gcc/tree-ssanames.c

   1 /* Generic routines for manipulating SSA_NAME expressions
   2    Copyright (C) 2003 Free Software Foundation, Inc.
   3
   4 This file is part of GCC.
   5
   6 GCC is free software; you can redistribute it and/or modify
   7 it under the terms of the GNU General Public License as published by
   8 the Free Software Foundation; either version 2, or (at your option)
   9 any later version.
  10
  11 GCC is distributed in the hope that it will be useful,
  12 but WITHOUT ANY WARRANTY; without even the implied warranty of
  13 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  14 GNU General Public License for more details.
  15
  16 You should have received a copy of the GNU General Public License
  17 along with GCC; see the file COPYING.  If not, write to
  18 the Free Software Foundation, 59 Temple Place - Suite 330,
  19 Boston, MA 02111-1307, USA.  */
  20
  21 #include "config.h"
  22 #include "system.h"
  23 #include "coretypes.h"
  24 #include "tm.h"
  25 #include "tree.h"
  26 #include "varray.h"
  27 #include "ggc.h"
  28 #include "tree-flow.h"
  29
  30 /* Rewriting a function into SSA form can create a huge number of SSA_NAMEs,
  31    many of which may be thrown away shortly after their creation if jumps
  32    were threaded through PHI nodes.
  33
  34    While our garbage collection mechanisms will handle this situation, it
  35    is extremely wasteful to create nodes and throw them away, especially
  36    when the nodes can be reused.
  37
  38    For PR 8361, we can significantly reduce the number of nodes allocated
  39    and thus the total amount of memory allocated by managing SSA_NAMEs a
  40    little.  This additionally helps reduce the amount of work done by the
  41    garbage collector.  Similar results have been seen on a wider variety
  42    of tests (such as the compiler itself).
  43
  44    Right now we maintain our free list on a per-function basis.  It may
  45    or may not make sense to maintain the free list for the duration of
  46    a compilation unit.
  47
  48    External code should rely solely upon HIGHEST_SSA_VERSION and the
  49    externally defined functions.  External code should not know about
  50    the details of the free list management.
  51
  52    External code should also not assume the version number on nodes is
  53    monotonically increasing.  We reuse the version number when we
  54    reuse an SSA_NAME expression.  This helps keep arrays and bitmaps
  55    more compact.
  56
  57    We could also use a zone allocator for these objects since they have
  58    a very well defined lifetime.  If someone wants to experiment with that
  59    this is the place to try it.  */
  60
  61 /* Array of all SSA_NAMEs used in the function.  */
  62 varray_type ssa_names;
  63
  64 /* Free list of SSA_NAMEs.  This list is wiped at the end of each function
  65    after we leave SSA form.  */
  66 static GTY (()) tree free_ssanames;
  67
  68 /* Version numbers with special meanings.  We start allocating new version
  69    numbers after the special ones.  */
  70 #define UNUSED_NAME_VERSION 0
  71
  72 #ifdef GATHER_STATISTICS
  73 unsigned int ssa_name_nodes_reused;
  74 unsigned int ssa_name_nodes_created;
  75 #endif
  76
  77 /* Initialize management of SSA_NAMEs.  */
  78
  79 void
  80 init_ssanames (void)
  81 {
  82   VARRAY_TREE_INIT (ssa_names, 50, "ssa_names table");
  83
  84   /* Version 0 is special, so reserve the first slot in the table.  Though
  85      currently unused, we may use version 0 in alias analysis as part of
  86      the heuristics used to group aliases when the alias sets are too
  87      large.  */
  88   VARRAY_PUSH_TREE (ssa_names, NULL_TREE);
  89   free_ssanames = NULL;
  90 }
  91
  92 /* Finalize management of SSA_NAMEs.  */
  93
  94 void
  95 fini_ssanames (void)
  96 {
  97   free_ssanames = NULL;
  98 }
  99
 100 /* Dump some simple statistics regarding the re-use of SSA_NAME nodes.  */
 101
 102 #ifdef GATHER_STATISTICS
 103 void
 104 ssanames_print_statistics (void)
 105 {
 106   fprintf (stderr, "SSA_NAME nodes allocated: %u\n", ssa_name_nodes_created);
 107   fprintf (stderr, "SSA_NAME nodes reused: %u\n", ssa_name_nodes_reused);
 108 }
 109 #endif
 110
 111 /* Return an SSA_NAME node for variable VAR defined in statement STMT.
 112    STMT may be an empty statement for artificial references (e.g., default
 113    definitions created when a variable is used without a preceding
 114    definition).  */
 115
 116 tree
 117 make_ssa_name (tree var, tree stmt)
 118 {
 119   tree t;
 120
 121 #if defined ENABLE_CHECKING
 122   if ((!DECL_P (var)
 123        && TREE_CODE (var) != INDIRECT_REF)
 124       || (!IS_EXPR_CODE_CLASS (TREE_CODE_CLASS (TREE_CODE (stmt)))
 125           && TREE_CODE (stmt) != PHI_NODE))
 126     abort ();
 127 #endif
 128
 129   /* If our free list has an element, then use it.  Also reuse the
 130      SSA version number of the element on the free list which helps
 131      keep sbitmaps and arrays sized HIGHEST_SSA_VERSION smaller.  */
 132   if (free_ssanames)
 133     {
 134       unsigned int save_version;
 135
 136       t = free_ssanames;
 137       free_ssanames = TREE_CHAIN (free_ssanames);
 138 #ifdef GATHER_STATISTICS
 139       ssa_name_nodes_reused++;
 140 #endif
 141
 142       /* Clear the node so that it looks just like one we would have
 143          received from make_node.  */
 144       save_version = SSA_NAME_VERSION (t);
 145       memset (t, 0, tree_size (t));
 146       TREE_SET_CODE (t, SSA_NAME);
 147       SSA_NAME_VERSION (t) = save_version;
 148     }
 149   else
 150     {
 151       t = make_node (SSA_NAME);
 152       SSA_NAME_VERSION (t) = num_ssa_names;
 153       VARRAY_PUSH_TREE (ssa_names, t);
 154 #ifdef GATHER_STATISTICS
 155       ssa_name_nodes_created++;
 156 #endif
 157     }
 158
 159   TREE_TYPE (t) = TREE_TYPE (var);
 160   SSA_NAME_VAR (t) = var;
 161   SSA_NAME_DEF_STMT (t) = stmt;
 162   SSA_NAME_PTR_INFO (t) = NULL;
 163
 164   return t;
 165 }
 166
 167
 168 /* We no longer need the SSA_NAME expression VAR, release it so that
 169    it may be reused.
 170
 171    Note it is assumed that no calls to make_ssa_name will be made
 172    until all uses of the ssa name are released and that the only
 173    use of the SSA_NAME expression is to check its SSA_NAME_VAR.  All
 174    other fields must be assumed clobbered.  */
 175
 176 void
 177 release_ssa_name (tree var)
 178 {
 179   /* release_ssa_name can be called multiple times on a single SSA_NAME.
 180      However, it should only end up on our free list one time.   We
 181      keep a status bit in the SSA_NAME node itself to indicate it has
 182      been put on the free list.
 183
 184      Note that once on the freelist you can not reference the SSA_NAME's
 185      defining statement.  */
 186   if (! SSA_NAME_IN_FREE_LIST (var))
 187     {
 188       SSA_NAME_IN_FREE_LIST (var) = 1;
 189       TREE_CHAIN (var) = free_ssanames;
 190       free_ssanames = var;
 191     }
 192 }
 193
 194 /* Creates a duplicate of a ssa name NAME defined in statement STMT.  */
 195
 196 tree
 197 duplicate_ssa_name (tree name, tree stmt)
 198 {
 199   tree new_name = make_ssa_name (SSA_NAME_VAR (name), stmt);
 200   struct ptr_info_def *old_ptr_info = SSA_NAME_PTR_INFO (name);
 201   struct ptr_info_def *new_ptr_info;
 202
 203   if (!old_ptr_info)
 204     return new_name;
 205
 206   new_ptr_info = ggc_alloc (sizeof (struct ptr_info_def));
 207   *new_ptr_info = *old_ptr_info;
 208
 209   if (old_ptr_info->pt_vars)
 210     {
 211       new_ptr_info->pt_vars = BITMAP_GGC_ALLOC ();
 212       bitmap_copy (new_ptr_info->pt_vars, old_ptr_info->pt_vars);
 213     }
 214
 215   SSA_NAME_PTR_INFO (new_name) = new_ptr_info;
 216   return new_name;
 217 }
 218
 219 #include "gt-tree-ssanames.h"