2007-06-12 Richard Guenther <rguenther@suse.de>

	PR tree-optimization/15353
	PR tree-optimization/31657
	* passes.c (init_optimization_passes): Add pass_tree_ifcombine.
	* timevar.def: Add TV_TREE_IFCOMBINE.
	* tree-pass.h (pass_tree_ifcombine): Declare.
	* tree-ssa-ifcombine.c: New file.
	* tree-ssa-phiopt.c (blocks_in_phiopt_order): Export.
	* tree-flow.h (blocks_in_phiopt_order): Declare.
	* Makefile.in (OBJS-common): Add tree-ssa-ifcombine.o.
	(tree-ssa-ifcombine.o): New dependencies.

	* gcc.c-torture/execute/20070424-1.c: New testcase.
	* gcc.dg/tree-ssa/ssa-ifcombine-1.c: Likewise.
	* gcc.dg/tree-ssa/ssa-ifcombine-2.c: Likewise.
	* gcc.dg/tree-ssa/ssa-ifcombine-3.c: Likewise.
	* gcc.dg/tree-ssa/ssa-ifcombine-4.c: Likewise.
	* gcc.dg/tree-ssa/ssa-ifcombine-5.c: Likewise.


git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@125644 138bc75d-0d04-0410-961f-82ee72b054a4

1 files changed, 614 insertions, 0 deletions

diff --git a/gcc/tree-ssa-ifcombine.c b/gcc/tree-ssa-ifcombine.c
new file mode 100644
index 00000000000..89fd61d2405
--- /dev/null
+++ b/gcc/tree-ssa-ifcombine.c
@@ -0,0 +1,614 @@
+/* Combining of if-expressions on trees.
+   Copyright (C) 2007 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 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 "basic-block.h"
+#include "timevar.h"
+#include "diagnostic.h"
+#include "tree-flow.h"
+#include "tree-pass.h"
+#include "tree-dump.h"
+
+/* This pass combines COND_EXPRs to simplify control flow.  It
+   currently recognizes bit tests and comparisons in chains that
+   represent logical and or logical or of two COND_EXPRs.
+
+   It does so by walking basic blocks in a approximate reverse
+   post-dominator order and trying to match CFG patterns that
+   represent logical and or logical or of two COND_EXPRs.
+   Transformations are done if the COND_EXPR conditions match
+   either
+
+     1. two single bit tests X & (1 << Yn) (for logical and)
+
+     2. two bit tests X & Yn (for logical or)
+
+     3. two comparisons X OPn Y (for logical or)
+
+   To simplify this pass, removing basic blocks and dead code
+   is left to CFG cleanup and DCE.  */
+
+
+/* Recognize a if-then-else CFG pattern starting to match with the
+   COND_BB basic-block containing the COND_EXPR.  The recognized
+   then end else blocks are stored to *THEN_BB and *ELSE_BB.  If
+   *THEN_BB and/or *ELSE_BB are already set, they are required to
+   match the then and else basic-blocks to make the pattern match.
+   Returns true if the pattern matched, false otherwise.  */
+
+static bool
+recognize_if_then_else (basic_block cond_bb,
+			basic_block *then_bb, basic_block *else_bb)
+{
+  edge t, e;
+
+  if (EDGE_COUNT (cond_bb->succs) != 2)
+    return false;
+
+  /* Find the then/else edges.  */
+  t = EDGE_SUCC (cond_bb, 0);
+  e = EDGE_SUCC (cond_bb, 1);
+  if (!(t->flags & EDGE_TRUE_VALUE))
+    {
+      edge tmp = t;
+      t = e;
+      e = tmp;
+    }
+  if (!(t->flags & EDGE_TRUE_VALUE)
+      || !(e->flags & EDGE_FALSE_VALUE))
+    return false;
+
+  /* Check if the edge destinations point to the required block.  */
+  if (*then_bb
+      && t->dest != *then_bb)
+    return false;
+  if (*else_bb
+      && e->dest != *else_bb)
+    return false;
+
+  if (!*then_bb)
+    *then_bb = t->dest;
+  if (!*else_bb)
+    *else_bb = e->dest;
+
+  return true;
+}
+
+/* Verify if the basic block BB does not have side-effects.  Return
+   true in this case, else false.  */
+
+static bool
+bb_no_side_effects_p (basic_block bb)
+{
+  block_stmt_iterator bsi;
+
+  for (bsi = bsi_start (bb); !bsi_end_p (bsi); bsi_next (&bsi))
+    {
+      tree stmt = bsi_stmt (bsi);
+      stmt_ann_t ann = stmt_ann (stmt);
+
+      if (ann->has_volatile_ops
+	  || !ZERO_SSA_OPERANDS (stmt, SSA_OP_ALL_VIRTUALS))
+	return false;
+    }
+
+  return true;
+}
+
+/* Verify if all PHI node arguments in DEST for edges from BB1 or
+   BB2 to DEST are the same.  This makes the CFG merge point
+   free from side-effects.  Return true in this case, else false.  */
+
+static bool
+same_phi_args_p (basic_block bb1, basic_block bb2, basic_block dest)
+{
+  edge e1 = find_edge (bb1, dest);
+  edge e2 = find_edge (bb2, dest);
+  tree phi;
+
+  for (phi = phi_nodes (dest); phi; phi = PHI_CHAIN (phi))
+    if (!operand_equal_p (PHI_ARG_DEF_FROM_EDGE (phi, e1),
+			  PHI_ARG_DEF_FROM_EDGE (phi, e2), 0))
+      return false;
+
+  return true;
+}
+
+/* Recognize a single bit test pattern in COND_EXPR and its defining
+   statements.  Store the name being tested in *NAME and the bit
+   in *BIT.  The COND_EXPR computes *NAME & (1 << *BIT).
+   Returns true if the pattern matched, false otherwise.  */
+
+static bool
+recognize_single_bit_test (tree cond_expr, tree *name, tree *bit)
+{
+  tree t;
+
+  /* Get at the definition of the result of the bit test.  */
+  t = TREE_OPERAND (cond_expr, 0);
+  if (TREE_CODE (t) == NE_EXPR
+      && integer_zerop (TREE_OPERAND (t, 1)))
+    t = TREE_OPERAND (t, 0);
+  if (TREE_CODE (t) != SSA_NAME)
+    return false;
+  t = SSA_NAME_DEF_STMT (t);
+  if (TREE_CODE (t) != GIMPLE_MODIFY_STMT)
+    return false;
+  t = GIMPLE_STMT_OPERAND (t, 1);
+
+  /* Look at which bit is tested.  One form to recognize is
+     D.1985_5 = state_3(D) >> control1_4(D);
+     D.1986_6 = (int) D.1985_5;
+     D.1987_7 = op0 & 1;
+     if (D.1987_7 != 0)  */
+  if (TREE_CODE (t) == BIT_AND_EXPR
+      && integer_onep (TREE_OPERAND (t, 1))
+      && TREE_CODE (TREE_OPERAND (t, 0)) == SSA_NAME)
+    {
+      t = TREE_OPERAND (t, 0);
+      do {
+	t = SSA_NAME_DEF_STMT (t);
+	if (TREE_CODE (t) != GIMPLE_MODIFY_STMT)
+	  return false;
+	t = GIMPLE_STMT_OPERAND (t, 1);
+	if (TREE_CODE (t) == NOP_EXPR
+	    || TREE_CODE (t) == CONVERT_EXPR)
+	  t = TREE_OPERAND (t, 0);
+      } while (TREE_CODE (t) == SSA_NAME);
+
+      if (TREE_CODE (t) == RSHIFT_EXPR)
+	{
+	  /* op0 & (1 << op1) */
+	  *bit = TREE_OPERAND (t, 1);
+	  *name = TREE_OPERAND (t, 0);
+	}
+      else
+	{
+	  /* t & 1 */
+	  *bit = integer_one_node;
+	  *name = t;
+	}
+
+      return true;
+    }
+
+  /* Another form is
+     D.1987_7 = op0 & (1 << CST)
+     if (D.1987_7 != 0)  */
+  if (TREE_CODE (t) == BIT_AND_EXPR
+      && TREE_CODE (TREE_OPERAND (t, 0)) == SSA_NAME
+      && integer_pow2p (TREE_OPERAND (t, 1)))
+    {
+      *name = TREE_OPERAND (t, 0);
+      *bit = build_int_cst (integer_type_node,
+			    tree_log2 (TREE_OPERAND (t, 1)));
+      return true;
+    }
+
+  /* Another form is
+     D.1986_6 = 1 << control1_4(D)
+     D.1987_7 = op0 & D.1986_6
+     if (D.1987_7 != 0)  */
+  if (TREE_CODE (t) == BIT_AND_EXPR
+      && TREE_CODE (TREE_OPERAND (t, 0)) == SSA_NAME
+      && TREE_CODE (TREE_OPERAND (t, 1)) == SSA_NAME)
+    {
+      tree tmp;
+
+      /* Both arguments of the BIT_AND_EXPR can be the single-bit
+	 specifying expression.  */
+      tmp = SSA_NAME_DEF_STMT (TREE_OPERAND (t, 0));
+      if (TREE_CODE (tmp) == GIMPLE_MODIFY_STMT
+	  && TREE_CODE (GIMPLE_STMT_OPERAND (tmp, 1)) == LSHIFT_EXPR
+	  && integer_onep (TREE_OPERAND (GIMPLE_STMT_OPERAND (tmp, 1), 0)))
+	{
+	  *name = TREE_OPERAND (t, 1);
+	  *bit = TREE_OPERAND (GIMPLE_STMT_OPERAND (tmp, 1), 1);
+	  return true;
+	}
+
+      tmp = SSA_NAME_DEF_STMT (TREE_OPERAND (t, 1));
+      if (TREE_CODE (tmp) == GIMPLE_MODIFY_STMT
+	  && TREE_CODE (GIMPLE_STMT_OPERAND (tmp, 1)) == LSHIFT_EXPR
+	  && integer_onep (TREE_OPERAND (GIMPLE_STMT_OPERAND (tmp, 1), 0)))
+	{
+	  *name = TREE_OPERAND (t, 0);
+	  *bit = TREE_OPERAND (GIMPLE_STMT_OPERAND (tmp, 1), 1);
+	  return true;
+	}
+    }
+
+  return false;
+}
+
+/* Recognize a bit test pattern in COND_EXPR and its defining
+   statements.  Store the name being tested in *NAME and the bits
+   in *BITS.  The COND_EXPR computes *NAME & *BITS.
+   Returns true if the pattern matched, false otherwise.  */
+
+static bool
+recognize_bits_test (tree cond_expr, tree *name, tree *bits)
+{
+  tree t;
+
+  /* Get at the definition of the result of the bit test.  */
+  t = TREE_OPERAND (cond_expr, 0);
+  if (TREE_CODE (t) == NE_EXPR
+      && integer_zerop (TREE_OPERAND (t, 1)))
+    t = TREE_OPERAND (t, 0);
+  if (TREE_CODE (t) != SSA_NAME)
+    return false;
+  t = SSA_NAME_DEF_STMT (t);
+  if (TREE_CODE (t) != GIMPLE_MODIFY_STMT)
+    return false;
+  t = GIMPLE_STMT_OPERAND (t, 1);
+
+  if (TREE_CODE (t) != BIT_AND_EXPR)
+    return false;
+
+  *name = TREE_OPERAND (t, 0);
+  *bits = TREE_OPERAND (t, 1);
+
+  return true;
+}
+
+/* If-convert on a and pattern with a common else block.  The inner
+   if is specified by its INNER_COND_BB, the outer by OUTER_COND_BB.
+   Returns true if the edges to the common else basic-block were merged.  */
+
+static bool
+ifcombine_ifandif (basic_block inner_cond_bb, basic_block outer_cond_bb)
+{
+  block_stmt_iterator bsi;
+  tree inner_cond, outer_cond;
+  tree name1, name2, bit1, bit2;
+
+  inner_cond = last_stmt (inner_cond_bb);
+  if (!inner_cond
+      || TREE_CODE (inner_cond) != COND_EXPR)
+    return false;
+
+  outer_cond = last_stmt (outer_cond_bb);
+  if (!outer_cond
+      || TREE_CODE (outer_cond) != COND_EXPR)
+    return false;
+
+  /* See if we test a single bit of the same name in both tests.  In
+     that case remove the outer test, merging both else edges,
+     and change the inner one to test for
+     name & (bit1 | bit2) == (bit1 | bit2).  */
+  if (recognize_single_bit_test (inner_cond, &name1, &bit1)
+      && recognize_single_bit_test (outer_cond, &name2, &bit2)
+      && name1 == name2)
+    {
+      tree t, t2;
+
+      /* Do it.  */
+      bsi = bsi_for_stmt (inner_cond);
+      t = fold_build2 (LSHIFT_EXPR, TREE_TYPE (name1),
+		       integer_one_node, bit1);
+      t2 = fold_build2 (LSHIFT_EXPR, TREE_TYPE (name1),
+		        integer_one_node, bit2);
+      t = fold_build2 (BIT_IOR_EXPR, TREE_TYPE (name1), t, t2);
+      t = force_gimple_operand_bsi (&bsi, t, true, NULL_TREE);
+      t2 = fold_build2 (BIT_AND_EXPR, TREE_TYPE (name1), name1, t);
+      t2 = force_gimple_operand_bsi (&bsi, t2, true, NULL_TREE);
+      COND_EXPR_COND (inner_cond) = fold_build2 (EQ_EXPR, boolean_type_node,
+						 t2, t);
+      update_stmt (inner_cond);
+
+      /* Leave CFG optimization to cfg_cleanup.  */
+      COND_EXPR_COND (outer_cond) = boolean_true_node;
+      update_stmt (outer_cond);
+
+      if (dump_file)
+	{
+	  fprintf (dump_file, "optimizing double bit test to ");
+	  print_generic_expr (dump_file, name1, 0);
+	  fprintf (dump_file, " & T == T\nwith temporary T = (1 << ");
+	  print_generic_expr (dump_file, bit1, 0);
+	  fprintf (dump_file, ") | (1 << ");
+	  print_generic_expr (dump_file, bit2, 0);
+	  fprintf (dump_file, ")\n");
+	}
+
+      return true;
+    }
+
+  return false;
+}
+
+/* If-convert on a or pattern with a common then block.  The inner
+   if is specified by its INNER_COND_BB, the outer by OUTER_COND_BB.
+   Returns true, if the edges leading to the common then basic-block
+   were merged.  */
+
+static bool
+ifcombine_iforif (basic_block inner_cond_bb, basic_block outer_cond_bb)
+{
+  tree inner_cond, outer_cond;
+  tree name1, name2, bits1, bits2;
+
+  inner_cond = last_stmt (inner_cond_bb);
+  if (!inner_cond
+      || TREE_CODE (inner_cond) != COND_EXPR)
+    return false;
+
+  outer_cond = last_stmt (outer_cond_bb);
+  if (!outer_cond
+      || TREE_CODE (outer_cond) != COND_EXPR)
+    return false;
+
+  /* See if we have two bit tests of the same name in both tests.
+     In that case remove the outer test and change the inner one to
+     test for name & (bits1 | bits2) != 0.  */
+  if (recognize_bits_test (inner_cond, &name1, &bits1)
+      && recognize_bits_test (outer_cond, &name2, &bits2))
+    {
+      block_stmt_iterator bsi;
+      tree t;
+
+      /* Find the common name which is bit-tested.  */
+      if (name1 == name2)
+	;
+      else if (bits1 == bits2)
+	{
+	  t = name2;
+	  name2 = bits2;
+	  bits2 = t;
+	  t = name1;
+	  name1 = bits1;
+	  bits1 = t;
+	}
+      else if (name1 == bits2)
+	{
+	  t = name2;
+	  name2 = bits2;
+	  bits2 = t;
+	}
+      else if (bits1 == name2)
+	{
+	  t = name1;
+	  name1 = bits1;
+	  bits1 = t;
+	}
+      else
+	return false;
+
+      /* Do it.  */
+      bsi = bsi_for_stmt (inner_cond);
+      t = fold_build2 (BIT_IOR_EXPR, TREE_TYPE (name1), bits1, bits2);
+      t = force_gimple_operand_bsi (&bsi, t, true, NULL_TREE);
+      t = fold_build2 (BIT_AND_EXPR, TREE_TYPE (name1), name1, t);
+      t = force_gimple_operand_bsi (&bsi, t, true, NULL_TREE);
+      COND_EXPR_COND (inner_cond) = fold_build2 (NE_EXPR, boolean_type_node, t,
+						 build_int_cst (TREE_TYPE (t), 0));
+      update_stmt (inner_cond);
+
+      /* Leave CFG optimization to cfg_cleanup.  */
+      COND_EXPR_COND (outer_cond) = boolean_false_node;
+      update_stmt (outer_cond);
+
+      if (dump_file)
+	{
+	  fprintf (dump_file, "optimizing bits or bits test to ");
+	  print_generic_expr (dump_file, name1, 0);
+	  fprintf (dump_file, " & T != 0\nwith temporary T = ");
+	  print_generic_expr (dump_file, bits1, 0);
+	  fprintf (dump_file, " | ");
+	  print_generic_expr (dump_file, bits2, 0);
+	  fprintf (dump_file, "\n");
+	}
+
+      return true;
+    }
+
+  /* See if we have two comparisons that we can merge into one.
+     This happens for C++ operator overloading where for example
+     GE_EXPR is implemented as GT_EXPR || EQ_EXPR.  */
+  else if (COMPARISON_CLASS_P (COND_EXPR_COND (inner_cond))
+	   && COMPARISON_CLASS_P (COND_EXPR_COND (outer_cond))
+	   && operand_equal_p (TREE_OPERAND (COND_EXPR_COND (inner_cond), 0),
+			       TREE_OPERAND (COND_EXPR_COND (outer_cond), 0), 0)
+	   && operand_equal_p (TREE_OPERAND (COND_EXPR_COND (inner_cond), 1),
+			       TREE_OPERAND (COND_EXPR_COND (outer_cond), 1), 0))
+    {
+      tree ccond1 = COND_EXPR_COND (inner_cond);
+      tree ccond2 = COND_EXPR_COND (outer_cond);
+      enum tree_code code1 = TREE_CODE (ccond1);
+      enum tree_code code2 = TREE_CODE (ccond2);
+      enum tree_code code;
+      tree t;
+
+#define CHK(a,b) ((code1 == a ## _EXPR && code2 == b ## _EXPR) \
+		  || (code2 == a ## _EXPR && code1 == b ## _EXPR))
+      /* Merge the two condition codes if possible.  */
+      if (code1 == code2)
+	code = code1;
+      else if (CHK (EQ, LT))
+	code = LE_EXPR;
+      else if (CHK (EQ, GT))
+	code = GE_EXPR;
+      else if (CHK (LT, LE))
+	code = LE_EXPR;
+      else if (CHK (GT, GE))
+	code = GE_EXPR;
+      else if (INTEGRAL_TYPE_P (TREE_TYPE (TREE_OPERAND (ccond1, 0)))
+	       || flag_unsafe_math_optimizations)
+	{
+	  if (CHK (LT, GT))
+	    code = NE_EXPR;
+	  else if (CHK (LT, NE))
+	    code = NE_EXPR;
+	  else if (CHK (GT, NE))
+	    code = NE_EXPR;
+	  else
+	    return false;
+	}
+      /* We could check for combinations leading to trivial true/false.  */
+      else
+	return false;
+#undef CHK
+
+      /* Do it.  */
+      t = fold_build2 (code, boolean_type_node,
+		       TREE_OPERAND (ccond2, 0), TREE_OPERAND (ccond2, 1));
+      COND_EXPR_COND (inner_cond) = t;
+      update_stmt (inner_cond);
+
+      /* Leave CFG optimization to cfg_cleanup.  */
+      COND_EXPR_COND (outer_cond) = boolean_false_node;
+      update_stmt (outer_cond);
+
+      if (dump_file)
+	{
+	  fprintf (dump_file, "optimizing two comparisons to ");
+	  print_generic_expr (dump_file, t, 0);
+	  fprintf (dump_file, "\n");
+	}
+
+      return true;
+    }
+
+  return false;
+}
+
+/* Recognize a CFG pattern and dispatch to the appropriate
+   if-conversion helper.  We start with BB as the innermost
+   worker basic-block.  Returns true if a transformation was done.  */
+
+static bool
+tree_ssa_ifcombine_bb (basic_block inner_cond_bb)
+{
+  basic_block then_bb = NULL, else_bb = NULL;
+
+  if (!recognize_if_then_else (inner_cond_bb, &then_bb, &else_bb))
+    return false;
+
+  /* Recognize && and || of two conditions with a common
+     then/else block which entry edges we can merge.  That is:
+       if (a || b)
+	 ;
+     and
+       if (a && b)
+	 ;
+     This requires a single predecessor of the inner cond_bb.  */
+  if (single_pred_p (inner_cond_bb))
+    {
+      basic_block outer_cond_bb = single_pred (inner_cond_bb);
+
+      /* The && form is characterized by a common else_bb with
+	 the two edges leading to it mergable.  The latter is
+	 guaranteed by matching PHI arguments in the else_bb and
+	 the inner cond_bb having no side-effects.  */
+      if (recognize_if_then_else (outer_cond_bb, &inner_cond_bb, &else_bb)
+	  && same_phi_args_p (outer_cond_bb, inner_cond_bb, else_bb)
+	  && bb_no_side_effects_p (inner_cond_bb))
+	{
+	  /* We have
+	       <outer_cond_bb>
+		 if (q) goto inner_cond_bb; else goto else_bb;
+	       <inner_cond_bb>
+		 if (p) goto ...; else goto else_bb;
+		 ...
+	       <else_bb>
+		 ...
+	   */
+	  return ifcombine_ifandif (inner_cond_bb, outer_cond_bb);
+	}
+
+      /* The || form is characterized by a common then_bb with the
+	 two edges leading to it mergable.  The latter is guaranteed
+         by matching PHI arguments in the then_bb and the inner cond_bb
+	 having no side-effects.  */
+      if (recognize_if_then_else (outer_cond_bb, &then_bb, &inner_cond_bb)
+	  && same_phi_args_p (outer_cond_bb, inner_cond_bb, then_bb)
+	  && bb_no_side_effects_p (inner_cond_bb))
+	{
+	  /* We have
+	       <outer_cond_bb>
+		 if (q) goto then_bb; else goto inner_cond_bb;
+	       <inner_cond_bb>
+		 if (q) goto then_bb; else goto ...;
+	       <then_bb>
+		 ...
+	   */
+	  return ifcombine_iforif (inner_cond_bb, outer_cond_bb);
+	}
+    }
+
+  return false;
+}
+
+/* Main entry for the tree if-conversion pass.  */
+
+static unsigned int
+tree_ssa_ifcombine (void)
+{
+  basic_block *bbs;
+  bool cfg_changed = false;
+  int i;
+
+  bbs = blocks_in_phiopt_order ();
+
+  for (i = 0; i < n_basic_blocks - NUM_FIXED_BLOCKS; ++i)
+    {
+      basic_block bb = bbs[i];
+      tree stmt = last_stmt (bb);
+
+      if (stmt
+	  && TREE_CODE (stmt) == COND_EXPR)
+	cfg_changed |= tree_ssa_ifcombine_bb (bb);
+    }
+
+  free (bbs);
+
+  return cfg_changed ? TODO_cleanup_cfg : 0;
+}
+
+static bool
+gate_ifcombine (void)
+{
+  return 1;
+}
+
+struct tree_opt_pass pass_tree_ifcombine = {
+  "ifcombine",			/* name */
+  gate_ifcombine,		/* gate */
+  tree_ssa_ifcombine,		/* execute */
+  NULL,				/* sub */
+  NULL,				/* next */
+  0,				/* static_pass_number */
+  TV_TREE_IFCOMBINE,		/* tv_id */
+  PROP_cfg | PROP_ssa,		/* properties_required */
+  0,				/* properties_provided */
+  0,				/* properties_destroyed */
+  0,				/* todo_flags_start */
+  TODO_dump_func
+  | TODO_ggc_collect
+  | TODO_update_ssa
+  | TODO_verify_ssa,		/* todo_flags_finish */
+  0				/* letter */
+};