gcc/combine-stack-adj.c

   1 /* Combine stack adjustments.
   2    Copyright (C) 1987, 1988, 1989, 1992, 1993, 1994, 1995, 1996, 1997,
   3    1998, 1999, 2000, 2001, 2002, 2003, 2004, 2005, 2006, 2007
   4    Free Software Foundation, Inc.
   5
   6 This file is part of GCC.
   7
   8 GCC is free software; you can redistribute it and/or modify it under
   9 the terms of the GNU General Public License as published by the Free
  10 Software Foundation; either version 3, or (at your option) any later
  11 version.
  12
  13 GCC is distributed in the hope that it will be useful, but WITHOUT ANY
  14 WARRANTY; without even the implied warranty of MERCHANTABILITY or
  15 FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
  16 for more details.
  17
  18 You should have received a copy of the GNU General Public License
  19 along with GCC; see the file COPYING3.  If not see
  20 <http://www.gnu.org/licenses/>.  */
  21
  22 /* Track stack adjustments and stack memory references.  Attempt to
  23    reduce the number of stack adjustments by back-propagating across
  24    the memory references.
  25
  26    This is intended primarily for use with targets that do not define
  27    ACCUMULATE_OUTGOING_ARGS.  It is of significantly more value to
  28    targets that define PREFERRED_STACK_BOUNDARY more aligned than
  29    STACK_BOUNDARY (e.g. x86), or if not all registers can be pushed
  30    (e.g. x86 fp regs) which would ordinarily have to be implemented
  31    as a sub/mov pair due to restrictions in calls.c.
  32
  33    Propagation stops when any of the insns that need adjusting are
  34    (a) no longer valid because we've exceeded their range, (b) a
  35    non-trivial push instruction, or (c) a call instruction.
  36
  37    Restriction B is based on the assumption that push instructions
  38    are smaller or faster.  If a port really wants to remove all
  39    pushes, it should have defined ACCUMULATE_OUTGOING_ARGS.  The
  40    one exception that is made is for an add immediately followed
  41    by a push.  */
  42
  43 #include "config.h"
  44 #include "system.h"
  45 #include "coretypes.h"
  46 #include "tm.h"
  47 #include "rtl.h"
  48 #include "tm_p.h"
  49 #include "insn-config.h"
  50 #include "recog.h"
  51 #include "output.h"
  52 #include "regs.h"
  53 #include "hard-reg-set.h"
  54 #include "flags.h"
  55 #include "function.h"
  56 #include "expr.h"
  57 #include "basic-block.h"
  58 #include "df.h"
  59 #include "except.h"
  60 #include "toplev.h"
  61 #include "reload.h"
  62 #include "timevar.h"
  63 #include "tree-pass.h"
  64
  65 \f
  66 /* Turn STACK_GROWS_DOWNWARD into a boolean.  */
  67 #ifdef STACK_GROWS_DOWNWARD
  68 #undef STACK_GROWS_DOWNWARD
  69 #define STACK_GROWS_DOWNWARD 1
  70 #else
  71 #define STACK_GROWS_DOWNWARD 0
  72 #endif
  73
  74 /* This structure records stack memory references between stack adjusting
  75    instructions.  */
  76
  77 struct csa_memlist
  78 {
  79   HOST_WIDE_INT sp_offset;
  80   rtx insn, *mem;
  81   struct csa_memlist *next;
  82 };
  83
  84 static int stack_memref_p (rtx);
  85 static rtx single_set_for_csa (rtx);
  86 static void free_csa_memlist (struct csa_memlist *);
  87 static struct csa_memlist *record_one_stack_memref (rtx, rtx *,
  88                                                     struct csa_memlist *);
  89 static int try_apply_stack_adjustment (rtx, struct csa_memlist *,
  90                                        HOST_WIDE_INT, HOST_WIDE_INT);
  91 static void combine_stack_adjustments_for_block (basic_block);
  92 static int record_stack_memrefs (rtx *, void *);
  93
  94
  95 /* Main entry point for stack adjustment combination.  */
  96
  97 static void
  98 combine_stack_adjustments (void)
  99 {
 100   basic_block bb;
 101
 102   FOR_EACH_BB (bb)
 103     combine_stack_adjustments_for_block (bb);
 104 }
 105
 106 /* Recognize a MEM of the form (sp) or (plus sp const).  */
 107
 108 static int
 109 stack_memref_p (rtx x)
 110 {
 111   if (!MEM_P (x))
 112     return 0;
 113   x = XEXP (x, 0);
 114
 115   if (x == stack_pointer_rtx)
 116     return 1;
 117   if (GET_CODE (x) == PLUS
 118       && XEXP (x, 0) == stack_pointer_rtx
 119       && GET_CODE (XEXP (x, 1)) == CONST_INT)
 120     return 1;
 121
 122   return 0;
 123 }
 124
 125 /* Recognize either normal single_set or the hack in i386.md for
 126    tying fp and sp adjustments.  */
 127
 128 static rtx
 129 single_set_for_csa (rtx insn)
 130 {
 131   int i;
 132   rtx tmp = single_set (insn);
 133   if (tmp)
 134     return tmp;
 135
 136   if (!NONJUMP_INSN_P (insn)
 137       || GET_CODE (PATTERN (insn)) != PARALLEL)
 138     return NULL_RTX;
 139
 140   tmp = PATTERN (insn);
 141   if (GET_CODE (XVECEXP (tmp, 0, 0)) != SET)
 142     return NULL_RTX;
 143
 144   for (i = 1; i < XVECLEN (tmp, 0); ++i)
 145     {
 146       rtx this = XVECEXP (tmp, 0, i);
 147
 148       /* The special case is allowing a no-op set.  */
 149       if (GET_CODE (this) == SET
 150           && SET_SRC (this) == SET_DEST (this))
 151         ;
 152       else if (GET_CODE (this) != CLOBBER
 153                && GET_CODE (this) != USE)
 154         return NULL_RTX;
 155     }
 156
 157   return XVECEXP (tmp, 0, 0);
 158 }
 159
 160 /* Free the list of csa_memlist nodes.  */
 161
 162 static void
 163 free_csa_memlist (struct csa_memlist *memlist)
 164 {
 165   struct csa_memlist *next;
 166   for (; memlist ; memlist = next)
 167     {
 168       next = memlist->next;
 169       free (memlist);
 170     }
 171 }
 172
 173 /* Create a new csa_memlist node from the given memory reference.
 174    It is already known that the memory is stack_memref_p.  */
 175
 176 static struct csa_memlist *
 177 record_one_stack_memref (rtx insn, rtx *mem, struct csa_memlist *next_memlist)
 178 {
 179   struct csa_memlist *ml;
 180
 181   ml = XNEW (struct csa_memlist);
 182
 183   if (XEXP (*mem, 0) == stack_pointer_rtx)
 184     ml->sp_offset = 0;
 185   else
 186     ml->sp_offset = INTVAL (XEXP (XEXP (*mem, 0), 1));
 187
 188   ml->insn = insn;
 189   ml->mem = mem;
 190   ml->next = next_memlist;
 191
 192   return ml;
 193 }
 194
 195 /* Attempt to apply ADJUST to the stack adjusting insn INSN, as well
 196    as each of the memories in MEMLIST.  Return true on success.  */
 197
 198 static int
 199 try_apply_stack_adjustment (rtx insn, struct csa_memlist *memlist, HOST_WIDE_INT new_adjust,
 200                             HOST_WIDE_INT delta)
 201 {
 202   struct csa_memlist *ml;
 203   rtx set;
 204
 205   set = single_set_for_csa (insn);
 206   validate_change (insn, &XEXP (SET_SRC (set), 1), GEN_INT (new_adjust), 1);
 207
 208   for (ml = memlist; ml ; ml = ml->next)
 209     validate_change
 210       (ml->insn, ml->mem,
 211        replace_equiv_address_nv (*ml->mem,
 212                                  plus_constant (stack_pointer_rtx,
 213                                                 ml->sp_offset - delta)), 1);
 214
 215   if (apply_change_group ())
 216     {
 217       /* Succeeded.  Update our knowledge of the memory references.  */
 218       for (ml = memlist; ml ; ml = ml->next)
 219         ml->sp_offset -= delta;
 220
 221       return 1;
 222     }
 223   else
 224     return 0;
 225 }
 226
 227 /* Called via for_each_rtx and used to record all stack memory references in
 228    the insn and discard all other stack pointer references.  */
 229 struct record_stack_memrefs_data
 230 {
 231   rtx insn;
 232   struct csa_memlist *memlist;
 233 };
 234
 235 static int
 236 record_stack_memrefs (rtx *xp, void *data)
 237 {
 238   rtx x = *xp;
 239   struct record_stack_memrefs_data *d =
 240     (struct record_stack_memrefs_data *) data;
 241   if (!x)
 242     return 0;
 243   switch (GET_CODE (x))
 244     {
 245     case MEM:
 246       if (!reg_mentioned_p (stack_pointer_rtx, x))
 247         return -1;
 248       /* We are not able to handle correctly all possible memrefs containing
 249          stack pointer, so this check is necessary.  */
 250       if (stack_memref_p (x))
 251         {
 252           d->memlist = record_one_stack_memref (d->insn, xp, d->memlist);
 253           return -1;
 254         }
 255       return 1;
 256     case REG:
 257       /* ??? We want be able to handle non-memory stack pointer
 258          references later.  For now just discard all insns referring to
 259          stack pointer outside mem expressions.  We would probably
 260          want to teach validate_replace to simplify expressions first.
 261
 262          We can't just compare with STACK_POINTER_RTX because the
 263          reference to the stack pointer might be in some other mode.
 264          In particular, an explicit clobber in an asm statement will
 265          result in a QImode clobber.  */
 266       if (REGNO (x) == STACK_POINTER_REGNUM)
 267         return 1;
 268       break;
 269     default:
 270       break;
 271     }
 272   return 0;
 273 }
 274
 275 /* Subroutine of combine_stack_adjustments, called for each basic block.  */
 276
 277 static void
 278 combine_stack_adjustments_for_block (basic_block bb)
 279 {
 280   HOST_WIDE_INT last_sp_adjust = 0;
 281   rtx last_sp_set = NULL_RTX;
 282   struct csa_memlist *memlist = NULL;
 283   rtx insn, next, set;
 284   struct record_stack_memrefs_data data;
 285   bool end_of_block = false;
 286
 287   for (insn = BB_HEAD (bb); !end_of_block ; insn = next)
 288     {
 289       end_of_block = insn == BB_END (bb);
 290       next = NEXT_INSN (insn);
 291
 292       if (! INSN_P (insn))
 293         continue;
 294
 295       set = single_set_for_csa (insn);
 296       if (set)
 297         {
 298           rtx dest = SET_DEST (set);
 299           rtx src = SET_SRC (set);
 300
 301           /* Find constant additions to the stack pointer.  */
 302           if (dest == stack_pointer_rtx
 303               && GET_CODE (src) == PLUS
 304               && XEXP (src, 0) == stack_pointer_rtx
 305               && GET_CODE (XEXP (src, 1)) == CONST_INT)
 306             {
 307               HOST_WIDE_INT this_adjust = INTVAL (XEXP (src, 1));
 308
 309               /* If we've not seen an adjustment previously, record
 310                  it now and continue.  */
 311               if (! last_sp_set)
 312                 {
 313                   last_sp_set = insn;
 314                   last_sp_adjust = this_adjust;
 315                   continue;
 316                 }
 317
 318               /* If not all recorded memrefs can be adjusted, or the
 319                  adjustment is now too large for a constant addition,
 320                  we cannot merge the two stack adjustments.
 321
 322                  Also we need to be careful to not move stack pointer
 323                  such that we create stack accesses outside the allocated
 324                  area.  We can combine an allocation into the first insn,
 325                  or a deallocation into the second insn.  We can not
 326                  combine an allocation followed by a deallocation.
 327
 328                  The only somewhat frequent occurrence of the later is when
 329                  a function allocates a stack frame but does not use it.
 330                  For this case, we would need to analyze rtl stream to be
 331                  sure that allocated area is really unused.  This means not
 332                  only checking the memory references, but also all registers
 333                  or global memory references possibly containing a stack
 334                  frame address.
 335
 336                  Perhaps the best way to address this problem is to teach
 337                  gcc not to allocate stack for objects never used.  */
 338
 339               /* Combine an allocation into the first instruction.  */
 340               if (STACK_GROWS_DOWNWARD ? this_adjust <= 0 : this_adjust >= 0)
 341                 {
 342                   if (try_apply_stack_adjustment (last_sp_set, memlist,
 343                                                   last_sp_adjust + this_adjust,
 344                                                   this_adjust))
 345                     {
 346                       /* It worked!  */
 347                       delete_insn (insn);
 348                       last_sp_adjust += this_adjust;
 349                       continue;
 350                     }
 351                 }
 352
 353               /* Otherwise we have a deallocation.  Do not combine with
 354                  a previous allocation.  Combine into the second insn.  */
 355               else if (STACK_GROWS_DOWNWARD
 356                        ? last_sp_adjust >= 0 : last_sp_adjust <= 0)
 357                 {
 358                   if (try_apply_stack_adjustment (insn, memlist,
 359                                                   last_sp_adjust + this_adjust,
 360                                                   -last_sp_adjust))
 361                     {
 362                       /* It worked!  */
 363                       delete_insn (last_sp_set);
 364                       last_sp_set = insn;
 365                       last_sp_adjust += this_adjust;
 366                       free_csa_memlist (memlist);
 367                       memlist = NULL;
 368                       continue;
 369                     }
 370                 }
 371
 372               /* Combination failed.  Restart processing from here.  If
 373                  deallocation+allocation conspired to cancel, we can
 374                  delete the old deallocation insn.  */
 375               if (last_sp_set && last_sp_adjust == 0)
 376                 delete_insn (insn);
 377               free_csa_memlist (memlist);
 378               memlist = NULL;
 379               last_sp_set = insn;
 380               last_sp_adjust = this_adjust;
 381               continue;
 382             }
 383
 384           /* Find a predecrement of exactly the previous adjustment and
 385              turn it into a direct store.  Obviously we can't do this if
 386              there were any intervening uses of the stack pointer.  */
 387           if (memlist == NULL
 388               && MEM_P (dest)
 389               && ((GET_CODE (XEXP (dest, 0)) == PRE_DEC
 390                    && (last_sp_adjust
 391                        == (HOST_WIDE_INT) GET_MODE_SIZE (GET_MODE (dest))))
 392                   || (GET_CODE (XEXP (dest, 0)) == PRE_MODIFY
 393                       && GET_CODE (XEXP (XEXP (dest, 0), 1)) == PLUS
 394                       && XEXP (XEXP (XEXP (dest, 0), 1), 0) == stack_pointer_rtx
 395                       && (GET_CODE (XEXP (XEXP (XEXP (dest, 0), 1), 1))
 396                           == CONST_INT)
 397                       && (INTVAL (XEXP (XEXP (XEXP (dest, 0), 1), 1))
 398                           == -last_sp_adjust)))
 399               && XEXP (XEXP (dest, 0), 0) == stack_pointer_rtx
 400               && ! reg_mentioned_p (stack_pointer_rtx, src)
 401               && memory_address_p (GET_MODE (dest), stack_pointer_rtx)
 402               && validate_change (insn, &SET_DEST (set),
 403                                   replace_equiv_address (dest,
 404                                                          stack_pointer_rtx),
 405                                   0))
 406             {
 407               delete_insn (last_sp_set);
 408               free_csa_memlist (memlist);
 409               memlist = NULL;
 410               last_sp_set = NULL_RTX;
 411               last_sp_adjust = 0;
 412               continue;
 413             }
 414         }
 415
 416       data.insn = insn;
 417       data.memlist = memlist;
 418       if (!CALL_P (insn) && last_sp_set
 419           && !for_each_rtx (&PATTERN (insn), record_stack_memrefs, &data))
 420         {
 421            memlist = data.memlist;
 422            continue;
 423         }
 424       memlist = data.memlist;
 425
 426       /* Otherwise, we were not able to process the instruction.
 427          Do not continue collecting data across such a one.  */
 428       if (last_sp_set
 429           && (CALL_P (insn)
 430               || reg_mentioned_p (stack_pointer_rtx, PATTERN (insn))))
 431         {
 432           if (last_sp_set && last_sp_adjust == 0)
 433             delete_insn (last_sp_set);
 434           free_csa_memlist (memlist);
 435           memlist = NULL;
 436           last_sp_set = NULL_RTX;
 437           last_sp_adjust = 0;
 438         }
 439     }
 440
 441   if (last_sp_set && last_sp_adjust == 0)
 442     delete_insn (last_sp_set);
 443
 444   if (memlist)
 445     free_csa_memlist (memlist);
 446 }
 447 \f
 448
 449 static bool
 450 gate_handle_stack_adjustments (void)
 451 {
 452   return (optimize > 0);
 453 }
 454
 455 static unsigned int
 456 rest_of_handle_stack_adjustments (void)
 457 {
 458   cleanup_cfg (flag_crossjumping ? CLEANUP_CROSSJUMP : 0);
 459
 460   /* This is kind of a heuristic.  We need to run combine_stack_adjustments
 461      even for machines with possibly nonzero RETURN_POPS_ARGS
 462      and ACCUMULATE_OUTGOING_ARGS.  We expect that only ports having
 463      push instructions will have popping returns.  */
 464 #ifndef PUSH_ROUNDING
 465   if (!ACCUMULATE_OUTGOING_ARGS)
 466 #endif
 467     {
 468       df_note_add_problem ();
 469       df_analyze ();
 470       combine_stack_adjustments ();
 471     }
 472   return 0;
 473 }
 474
 475 struct tree_opt_pass pass_stack_adjustments =
 476 {
 477   "csa",                                /* name */
 478   gate_handle_stack_adjustments,        /* gate */
 479   rest_of_handle_stack_adjustments,     /* execute */
 480   NULL,                                 /* sub */
 481   NULL,                                 /* next */
 482   0,                                    /* static_pass_number */
 483   0,                                    /* tv_id */
 484   0,                                    /* properties_required */
 485   0,                                    /* properties_provided */
 486   0,                                    /* properties_destroyed */
 487   0,                                    /* todo_flags_start */
 488   TODO_df_finish | TODO_verify_rtl_sharing |
 489   TODO_dump_func |
 490   TODO_ggc_collect,                     /* todo_flags_finish */
 491   0                                     /* letter */
 492 };
 493