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/* Generic SSA value propagation engine.

   Copyright (C) 2004, 2005 Free Software Foundation, Inc.

   Contributed by Diego Novillo <dnovillo@redhat.com>

   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, 51 Franklin Street, Fifth Floor, Boston, MA

   02110-1301, USA.  */

#include "config.h"

#include "system.h"

#include "coretypes.h"

#include "tm.h"

#include "tree.h"

#include "flags.h"

#include "rtl.h"

#include "tm_p.h"

#include "ggc.h"

#include "basic-block.h"

#include "output.h"

#include "expr.h"

#include "function.h"

#include "diagnostic.h"

#include "timevar.h"

#include "tree-dump.h"

#include "tree-flow.h"

#include "tree-pass.h"

#include "tree-ssa-propagate.h"

#include "langhooks.h"

#include "varray.h"

#include "vec.h"

/* This file implements a generic value propagation engine based on

   the same propagation used by the SSA-CCP algorithm [1].

   Propagation is performed by simulating the execution of every

   statement that produces the value being propagated.  Simulation

   proceeds as follows:

   1- Initially, all edges of the CFG are marked not executable and

      the CFG worklist is seeded with all the statements in the entry

      basic block (block 0).

   2- Every statement S is simulated with a call to the call-back

      function SSA_PROP_VISIT_STMT.  This evaluation may produce 3

      results:

        SSA_PROP_NOT_INTERESTING: Statement S produces nothing of

            interest and does not affect any of the work lists.

        SSA_PROP_VARYING: The value produced by S cannot be determined

            at compile time.  Further simulation of S is not required.

            If S is a conditional jump, all the outgoing edges for the

            block are considered executable and added to the work

            list.

        SSA_PROP_INTERESTING: S produces a value that can be computed

            at compile time.  Its result can be propagated into the

            statements that feed from S.  Furthermore, if S is a

            conditional jump, only the edge known to be taken is added

            to the work list.  Edges that are known not to execute are

            never simulated.

   3- PHI nodes are simulated with a call to SSA_PROP_VISIT_PHI.  The

      return value from SSA_PROP_VISIT_PHI has the same semantics as

      described in #2.

   4- Three work lists are kept.  Statements are only added to these

      lists if they produce one of SSA_PROP_INTERESTING or

      SSA_PROP_VARYING.

        CFG_BLOCKS contains the list of blocks to be simulated.

            Blocks are added to this list if their incoming edges are

            found executable.

        VARYING_SSA_EDGES contains the list of statements that feed

            from statements that produce an SSA_PROP_VARYING result.

            These are simulated first to speed up processing.

        INTERESTING_SSA_EDGES contains the list of statements that

            feed from statements that produce an SSA_PROP_INTERESTING

            result.

   5- Simulation terminates when all three work lists are drained.

   Before calling ssa_propagate, it is important to clear

   DONT_SIMULATE_AGAIN for all the statements in the program that

   should be simulated.  This initialization allows an implementation

   to specify which statements should never be simulated.

   It is also important to compute def-use information before calling

   ssa_propagate.

   References:

     [1] Constant propagation with conditional branches,

         Wegman and Zadeck, ACM TOPLAS 13(2):181-210.

     [2] Building an Optimizing Compiler,

         Robert Morgan, Butterworth-Heinemann, 1998, Section 8.9.

     [3] Advanced Compiler Design and Implementation,

         Steven Muchnick, Morgan Kaufmann, 1997, Section 12.6  */

/* Function pointers used to parameterize the propagation engine.  */

static ssa_prop_visit_stmt_fn ssa_prop_visit_stmt;

static ssa_prop_visit_phi_fn ssa_prop_visit_phi;

/* Use the TREE_DEPRECATED bitflag to mark statements that have been

   added to one of the SSA edges worklists.  This flag is used to

   avoid visiting statements unnecessarily when draining an SSA edge

   worklist.  If while simulating a basic block, we find a statement with

   STMT_IN_SSA_EDGE_WORKLIST set, we clear it to prevent SSA edge

   processing from visiting it again.  */

#define STMT_IN_SSA_EDGE_WORKLIST(T)    TREE_DEPRECATED (T)

/* A bitmap to keep track of executable blocks in the CFG.  */

static sbitmap executable_blocks;

/* Array of control flow edges on the worklist.  */

static GTY(()) varray_type cfg_blocks = NULL;

static unsigned int cfg_blocks_num = 0;

static int cfg_blocks_tail;

static int cfg_blocks_head;

static sbitmap bb_in_list;

/* Worklist of SSA edges which will need reexamination as their

   definition has changed.  SSA edges are def-use edges in the SSA

   web.  For each D-U edge, we store the target statement or PHI node

   U.  */

static GTY(()) VEC(tree,gc) *interesting_ssa_edges;

/* Identical to INTERESTING_SSA_EDGES.  For performance reasons, the

   list of SSA edges is split into two.  One contains all SSA edges

   who need to be reexamined because their lattice value changed to

   varying (this worklist), and the other contains all other SSA edges

   to be reexamined (INTERESTING_SSA_EDGES).

   Since most values in the program are VARYING, the ideal situation

   is to move them to that lattice value as quickly as possible.

   Thus, it doesn't make sense to process any other type of lattice

   value until all VARYING values are propagated fully, which is one

   thing using the VARYING worklist achieves.  In addition, if we

   don't use a separate worklist for VARYING edges, we end up with

   situations where lattice values move from

   UNDEFINED->INTERESTING->VARYING instead of UNDEFINED->VARYING.  */

static GTY(()) VEC(tree,gc) *varying_ssa_edges;

/* Return true if the block worklist empty.  */

static inline bool

cfg_blocks_empty_p (void)

{

  return (cfg_blocks_num == 0);

}

/* Add a basic block to the worklist.  The block must not be already

   in the worklist, and it must not be the ENTRY or EXIT block.  */

static void

cfg_blocks_add (basic_block bb)

{

  gcc_assert (bb != ENTRY_BLOCK_PTR && bb != EXIT_BLOCK_PTR);

  gcc_assert (!TEST_BIT (bb_in_list, bb->index));

  if (cfg_blocks_empty_p ())

    {

      cfg_blocks_tail = cfg_blocks_head = 0;

      cfg_blocks_num = 1;

    }

  else

    {

      cfg_blocks_num++;

      if (cfg_blocks_num > VARRAY_SIZE (cfg_blocks))

        {

          /* We have to grow the array now.  Adjust to queue to occupy the

             full space of the original array.  */

          cfg_blocks_tail = VARRAY_SIZE (cfg_blocks);

          cfg_blocks_head = 0;

          VARRAY_GROW (cfg_blocks, 2 * VARRAY_SIZE (cfg_blocks));

        }

      else

        cfg_blocks_tail = (cfg_blocks_tail + 1) % VARRAY_SIZE (cfg_blocks);

    }

  VARRAY_BB (cfg_blocks, cfg_blocks_tail) = bb;

  SET_BIT (bb_in_list, bb->index);

}

/* Remove a block from the worklist.  */

static basic_block

cfg_blocks_get (void)

{

  basic_block bb;

  bb = VARRAY_BB (cfg_blocks, cfg_blocks_head);

  gcc_assert (!cfg_blocks_empty_p ());

  gcc_assert (bb);

  cfg_blocks_head = (cfg_blocks_head + 1) % VARRAY_SIZE (cfg_blocks);

  --cfg_blocks_num;

  RESET_BIT (bb_in_list, bb->index);

  return bb;

}

/* We have just defined a new value for VAR.  If IS_VARYING is true,

   add all immediate uses of VAR to VARYING_SSA_EDGES, otherwise add

   them to INTERESTING_SSA_EDGES.  */

static void

add_ssa_edge (tree var, bool is_varying)

{

  imm_use_iterator iter;

  use_operand_p use_p;

  FOR_EACH_IMM_USE_FAST (use_p, iter, var)

    {

      tree use_stmt = USE_STMT (use_p);

      if (!DONT_SIMULATE_AGAIN (use_stmt)

          && !STMT_IN_SSA_EDGE_WORKLIST (use_stmt))

        {

          STMT_IN_SSA_EDGE_WORKLIST (use_stmt) = 1;

          if (is_varying)

            VEC_safe_push (tree, gc, varying_ssa_edges, use_stmt);

          else

            VEC_safe_push (tree, gc, interesting_ssa_edges, use_stmt);

        }

    }

}

/* Add edge E to the control flow worklist.  */

static void

add_control_edge (edge e)

{

  basic_block bb = e->dest;

  if (bb == EXIT_BLOCK_PTR)

    return;

  /* If the edge had already been executed, skip it.  */

  if (e->flags & EDGE_EXECUTABLE)

    return;

  e->flags |= EDGE_EXECUTABLE;

  /* If the block is already in the list, we're done.  */

  if (TEST_BIT (bb_in_list, bb->index))

    return;

  cfg_blocks_add (bb);

  if (dump_file && (dump_flags & TDF_DETAILS))

    fprintf (dump_file, "Adding Destination of edge (%d -> %d) to worklist\n\n",

        e->src->index, e->dest->index);

}

/* Simulate the execution of STMT and update the work lists accordingly.  */

static void

simulate_stmt (tree stmt)

{

  enum ssa_prop_result val = SSA_PROP_NOT_INTERESTING;

  edge taken_edge = NULL;

  tree output_name = NULL_TREE;

  /* Don't bother visiting statements that are already

     considered varying by the propagator.  */

  if (DONT_SIMULATE_AGAIN (stmt))

    return;

  if (TREE_CODE (stmt) == PHI_NODE)

    {

      val = ssa_prop_visit_phi (stmt);

      output_name = PHI_RESULT (stmt);

    }

  else

    val = ssa_prop_visit_stmt (stmt, &taken_edge, &output_name);

  if (val == SSA_PROP_VARYING)

    {

      DONT_SIMULATE_AGAIN (stmt) = 1;

      /* If the statement produced a new varying value, add the SSA

         edges coming out of OUTPUT_NAME.  */

      if (output_name)

        add_ssa_edge (output_name, true);

      /* If STMT transfers control out of its basic block, add

         all outgoing edges to the work list.  */

      if (stmt_ends_bb_p (stmt))

        {

          edge e;

          edge_iterator ei;

          basic_block bb = bb_for_stmt (stmt);

          FOR_EACH_EDGE (e, ei, bb->succs)

            add_control_edge (e);

        }

    }

  else if (val == SSA_PROP_INTERESTING)

    {

      /* If the statement produced new value, add the SSA edges coming

         out of OUTPUT_NAME.  */

      if (output_name)

        add_ssa_edge (output_name, false);

      /* If we know which edge is going to be taken out of this block,

         add it to the CFG work list.  */

      if (taken_edge)

        add_control_edge (taken_edge);

    }

}

/* Process an SSA edge worklist.  WORKLIST is the SSA edge worklist to

   drain.  This pops statements off the given WORKLIST and processes

   them until there are no more statements on WORKLIST.

   We take a pointer to WORKLIST because it may be reallocated when an

   SSA edge is added to it in simulate_stmt.  */

static void

process_ssa_edge_worklist (VEC(tree,gc) **worklist)

{

  /* Drain the entire worklist.  */

  while (VEC_length (tree, *worklist) > 0)

    {

      basic_block bb;

      /* Pull the statement to simulate off the worklist.  */

      tree stmt = VEC_pop (tree, *worklist);

      /* If this statement was already visited by simulate_block, then

         we don't need to visit it again here.  */

      if (!STMT_IN_SSA_EDGE_WORKLIST (stmt))

        continue;

      /* STMT is no longer in a worklist.  */

      STMT_IN_SSA_EDGE_WORKLIST (stmt) = 0;

      if (dump_file && (dump_flags & TDF_DETAILS))

        {

          fprintf (dump_file, "\nSimulating statement (from ssa_edges): ");

          print_generic_stmt (dump_file, stmt, dump_flags);

        }

      bb = bb_for_stmt (stmt);

      /* PHI nodes are always visited, regardless of whether or not

         the destination block is executable.  Otherwise, visit the

         statement only if its block is marked executable.  */

      if (TREE_CODE (stmt) == PHI_NODE

          || TEST_BIT (executable_blocks, bb->index))

        simulate_stmt (stmt);

    }

}

/* Simulate the execution of BLOCK.  Evaluate the statement associated

   with each variable reference inside the block.  */

static void

simulate_block (basic_block block)

{

  tree phi;

  /* There is nothing to do for the exit block.  */

  if (block == EXIT_BLOCK_PTR)

    return;

  if (dump_file && (dump_flags & TDF_DETAILS))

    fprintf (dump_file, "\nSimulating block %d\n", block->index);

  /* Always simulate PHI nodes, even if we have simulated this block

     before.  */

  for (phi = phi_nodes (block); phi; phi = PHI_CHAIN (phi))

    simulate_stmt (phi);

  /* If this is the first time we've simulated this block, then we

     must simulate each of its statements.  */

  if (!TEST_BIT (executable_blocks, block->index))

    {

      block_stmt_iterator j;

      unsigned int normal_edge_count;

      edge e, normal_edge;

      edge_iterator ei;

      /* Note that we have simulated this block.  */

      SET_BIT (executable_blocks, block->index);

      for (j = bsi_start (block); !bsi_end_p (j); bsi_next (&j))

        {

          tree stmt = bsi_stmt (j);

          /* If this statement is already in the worklist then

             "cancel" it.  The reevaluation implied by the worklist

             entry will produce the same value we generate here and

             thus reevaluating it again from the worklist is

             pointless.  */

          if (STMT_IN_SSA_EDGE_WORKLIST (stmt))

            STMT_IN_SSA_EDGE_WORKLIST (stmt) = 0;

          simulate_stmt (stmt);

        }

      /* We can not predict when abnormal edges will be executed, so

         once a block is considered executable, we consider any

         outgoing abnormal edges as executable.

         At the same time, if this block has only one successor that is

         reached by non-abnormal edges, then add that successor to the

         worklist.  */

      normal_edge_count = 0;

      normal_edge = NULL;

      FOR_EACH_EDGE (e, ei, block->succs)

        {

          if (e->flags & EDGE_ABNORMAL)

            add_control_edge (e);

          else

            {

              normal_edge_count++;

              normal_edge = e;

            }

        }

      if (normal_edge_count == 1)

        add_control_edge (normal_edge);

    }

}

/* Initialize local data structures and work lists.  */

static void

ssa_prop_init (void)

{

  edge e;

  edge_iterator ei;

  basic_block bb;

  size_t i;

  /* Worklists of SSA edges.  */

  interesting_ssa_edges = VEC_alloc (tree, gc, 20);

  varying_ssa_edges = VEC_alloc (tree, gc, 20);

  executable_blocks = sbitmap_alloc (last_basic_block);

  sbitmap_zero (executable_blocks);

  bb_in_list = sbitmap_alloc (last_basic_block);

  sbitmap_zero (bb_in_list);

  if (dump_file && (dump_flags & TDF_DETAILS))

    dump_immediate_uses (dump_file);

  VARRAY_BB_INIT (cfg_blocks, 20, "cfg_blocks");

  /* Initialize the values for every SSA_NAME.  */

  for (i = 1; i < num_ssa_names; i++)

    if (ssa_name (i))

      SSA_NAME_VALUE (ssa_name (i)) = NULL_TREE;

  /* Initially assume that every edge in the CFG is not executable.

     (including the edges coming out of ENTRY_BLOCK_PTR).  */

  FOR_ALL_BB (bb)

    {

      block_stmt_iterator si;

      for (si = bsi_start (bb); !bsi_end_p (si); bsi_next (&si))

        STMT_IN_SSA_EDGE_WORKLIST (bsi_stmt (si)) = 0;

      FOR_EACH_EDGE (e, ei, bb->succs)

        e->flags &= ~EDGE_EXECUTABLE;

    }

  /* Seed the algorithm by adding the successors of the entry block to the

     edge worklist.  */

  FOR_EACH_EDGE (e, ei, ENTRY_BLOCK_PTR->succs)

    add_control_edge (e);

}

/* Free allocated storage.  */

static void

ssa_prop_fini (void)

{

  VEC_free (tree, gc, interesting_ssa_edges);

  VEC_free (tree, gc, varying_ssa_edges);

  cfg_blocks = NULL;

  sbitmap_free (bb_in_list);

  sbitmap_free (executable_blocks);

}

/* Get the main expression from statement STMT.  */

tree

get_rhs (tree stmt)

{

  enum tree_code code = TREE_CODE (stmt);

  switch (code)

    {

    case RETURN_EXPR:

      stmt = TREE_OPERAND (stmt, 0);

      if (!stmt || TREE_CODE (stmt) != MODIFY_EXPR)

        return stmt;

      /* FALLTHRU */

    case MODIFY_EXPR:

      stmt = TREE_OPERAND (stmt, 1);

      if (TREE_CODE (stmt) == WITH_SIZE_EXPR)

        return TREE_OPERAND (stmt, 0);

      else

        return stmt;

    case COND_EXPR:

      return COND_EXPR_COND (stmt);

    case SWITCH_EXPR:

      return SWITCH_COND (stmt);

    case GOTO_EXPR:

      return GOTO_DESTINATION (stmt);

    case LABEL_EXPR:

      return LABEL_EXPR_LABEL (stmt);

    default:

      return stmt;

    }

}

/* Set the main expression of *STMT_P to EXPR.  If EXPR is not a valid

   GIMPLE expression no changes are done and the function returns

   false.  */

bool

set_rhs (tree *stmt_p, tree expr)

{

  tree stmt = *stmt_p, op;

  enum tree_code code = TREE_CODE (expr);

  stmt_ann_t ann;

  tree var;

  ssa_op_iter iter;

  /* Verify the constant folded result is valid gimple.  */

  if (TREE_CODE_CLASS (code) == tcc_binary)

    {

      if (!is_gimple_val (TREE_OPERAND (expr, 0))

          || !is_gimple_val (TREE_OPERAND (expr, 1)))

        return false;

    }

  else if (TREE_CODE_CLASS (code) == tcc_unary)

    {

      if (!is_gimple_val (TREE_OPERAND (expr, 0)))

        return false;

    }

  else if (code == ADDR_EXPR)

    {

      if (TREE_CODE (TREE_OPERAND (expr, 0)) == ARRAY_REF

          && !is_gimple_val (TREE_OPERAND (TREE_OPERAND (expr, 0), 1)))

        return false;

    }

  else if (code == COMPOUND_EXPR)

    return false;

  switch (TREE_CODE (stmt))

    {

    case RETURN_EXPR:

      op = TREE_OPERAND (stmt, 0);

      if (TREE_CODE (op) != MODIFY_EXPR)

        {

          TREE_OPERAND (stmt, 0) = expr;

          break;

        }

      stmt = op;

      /* FALLTHRU */

    case MODIFY_EXPR:

      op = TREE_OPERAND (stmt, 1);

      if (TREE_CODE (op) == WITH_SIZE_EXPR)

        stmt = op;

      TREE_OPERAND (stmt, 1) = expr;

      break;

    case COND_EXPR:

      if (!is_gimple_condexpr (expr))

        return false;

      COND_EXPR_COND (stmt) = expr;

      break;

    case SWITCH_EXPR:

      SWITCH_COND (stmt) = expr;

      break;

    case GOTO_EXPR:

      GOTO_DESTINATION (stmt) = expr;

      break;

    case LABEL_EXPR:

      LABEL_EXPR_LABEL (stmt) = expr;

      break;

    default:

      /* Replace the whole statement with EXPR.  If EXPR has no side

         effects, then replace *STMT_P with an empty statement.  */

      ann = stmt_ann (stmt);

      *stmt_p = TREE_SIDE_EFFECTS (expr) ? expr : build_empty_stmt ();

      (*stmt_p)->common.ann = (tree_ann_t) ann;

      if (in_ssa_p

          && TREE_SIDE_EFFECTS (expr))

        {

          /* Fix all the SSA_NAMEs created by *STMT_P to point to its new

             replacement.  */

          FOR_EACH_SSA_TREE_OPERAND (var, stmt, iter, SSA_OP_ALL_DEFS)

            {

              if (TREE_CODE (var) == SSA_NAME)

                SSA_NAME_DEF_STMT (var) = *stmt_p;

            }

        }

      break;

    }

  return true;

}

/* Entry point to the propagation engine.

   VISIT_STMT is called for every statement visited.

   VISIT_PHI is called for every PHI node visited.  */

void

ssa_propagate (ssa_prop_visit_stmt_fn visit_stmt,

               ssa_prop_visit_phi_fn visit_phi)

{

  ssa_prop_visit_stmt = visit_stmt;

  ssa_prop_visit_phi = visit_phi;

  ssa_prop_init ();

  /* Iterate until the worklists are empty.  */

  while (!cfg_blocks_empty_p ()

         || VEC_length (tree, interesting_ssa_edges) > 0

         || VEC_length (tree, varying_ssa_edges) > 0)

    {

      if (!cfg_blocks_empty_p ())

        {

          /* Pull the next block to simulate off the worklist.  */

          basic_block dest_block = cfg_blocks_get ();

          simulate_block (dest_block);

        }

      /* In order to move things to varying as quickly as

         possible,process the VARYING_SSA_EDGES worklist first.  */

      process_ssa_edge_worklist (&varying_ssa_edges);

      /* Now process the INTERESTING_SSA_EDGES worklist.  */

      process_ssa_edge_worklist (&interesting_ssa_edges);

    }

  ssa_prop_fini ();

}

/* Return the first V_MAY_DEF or V_MUST_DEF operand for STMT.  */

tree

first_vdef (tree stmt)

{

  ssa_op_iter iter;

  tree op;

  /* Simply return the first operand we arrive at.  */

  FOR_EACH_SSA_TREE_OPERAND (op, stmt, iter, SSA_OP_VIRTUAL_DEFS)

    return (op);

  gcc_unreachable ();

}

/* Return true if STMT is of the form 'LHS = mem_ref', where 'mem_ref'

   is a non-volatile pointer dereference, a structure reference or a

   reference to a single _DECL.  Ignore volatile memory references

   because they are not interesting for the optimizers.  */

bool

stmt_makes_single_load (tree stmt)

{

  tree rhs;

  if (TREE_CODE (stmt) != MODIFY_EXPR)

    return false;

  if (ZERO_SSA_OPERANDS (stmt, SSA_OP_VMAYDEF|SSA_OP_VUSE))

    return false;

  rhs = TREE_OPERAND (stmt, 1);

  STRIP_NOPS (rhs);

  return (!TREE_THIS_VOLATILE (rhs)

          && (DECL_P (rhs)

              || REFERENCE_CLASS_P (rhs)));

}

/* Return true if STMT is of the form 'mem_ref = RHS', where 'mem_ref'

   is a non-volatile pointer dereference, a structure reference or a

   reference to a single _DECL.  Ignore volatile memory references

   because they are not interesting for the optimizers.  */

bool

stmt_makes_single_store (tree stmt)

{

  tree lhs;

  if (TREE_CODE (stmt) != MODIFY_EXPR)

    return false;

  if (ZERO_SSA_OPERANDS (stmt, SSA_OP_VMAYDEF|SSA_OP_VMUSTDEF))

    return false;

  lhs = TREE_OPERAND (stmt, 0);

  STRIP_NOPS (lhs);

  return (!TREE_THIS_VOLATILE (lhs)

          && (DECL_P (lhs)

              || REFERENCE_CLASS_P (lhs)));