gcc/web.c

   1 /* Web construction code for GNU compiler.
   2    Contributed by Jan Hubicka.
   3    Copyright (C) 2001, 2002, 2004 Free Software Foundation, Inc.
   4
   5 This file is part of GCC.
   6
   7 GCC is free software; you can redistribute it and/or modify it under
   8 the terms of the GNU General Public License as published by the Free
   9 Software Foundation; either version 2, or (at your option) any later
  10 version.
  11
  12 GCC is distributed in the hope that it will be useful, but WITHOUT ANY
  13 WARRANTY; without even the implied warranty of MERCHANTABILITY or
  14 FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
  15 for more details.
  16
  17 You should have received a copy of the GNU General Public License
  18 along with GCC; see the file COPYING.  If not, write to the Free
  19 Software Foundation, 59 Temple Place - Suite 330, Boston, MA
  20 02111-1307, USA.  */
  21
  22 /* Simple optimization pass that splits independent uses of each pseudo,
  23    increasing effectiveness of other optimizations.  The optimization can
  24    serve as an example of use for the dataflow module.
  25
  26    We don't split registers with REG_USERVAR set unless -fmessy-debugging
  27    is specified, because debugging information about such split variables
  28    is almost unusable.
  29
  30    TODO
  31     - Add code to keep debugging up-to-date after splitting user variable
  32       pseudos.  This can be done by keeping track of all the pseudos used
  33       for the variable and using life analysis information before reload
  34       to determine which one is live and, in case more than one are live,
  35       choose the one with the latest definition.
  36
  37       Other optimization passes can benefit from the infrastructure too.
  38
  39     - We may use profile information and ignore infrequent use for the
  40       purpose of web unifying, inserting the compensation code later to
  41       implement full induction variable expansion for loops (currently
  42       we expand only if the induction variable is dead afterward, which
  43       is often the case).  */
  44
  45 #include "config.h"
  46 #include "system.h"
  47 #include "coretypes.h"
  48 #include "tm.h"
  49 #include "toplev.h"
  50
  51 #include "rtl.h"
  52 #include "hard-reg-set.h"
  53 #include "flags.h"
  54 #include "basic-block.h"
  55 #include "output.h"
  56 #include "df.h"
  57 #include "function.h"
  58
  59
  60 /* This entry is allocated for each reference in the insn stream.  */
  61 struct web_entry
  62 {
  63   /* Pointer to the parent in the union/find tree.  */
  64   struct web_entry *pred;
  65   /* Newly assigned register to the entry.  Set only for roots.  */
  66   rtx reg;
  67 };
  68
  69 static struct web_entry *unionfind_root (struct web_entry *);
  70 static void unionfind_union (struct web_entry *, struct web_entry *);
  71 static void union_defs (struct df *, struct ref *, struct web_entry *,
  72                         struct web_entry *);
  73 static rtx entry_register (struct web_entry *, struct ref *, char *);
  74 static void replace_ref (struct ref *, rtx);
  75
  76 /* Find the root of unionfind tree (the representative of set).  */
  77
  78 static struct web_entry *
  79 unionfind_root (struct web_entry *element)
  80 {
  81   struct web_entry *element1 = element, *element2;
  82
  83   while (element->pred)
  84     element = element->pred;
  85   while (element1->pred)
  86     {
  87       element2 = element1->pred;
  88       element1->pred = element;
  89       element1 = element2;
  90     }
  91   return element;
  92 }
  93
  94 /* Union sets.  */
  95
  96 static void
  97 unionfind_union (struct web_entry *first, struct web_entry *second)
  98 {
  99   first = unionfind_root (first);
 100   second = unionfind_root (second);
 101   if (first == second)
 102     return;
 103   second->pred = first;
 104 }
 105
 106 /* For each use, all possible defs reaching it must come in the same
 107    register, union them.  */
 108
 109 static void
 110 union_defs (struct df *df, struct ref *use, struct web_entry *def_entry,
 111             struct web_entry *use_entry)
 112 {
 113   rtx insn = DF_REF_INSN (use);
 114   struct df_link *link = DF_REF_CHAIN (use);
 115   struct df_link *use_link = DF_INSN_USES (df, insn);
 116   struct df_link *def_link = DF_INSN_DEFS (df, insn);
 117   rtx set = single_set (insn);
 118
 119   /* Some instructions may use match_dup for their operands.  In case the
 120      operands are dead, we will assign them different pseudos, creating
 121      invalid instructions, so union all uses of the same operand for each
 122      insn.  */
 123
 124   while (use_link)
 125     {
 126       if (use != use_link->ref
 127           && DF_REF_REAL_REG (use) == DF_REF_REAL_REG (use_link->ref))
 128         unionfind_union (use_entry + DF_REF_ID (use),
 129                          use_entry + DF_REF_ID (use_link->ref));
 130       use_link = use_link->next;
 131     }
 132
 133   /* Recognize trivial noop moves and attempt to keep them as noop.
 134      While most of noop moves should be removed, we still keep some
 135      of them at libcall boundaries and such.  */
 136
 137   if (set
 138       && SET_SRC (set) == DF_REF_REG (use)
 139       && SET_SRC (set) == SET_DEST (set))
 140     {
 141       while (def_link)
 142         {
 143           if (DF_REF_REAL_REG (use) == DF_REF_REAL_REG (def_link->ref))
 144             unionfind_union (use_entry + DF_REF_ID (use),
 145                              def_entry + DF_REF_ID (def_link->ref));
 146           def_link = def_link->next;
 147         }
 148     }
 149   while (link)
 150     {
 151       unionfind_union (use_entry + DF_REF_ID (use),
 152                        def_entry + DF_REF_ID (link->ref));
 153       link = link->next;
 154     }
 155
 156   /* A READ_WRITE use requires the corresponding def to be in the same
 157      register.  Find it and union.  */
 158   if (use->flags & DF_REF_READ_WRITE)
 159     {
 160       struct df_link *link = DF_INSN_DEFS (df, DF_REF_INSN (use));
 161
 162       while (link)
 163         {
 164           if (DF_REF_REAL_REG (link->ref) == DF_REF_REAL_REG (use))
 165             unionfind_union (use_entry + DF_REF_ID (use),
 166                              def_entry + DF_REF_ID (link->ref));
 167           link = link->next;
 168         }
 169     }
 170 }
 171
 172 /* Find the corresponding register for the given entry.  */
 173
 174 static rtx
 175 entry_register (struct web_entry *entry, struct ref *ref, char *used)
 176 {
 177   struct web_entry *root;
 178   rtx reg, newreg;
 179
 180   /* Find the corresponding web and see if it has been visited.  */
 181   root = unionfind_root (entry);
 182   if (root->reg)
 183     return root->reg;
 184
 185   /* We are seeing this web for the first time, do the assignment.  */
 186   reg = DF_REF_REAL_REG (ref);
 187
 188   /* In case the original register is already assigned, generate new one.  */
 189   if (!used[REGNO (reg)])
 190     newreg = reg, used[REGNO (reg)] = 1;
 191   else if (REG_USERVAR_P (reg) && 0/*&& !flag_messy_debugging*/)
 192     {
 193       newreg = reg;
 194       if (dump_file)
 195         fprintf (dump_file,
 196                  "New web forced to keep reg=%i (user variable)\n",
 197                  REGNO (reg));
 198     }
 199   else
 200     {
 201       newreg = gen_reg_rtx (GET_MODE (reg));
 202       REG_USERVAR_P (newreg) = REG_USERVAR_P (reg);
 203       REG_POINTER (newreg) = REG_POINTER (reg);
 204       REG_LOOP_TEST_P (newreg) = REG_LOOP_TEST_P (reg);
 205       RTX_UNCHANGING_P (newreg) = RTX_UNCHANGING_P (reg);
 206       REG_ATTRS (newreg) = REG_ATTRS (reg);
 207       if (dump_file)
 208         fprintf (dump_file, "Web oldreg=%i newreg=%i\n", REGNO (reg),
 209                  REGNO (newreg));
 210     }
 211
 212   root->reg = newreg;
 213   return newreg;
 214 }
 215
 216 /* Replace the reference by REG.  */
 217
 218 static void
 219 replace_ref (struct ref *ref, rtx reg)
 220 {
 221   rtx oldreg = DF_REF_REAL_REG (ref);
 222   rtx *loc = DF_REF_REAL_LOC (ref);
 223
 224   if (oldreg == reg)
 225     return;
 226   if (dump_file)
 227     fprintf (dump_file, "Updating insn %i (%i->%i)\n",
 228              INSN_UID (DF_REF_INSN (ref)), REGNO (oldreg), REGNO (reg));
 229   *loc = reg;
 230 }
 231
 232 /* Main entry point.  */
 233
 234 void
 235 web_main (void)
 236 {
 237   struct df *df;
 238   struct web_entry *def_entry;
 239   struct web_entry *use_entry;
 240   unsigned int i;
 241   int max = max_reg_num ();
 242   char *used;
 243
 244   df = df_init ();
 245   df_analyze (df, 0, DF_UD_CHAIN | DF_EQUIV_NOTES);
 246
 247   def_entry = xcalloc (df->n_defs, sizeof (struct web_entry));
 248   use_entry = xcalloc (df->n_uses, sizeof (struct web_entry));
 249   used = xcalloc (max, sizeof (char));
 250
 251   if (dump_file)
 252     df_dump (df, DF_UD_CHAIN | DF_DU_CHAIN, dump_file);
 253
 254   /* Produce the web.  */
 255   for (i = 0; i < df->n_uses; i++)
 256     union_defs (df, df->uses[i], def_entry, use_entry);
 257
 258   /* Update the instruction stream, allocating new registers for split pseudos
 259      in progress.  */
 260   for (i = 0; i < df->n_uses; i++)
 261     replace_ref (df->uses[i], entry_register (use_entry + i, df->uses[i],
 262                                               used));
 263   for (i = 0; i < df->n_defs; i++)
 264     replace_ref (df->defs[i], entry_register (def_entry + i, df->defs[i],
 265                                               used));
 266
 267   /* Dataflow information is corrupt here, but it can be easily updated
 268      by creating new entries for new registers and updates or calling
 269      df_insns_modify.  */
 270   free (def_entry);
 271   free (use_entry);
 272   free (used);
 273   df_finish (df);
 274 }