution order due to aggregate copies having VDEFs
	     and we emit loads on the edges in the very same order.
	     We get multiple copies (or intermediate register loads) handled
	     only by walking PHIs or immediate uses in a lucky order though,
	     so we could signal the caller to re-start iterating over PHIs
	     when we come here which would make it quadratic in the number
	     of PHIs.  */
	  unlink_stmt_vdef (use_stmt);
	  gsi_remove (&gsi, true);

	  changed = true;
	}

      /* Found a proper dereference.  Insert a phi node if this
	 is the first load transformation.  */
      else if (!phi_inserted)
	{
	  res = phiprop_insert_phi (bb, phi, use_stmt, phivn, n);
	  type = TREE_TYPE (res);

	  /* Remember the value we created for *ptr.  */
	  phivn[SSA_NAME_VERSION (ptr)].value = res;
	  phivn[SSA_NAME_VERSION (ptr)].vuse = vuse;

	  /* Remove old stmt.  The phi is taken care of by DCE, if we
	     want to delete it here we also have to delete all intermediate
	     copies.  */
	  gsi = gsi_for_stmt (use_stmt);
	  gsi_remove (&gsi, true);

	  phi_inserted = true;
	  changed = true;
	}
      else
	{
	  /* Further replacements are easy, just make a copy out of the
	     load.  */
	  gimple_assign_set_rhs1 (use_stmt, res);
	  update_stmt (use_stmt);
	  changed = true;
	}

next:;
      /* Continue searching for a proper dereference.  */
    }

  return changed;
}

/* Main entry for phiprop pass.  */

namespace {

const pass_data pass_data_phiprop =
{
  GIMPLE_PASS, /* type */
  "phiprop", /* name */
  OPTGROUP_NONE, /* optinfo_flags */
  TV_TREE_PHIPROP, /* tv_id */
  ( PROP_cfg | PROP_ssa ), /* properties_required */
  0, /* properties_provided */
  0, /* properties_destroyed */
  0, /* todo_flags_start */
  TODO_update_ssa, /* todo_flags_finish */
};

class pass_phiprop : public gimple_opt_pass
{
public:
  pass_phiprop (gcc::context *ctxt)
    : gimple_opt_pass (pass_data_phiprop, ctxt)
  {}

  /* opt_pass methods: */
  virtual bool gate (function *) { return flag_tree_phiprop; }
  virtual unsigned int execute (function *);

}; // class pass_phiprop

unsigned int
pass_phiprop::execute (function *fun)
{
  struct phiprop_d *phivn;
  bool did_something = false;
  basic_block bb;
  gphi_iterator gsi;
  unsigned i;
  size_t n;

  calculate_dominance_info (CDI_DOMINATORS);
  calculate_dominance_info (CDI_POST_DOMINATORS);

  n = num_ssa_names;
  phivn = XCNEWVEC (struct phiprop_d, n);

  /* Walk the dominator tree in preorder.  */
  auto_vec<basic_block> bbs
  = get_all_dominated_blocks (CDI_DOMINATORS,
			      single_succ (ENTRY_BLOCK_PTR_FOR_FN (fun)));
  FOR_EACH_VEC_ELT (bbs, i, bb)
    {
      /* Since we're going to move dereferences across predecessor
         edges avoid blocks with abnormal predecessors.  */
      if (bb_has_abnormal_pred (bb))
	continue;
      for (gsi = gsi_start_phis (bb); !gsi_end_p (gsi); gsi_next (&gsi))
	did_something |= propagate_with_phi (bb, gsi.phi (), phivn, n);
    }

  if (did_something)
    gsi_commit_edge_inserts ();

  free (phivn);

  free_dominance_info (CDI_POST_DOMINATORS);

  return 0;
}

} // anon namespace

gimple_opt_pass *
make_pass_phiprop (gcc::context *ctxt)
{
  return new pass_phiprop (ctxt);
}