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/* Loop unswitching.
   Copyright (C) 2004-2020 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 "tree.h"
#include "gimple.h"
#include "tree-pass.h"
#include "ssa.h"
#include "fold-const.h"
#include "gimplify.h"
#include "tree-cfg.h"
#include "tree-ssa.h"
#include "tree-ssa-loop-niter.h"
#include "tree-ssa-loop.h"
#include "tree-into-ssa.h"
#include "cfgloop.h"
#include "tree-inline.h"
#include "gimple-iterator.h"
#include "cfghooks.h"
#include "tree-ssa-loop-manip.h"

/* This file implements the loop unswitching, i.e. transformation of loops like

   while (A)
     {
       if (inv)
         B;

       X;

       if (!inv)
	 C;
     }

   where inv is the loop invariant, into

   if (inv)
     {
       while (A)
	 {
           B;
	   X;
	 }
     }
   else
     {
       while (A)
	 {
	   X;
	   C;
	 }
     }

   Inv is considered invariant iff the values it compares are both invariant;
   tree-ssa-loop-im.c ensures that all the suitable conditions are in this
   shape.  */

static class loop *tree_unswitch_loop (class loop *, basic_block, tree);
static bool tree_unswitch_single_loop (class loop *, int);
static tree tree_may_unswitch_on (basic_block, class loop *);
static bool tree_unswitch_outer_loop (class loop *);
static edge find_loop_guard (class loop *);
static bool empty_bb_without_guard_p (class loop *, basic_block);
static bool used_outside_loop_p (class loop *, tree);
static void hoist_guard (class loop *, edge);
static bool check_exit_phi (class loop *);
static tree get_vop_from_header (class loop *);

/* Main entry point.  Perform loop unswitching on all suitable loops.  */

unsigned int
tree_ssa_unswitch_loops (void)
{
  class loop *loop;
  bool changed = false;

  /* Go through all loops starting from innermost.  */
  FOR_EACH_LOOP (loop, LI_FROM_INNERMOST)
    {
      if (!loop->inner)
	/* Unswitch innermost loop.  */
	changed |= tree_unswitch_single_loop (loop, 0);
      else
	changed |= tree_unswitch_outer_loop (loop);
    }

  if (changed)
    return TODO_cleanup_cfg;
  return 0;
}

/* Return TRUE if an SSA_NAME maybe undefined and is therefore
   unsuitable for unswitching.  STMT is the statement we are
   considering for unswitching and LOOP is the loop it appears in.  */

static bool
is_maybe_undefined (const tree name, gimple *stmt, class loop *loop)
{
  /* The loop header is the only block we can trivially determine that
     will always be executed.  If the comparison is in the loop
     header, we know it's OK to unswitch on it.  */
  if (gimple_bb (stmt) == loop->header)
    return false;

  auto_bitmap visited_ssa;
  auto_vec<tree> worklist;
  worklist.safe_push (name);
  bitmap_set_bit (visited_ssa, SSA_NAME_VERSION (name));
  while (!worklist.is_empty ())
    {
      tree t = worklist.pop ();

      /* If it's obviously undefined, avoid further computations.  */
      if (ssa_undefined_value_p (t, true))
	return true;

      if (ssa_defined_default_def_p (t))
	continue;

      gimple *def = SSA_NAME_DEF_STMT (t);

      /* Check that all the PHI args are fully defined.  */
      if (gphi *phi = dyn_cast <gphi *> (def))
	{
	  for (unsigned i = 0; i < gimple_phi_num_args (phi); ++i)
	    {
	      tree t = gimple_phi_arg_def (phi, i);
	      /* If an SSA has already been seen, it may be a loop,
		 but we can continue and ignore this use.  Otherwise,
		 add the SSA_NAME to the queue and visit it later.  */
	      if (TREE_CODE (t) == SSA_NAME
		  && bitmap_set_bit (visited_ssa, SSA_NAME_VERSION (t)))
		worklist.safe_push (t);
	    }
	  continue;
	}

      /* Uses in stmts always executed when the region header executes
	 are fine.  */
      if (dominated_by_p (CDI_DOMINATORS, loop->header, gimple_bb (def)))
	continue;

      /* Handle calls and memory loads conservatively.  */
      if (!is_gimple_assign (def)
	  || (gimple_assign_single_p (def)
	      && gimple_vuse (def)))
	return true;

      /* Check that any SSA names used to define NAME are also fully
	 defined.  */
      use_operand_p use_p;
      ssa_op_iter iter;
      FOR_EACH_SSA_USE_OPERAND (use_p, def, iter, SSA_OP_USE)
	{
	  tree t = USE_FROM_PTR (use_p);
	  /* If an SSA has already been seen, it may be a loop,
	     but we can continue and ignore this use.  Otherwise,
	     add the SSA_NAME to the queue and visit it later.  */
	  if (bitmap_set_bit (visited_ssa, SSA_NAME_VERSION (t)))
	    worklist.safe_push (t);
	}
    }
  return false;
}

/* Checks whether we can unswitch LOOP on condition at end of BB -- one of its
   basic blocks (for what it means see comments below).  */

static tree
tree_may_unswitch_on (basic_block bb, class loop *loop)
{
  gimple *last, *def;
  gcond *stmt;
  tree cond, use;
  basic_block def_bb;
  ssa_op_iter iter;

  /* BB must end in a simple conditional jump.  */
  last = last_stmt (bb);
  if (!last || gimple_code (last) != GIMPLE_COND)
    return NULL_TREE;
  stmt = as_a <gcond *> (last);

  /* To keep the things simple, we do not directly remove the conditions,
     but just replace tests with 0 != 0 resp. 1 != 0.  Prevent the infinite
     loop where we would unswitch again on such a condition.  */
  if (gimple_cond_true_p (stmt) || gimple_cond_false_p (stmt))
    return NULL_TREE;

  /* Condition must be invariant.  */
  FOR_EACH_SSA_TREE_OPERAND (use, stmt, iter, SSA_OP_USE)
    {
      def = SSA_NAME_DEF_STMT (use);
      def_bb = gimple_bb (def);
      if (def_bb
	  && flow_bb_inside_loop_p (loop, def_bb))
	return NULL_TREE;
      /* Unswitching on undefined values would introduce undefined
	 behavior that the original program might never exercise.  */
      if (is_maybe_undefined (use, stmt, loop))
	return NULL_TREE;
    }

  cond = build2 (gimple_cond_code (stmt), boolean_type_node,
		 gimple_cond_lhs (stmt), gimple_cond_rhs (stmt));

  return cond;
}

/* Simplifies COND using checks in front of the entry of the LOOP.  Just very
   simplish (sufficient to prevent us from duplicating loop in unswitching
   unnecessarily).  */

static tree
simplify_using_entry_checks (class loop *loop, tree cond)
{
  edge e = loop_preheader_edge (loop);
  gimple *stmt;

  while (1)
    {
      stmt = last_stmt (e->src);
      if (stmt
	  && gimple_code (stmt) == GIMPLE_COND
	  && gimple_cond_code (stmt) == TREE_CODE (cond)
	  && operand_equal_p (gimple_cond_lhs (stmt),
			      TREE_OPERAND (cond, 0), 0)
	  && operand_equal_p (gimple_cond_rhs (stmt),
			      TREE_OPERAND (cond, 1), 0))
	return (e->flags & EDGE_TRUE_VALUE
		? boolean_true_node
		: boolean_false_node);

      if (!single_pred_p (e->src))
	return cond;

      e = single_pred_edge (e->src);
      if (e->src == ENTRY_BLOCK_PTR_FOR_FN (cfun))
	return cond;
    }
}

/* Unswitch single LOOP.  NUM is number of unswitchings done; we do not allow
   it to grow too much, it is too easy to create example on that the code would
   grow exponentially.  */

static bool
tree_unswitch_single_loop (class loop *loop, int num)
{
  basic_block *bbs;
  class loop *nloop;
  unsigned i, found;
  tree cond = NULL_TREE;
  gimple *stmt;
  bool changed = false;
  HOST_WIDE_INT iterations;

  /* Perform initial tests if unswitch is eligible.  */
  if (num == 0)
    {
      /* Do not unswitch in cold regions. */
      if (optimize_loop_for_size_p (loop))
	{
	  if (dump_file && (dump_flags & TDF_DETAILS))
	    fprintf (dump_file, ";; Not unswitching cold loops\n");
	  return false;
	}

      /* The loop should not be too large, to limit code growth. */
      if (tree_num_loop_insns (loop, &eni_size_weights)
	  > (unsigned) param_max_unswitch_insns)
	{
	  if (dump_file && (dump_flags & TDF_DETAILS))
	    fprintf (dump_file, ";; Not unswitching, loop too big\n");
	  return false;
	}

      /* If the loop is not expected to iterate, there is no need
	 for unswitching.  */
      iterations = estimated_loop_iterations_int (loop);
      if (iterations < 0)
        iterations = likely_max_loop_iterations_int (loop);
      if (iterations >= 0 && iterations <= 1)
	{
	  if (dump_file && (dump_flags & TDF_DETAILS))
	    fprintf (dump_file, ";; Not unswitching, loop is not expected"
		     " to iterate\n");
	  return false;
	}
    }

  i = 0;
  bbs = get_loop_body (loop);
  found = loop->num_nodes;

  while (1)
    {
      /* Find a bb to unswitch on.  */
      for (; i < loop->num_nodes; i++)
	if ((cond = tree_may_unswitch_on (bbs[i], loop)))
	  break;

      if (i == loop->num_nodes)
	{
	  if (dump_file
	      && num > param_max_unswitch_level
	      && (dump_flags & TDF_DETAILS))
	    fprintf (dump_file, ";; Not unswitching anymore, hit max level\n");

	  if (found == loop->num_nodes)
	    {
	      free (bbs);
	      return changed;
	    }
	  break;
	}

      cond = simplify_using_entry_checks (loop, cond);
      stmt = last_stmt (bbs[i]);
      if (integer_nonzerop (cond))
	{
	  /* Remove false path.  */
	  gimple_cond_set_condition_from_tree (as_a <gcond *> (stmt),
					       boolean_true_node);
	  changed = true;
	}
      else if (integer_zerop (cond))
	{
	  /* Remove true path.  */
	  gimple_cond_set_condition_from_tree (as_a <gcond *> (stmt),
					       boolean_false_node);
	  changed = true;
	}
      /* Do not unswitch too much.  */
      else if (num > param_max_unswitch_level)
	{
	  i++;
	  continue;
	}
      /* In nested tree_unswitch_single_loop first optimize all conditions
	 using entry checks, then discover still reachable blocks in the
	 loop and find the condition only among those still reachable bbs.  */
      else if (num != 0)
	{
	  if (found == loop->num_nodes)
	    found = i;
	  i++;
	  continue;
	}
      else
	{
	  found = i;
	  break;
	}

      update_stmt (stmt);
      i++;
    }

  if (num != 0)
    {
      basic_block *tos, *worklist;

      /* When called recursively, first do a quick discovery
	 of reachable bbs after the above changes and only
	 consider conditions in still reachable bbs.  */
      tos = worklist = XNEWVEC (basic_block, loop->num_nodes);

      for (i = 0; i < loop->num_nodes; i++)
	bbs[i]->flags &= ~BB_REACHABLE;

      /* Start with marking header.  */
      *tos++ = bbs[0];
      bbs[0]->flags |= BB_REACHABLE;

      /* Iterate: find everything reachable from what we've already seen
	 within the same innermost loop.  Don't look through false edges
	 if condition is always true or true edges if condition is