path: root/gcc/gimple-low.c

/* Tree lowering pass.  Lowers GIMPLE into unstructured form.

   Copyright (C) 2003 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 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, 59 Temple Place - Suite 330, Boston, MA
02111-1307, USA.  */

#include "config.h"
#include "system.h"
#include "coretypes.h"
#include "tm.h"
#include "tree.h"
#include "rtl.h"
#include "errors.h"
#include "varray.h"
#include "tree-gimple.h"
#include "tree-inline.h"
#include "diagnostic.h"
#include "langhooks.h"
#include "langhooks-def.h"
#include "tree-flow.h"
#include "timevar.h"
#include "except.h"
#include "hashtab.h"
#include "flags.h"
#include "function.h"
#include "expr.h"
#include "toplev.h"
#include "tree-pass.h"

struct lower_data
{
  /* Block the current statement belongs to.  */
  tree block;

  /* A TREE_LIST of label and return statements to be moved to the end
     of the function.  */
  tree return_statements;
};

static void lower_stmt (tree_stmt_iterator *, struct lower_data *);
static void lower_bind_expr (tree_stmt_iterator *, struct lower_data *);
static void lower_cond_expr (tree_stmt_iterator *, struct lower_data *);
static void lower_return_expr (tree_stmt_iterator *, struct lower_data *);
static bool expand_var_p (tree);

/* Lowers the body of current_function_decl.  */

static void
lower_function_body (void)
{
  struct lower_data data;
  tree *body_p = &DECL_SAVED_TREE (current_function_decl);
  tree bind = *body_p;
  tree_stmt_iterator i;
  tree t, x;

  if (TREE_CODE (bind) != BIND_EXPR)
    abort ();

  data.block = DECL_INITIAL (current_function_decl);
  BLOCK_SUBBLOCKS (data.block) = NULL_TREE;
  BLOCK_CHAIN (data.block) = NULL_TREE;
  TREE_ASM_WRITTEN (data.block) = 1;

  data.return_statements = NULL_TREE;

  *body_p = alloc_stmt_list ();
  i = tsi_start (*body_p);
  tsi_link_after (&i, bind, TSI_NEW_STMT);
  lower_bind_expr (&i, &data);

  i = tsi_last (*body_p);

  /* If the function falls off the end, we need a null return statement.
     If we've already got one in the return_statements list, we don't
     need to do anything special.  Otherwise build one by hand.  */
  if (block_may_fallthru (*body_p)
      && (data.return_statements == NULL
          || TREE_OPERAND (TREE_VALUE (data.return_statements), 0) != NULL))
    {
      x = build (RETURN_EXPR, void_type_node, NULL);
      SET_EXPR_LOCATION (x, cfun->function_end_locus);
      tsi_link_after (&i, x, TSI_CONTINUE_LINKING);
    }

  /* If we lowered any return statements, emit the representative
     at the end of the function.  */
  for (t = data.return_statements ; t ; t = TREE_CHAIN (t))
    {
      x = build (LABEL_EXPR, void_type_node, TREE_PURPOSE (t));
      tsi_link_after (&i, x, TSI_CONTINUE_LINKING);

      /* Remove the line number from the representative return statement.
	 It now fills in for many such returns.  Failure to remove this
	 will result in incorrect results for coverage analysis.  */
      x = TREE_VALUE (t);
#ifdef USE_MAPPED_LOCATION
      SET_EXPR_LOCATION (x, UNKNOWN_LOCATION);
#else
      SET_EXPR_LOCUS (x, NULL);
#endif
      tsi_link_after (&i, x, TSI_CONTINUE_LINKING);
    }

  if (data.block != DECL_INITIAL (current_function_decl))
    abort ();
  BLOCK_SUBBLOCKS (data.block)
    = blocks_nreverse (BLOCK_SUBBLOCKS (data.block));

  clear_block_marks (data.block);
}

struct tree_opt_pass pass_lower_cf = 
{
  "lower",				/* name */
  NULL,					/* gate */
  lower_function_body,			/* execute */
  NULL,					/* sub */
  NULL,					/* next */
  0,					/* static_pass_number */
  0,					/* tv_id */
  PROP_gimple_any,			/* properties_required */
  PROP_gimple_lcf,			/* properties_provided */
  PROP_gimple_any,			/* properties_destroyed */
  0,					/* todo_flags_start */
  TODO_dump_func,			/* todo_flags_finish */
  0					/* letter */
};


/* Lowers the EXPR.  Unlike gimplification the statements are not relowered
   when they are changed -- if this has to be done, the lowering routine must
   do it explicitly.  DATA is passed through the recursion.  */

void
lower_stmt_body (tree expr, struct lower_data *data)
{
  tree_stmt_iterator tsi;

  for (tsi = tsi_start (expr); !tsi_end_p (tsi); )
    lower_stmt (&tsi, data);
}

/* Lowers statement TSI.  DATA is passed through the recursion.  */

static void
lower_stmt (tree_stmt_iterator *tsi, struct lower_data *data)
{
  tree stmt = tsi_stmt (*tsi);

  if (EXPR_HAS_LOCATION (stmt) && data)
    TREE_BLOCK (stmt) = data->block;

  switch (TREE_CODE (stmt))
    {
    case BIND_EXPR:
      lower_bind_expr (tsi, data);
      return;
    case COND_EXPR:
      lower_cond_expr (tsi, data);
      return;
    case RETURN_EXPR:
      lower_return_expr (tsi, data);
      return;

    case TRY_FINALLY_EXPR:
    case TRY_CATCH_EXPR:
      lower_stmt_body (TREE_OPERAND (stmt, 0), data);
      lower_stmt_body (TREE_OPERAND (stmt, 1), data);
      break;
    case CATCH_EXPR:
      lower_stmt_body (CATCH_BODY (stmt), data);
      break;
    case EH_FILTER_EXPR:
      lower_stmt_body (EH_FILTER_FAILURE (stmt), data);
      break;
      
    case NOP_EXPR:
    case ASM_EXPR:
    case MODIFY_EXPR:
    case CALL_EXPR:
    case GOTO_EXPR:
    case LABEL_EXPR:
    case SWITCH_EXPR:
      break;

    default:
      print_node_brief (stderr, "", stmt, 0);
    case COMPOUND_EXPR:
      abort ();
    }

  tsi_next (tsi);
}

/* Lowers a bind_expr TSI.  DATA is passed through the recursion.  */

static void
lower_bind_expr (tree_stmt_iterator *tsi, struct lower_data *data)
{
  tree old_block = data->block;
  tree stmt = tsi_stmt (*tsi);
  tree new_block = BIND_EXPR_BLOCK (stmt);

  if (new_block)
    {
      if (new_block == old_block)
	{
	  /* The outermost block of the original function may not be the
	     outermost statement chain of the gimplified function.  So we
	     may see the outermost block just inside the function.  */
	  if (new_block != DECL_INITIAL (current_function_decl))
	    abort ();
	  new_block = NULL;
	}
      else
	{
	  /* We do not expect to handle duplicate blocks.  */
	  if (TREE_ASM_WRITTEN (new_block))
	    abort ();
	  TREE_ASM_WRITTEN (new_block) = 1;

	  /* Block tree may get clobbered by inlining.  Normally this would
	     be fixed in rest_of_decl_compilation using block notes, but
	     since we are not going to emit them, it is up to us.  */
	  BLOCK_CHAIN (new_block) = BLOCK_SUBBLOCKS (old_block);
	  BLOCK_SUBBLOCKS (old_block) = new_block;
	  BLOCK_SUBBLOCKS (new_block) = NULL_TREE;
	  BLOCK_SUPERCONTEXT (new_block) = old_block;

	  data->block = new_block;
	}
    }

  record_vars (BIND_EXPR_VARS (stmt));
  lower_stmt_body (BIND_EXPR_BODY (stmt), data);

  if (new_block)
    {
      if (data->block != new_block)
	abort ();

      BLOCK_SUBBLOCKS (new_block)
	= blocks_nreverse (BLOCK_SUBBLOCKS (new_block));
      data->block = old_block;
    }

  /* The BIND_EXPR no longer carries any useful information -- kill it.  */
  tsi_link_before (tsi, BIND_EXPR_BODY (stmt), TSI_SAME_STMT);
  tsi_delink (tsi);
}

/* Try to determine if we can fall out of the bottom of BLOCK.  This guess
   need not be 100% accurate; simply be conservative and return true if we
   don't know.  This is used only to avoid stupidly generating extra code.
   If we're wrong, we'll just delete the extra code later.  */

bool
block_may_fallthru (tree block)
{
  tree stmt = expr_last (block);

  switch (stmt ? TREE_CODE (stmt) : ERROR_MARK)
    {
    case GOTO_EXPR:
    case RETURN_EXPR:
    case RESX_EXPR:
    case SWITCH_EXPR:
      /* Easy cases.  If the last statement of the block implies 
	 control transfer, then we can't fall through.  */
      return false;

    case COND_EXPR:
      if (block_may_fallthru (COND_EXPR_THEN (stmt)))
	return true;
      return block_may_fallthru (COND_EXPR_ELSE (stmt));

    case BIND_EXPR:
      return block_may_fallthru (BIND_EXPR_BODY (stmt));

    case TRY_FINALLY_EXPR:
      return block_may_fallthru (TREE_OPERAND (stmt, 1));

    case MODIFY_EXPR:
      if (TREE_CODE (TREE_OPERAND (stmt, 1)) == CALL_EXPR)
	stmt = TREE_OPERAND (stmt, 1);
      else
	return true;
      /* FALLTHRU */

    case CALL_EXPR:
      /* Functions that do not return do not fall through.  */
      return (call_expr_flags (stmt) & ECF_NORETURN) == 0;

    default:
      return true;
    }
}

/* Lowers a cond_expr TSI.  DATA is passed through the recursion.  */

static void
lower_cond_expr (tree_stmt_iterator *tsi, struct lower_data *data)
{
  tree stmt = tsi_stmt (*tsi);
  bool then_is_goto, else_is_goto;
  tree then_branch, else_branch;
  tree then_goto, else_goto;
  
  then_branch = COND_EXPR_THEN (stmt);
  else_branch = COND_EXPR_ELSE (stmt);

  lower_stmt_body (then_branch, data);
  lower_stmt_body (else_branch, data);

  then_goto = expr_only (then_branch);
  then_is_goto = then_goto && simple_goto_p (then_goto);

  else_goto = expr_only (else_branch);
  else_is_goto = else_goto && simple_goto_p (else_goto);

  if (!then_is_goto || !else_is_goto)
    {
      tree then_label, else_label, end_label, t;

      then_label = NULL_TREE;
      else_label = NULL_TREE;
      end_label = NULL_TREE;
 
      /* Replace the cond_expr with explicit gotos.  */
      if (!then_is_goto)
	{
	  t = build1 (LABEL_EXPR, void_type_node, NULL_TREE);
	  if (TREE_SIDE_EFFECTS (then_branch))
	    then_label = t;
	  else
	    end_label = t;
	  then_goto = build_and_jump (&LABEL_EXPR_LABEL (t));
	}

      if (!else_is_goto)
	{
	  t = build1 (LABEL_EXPR, void_type_node, NULL_TREE);
	  if (TREE_SIDE_EFFECTS (else_branch))
	    else_label = t;
	  else
	    {
	      /* Both THEN and ELSE can be no-ops if one or both contained an
	         empty BIND_EXPR that was associated with the toplevel block
	         of an inlined function.  In that case remove_useless_stmts
	         can't have cleaned things up for us; kill the whole 
	         conditional now.  */
	      if (end_label)
		{
		  tsi_delink (tsi);
		  return;
		}
	      else
		end_label = t;
	    }
	  else_goto = build_and_jump (&LABEL_EXPR_LABEL (t));
	}

      if (then_label)
	{
	  bool may_fallthru = block_may_fallthru (then_branch);

	  tsi_link_after (tsi, then_label, TSI_CONTINUE_LINKING);
	  tsi_link_after (tsi, then_branch, TSI_CONTINUE_LINKING);
  
	  if (else_label && may_fallthru)
	    {
	      end_label = build1 (LABEL_EXPR, void_type_node, NULL_TREE);
	      t = build_and_jump (&LABEL_EXPR_LABEL (end_label));
	      tsi_link_after (tsi, t, TSI_CONTINUE_LINKING);
	    }
	}
  
      if (else_label)
	{
	  tsi_link_after (tsi, else_label, TSI_CONTINUE_LINKING);
	  tsi_link_after (tsi, else_branch, TSI_CONTINUE_LINKING);
	}

      if (end_label)
	tsi_link_after (tsi, end_label, TSI_CONTINUE_LINKING);
    }

  COND_EXPR_THEN (stmt) = then_goto;
  COND_EXPR_ELSE (stmt) = else_goto;

  tsi_next (tsi);
}

static void
lower_return_expr (tree_stmt_iterator *tsi, struct lower_data *data)
{
  tree stmt = tsi_stmt (*tsi);
  tree value, t, label;

  /* Extract the value being returned.  */
  value = TREE_OPERAND (stmt, 0);
  if (value && TREE_CODE (value) == MODIFY_EXPR)
    value = TREE_OPERAND (value, 1);

  /* Match this up with an existing return statement that's been created.  */
  for (t = data->return_statements; t ; t = TREE_CHAIN (t))
    {
      tree tvalue = TREE_OPERAND (TREE_VALUE (t), 0);
      if (tvalue && TREE_CODE (tvalue) == MODIFY_EXPR)
	tvalue = TREE_OPERAND (tvalue, 1);

      if (value == tvalue)
	{
	  label = TREE_PURPOSE (t);
	  goto found;
	}
    }

  /* Not found.  Create a new label and record the return statement.  */
  label = create_artificial_label ();
  data->return_statements = tree_cons (label, stmt, data->return_statements);

  /* Generate a goto statement and remove the return statement.  */
 found:
  t = build (GOTO_EXPR, void_type_node, label);
  SET_EXPR_LOCUS (t, EXPR_LOCUS (stmt));
  tsi_link_before (tsi, t, TSI_SAME_STMT);
  tsi_delink (tsi);
}


/* Record the variables in VARS.  */

void
record_vars (tree vars)
{
  for (; vars; vars = TREE_CHAIN (vars))
    {
      tree var = vars;

      /* Nothing to do in this case.  */
      if (DECL_EXTERNAL (var))
	continue;
      if (TREE_CODE (var) == FUNCTION_DECL)
	continue;

      /* Record the variable.  */
      cfun->unexpanded_var_list = tree_cons (NULL_TREE, var,
					     cfun->unexpanded_var_list);
    }
}

/* Check whether to expand a variable VAR.  */

static bool
expand_var_p (tree var)
{
  struct var_ann_d *ann;

  if (TREE_CODE (var) != VAR_DECL)
    return true;

  /* Leave statics and externals alone.  */
  if (TREE_STATIC (var) || DECL_EXTERNAL (var))
    return true;

  /* Remove all unused local variables.  */
  ann = var_ann (var);
  if (!ann || !ann->used)
    return false;

  return true;
}

/* Throw away variables that are unused.  */

static void
remove_useless_vars (void)
{
  tree var, *cell;
  FILE *df = NULL;

  if (dump_file && (dump_flags & TDF_DETAILS))
    {
      df = dump_file;
      fputs ("Discarding as unused:\n", df);
    }

  for (cell = &cfun->unexpanded_var_list; *cell; )
    {
      var = TREE_VALUE (*cell);

      if (!expand_var_p (var))
	{
	  if (df)
	    {
	      fputs ("  ", df);
	      print_generic_expr (df, var, dump_flags);
	      fputc ('\n', df);
	    }

	  *cell = TREE_CHAIN (*cell);
	  continue;
	}

      cell = &TREE_CHAIN (*cell);
    }

  if (df)
    fputc ('\n', df);
}

struct tree_opt_pass pass_remove_useless_vars = 
{
  "vars",				/* name */
  NULL,					/* gate */
  remove_useless_vars,			/* execute */
  NULL,					/* sub */
  NULL,					/* next */
  0,					/* static_pass_number */
  0,					/* tv_id */
  0,					/* properties_required */
  0,					/* properties_provided */
  0,					/* properties_destroyed */
  0,					/* todo_flags_start */
  TODO_dump_func,			/* todo_flags_finish */
  0					/* letter */
};