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, 2008, 2009,
   4    2010 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  34    (a) no longer valid because we've exceeded their range, (b) a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 "reload.h"
  61 #include "timevar.h"
  62 #include "tree-pass.h"
  63
  64 \f
  65 /* Turn STACK_GROWS_DOWNWARD into a boolean.  */
  66 #ifdef STACK_GROWS_DOWNWARD
  67 #undef STACK_GROWS_DOWNWARD
  68 #define STACK_GROWS_DOWNWARD 1
  69 #else
  70 #define STACK_GROWS_DOWNWARD 0
  71 #endif
  72
  73 /* This structure records two kinds of stack references between stack
  74    adjusting instructions: stack references in memory addresses for
  75    regular insns and all stack references for debug insns.  */
  76
  77 struct csa_reflist
  78 {
  79   HOST_WIDE_INT sp_offset;
  80   rtx insn, *ref;
  81   struct csa_reflist *next;
  82 };
  83
  84 static int stack_memref_p (rtx);
  85 static rtx single_set_for_csa (rtx);
  86 static void free_csa_reflist (struct csa_reflist *);
  87 static struct csa_reflist *record_one_stack_ref (rtx, rtx *,
  88                                                  struct csa_reflist *);
  89 static int try_apply_stack_adjustment (rtx, struct csa_reflist *,
  90                                        HOST_WIDE_INT, HOST_WIDE_INT);
  91 static void combine_stack_adjustments_for_block (basic_block);
  92 static int record_stack_refs (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       && CONST_INT_P (XEXP (x, 1)))
 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_rtx = XVECEXP (tmp, 0, i);
 147
 148       /* The special case is allowing a no-op set.  */
 149       if (GET_CODE (this_rtx) == SET
 150           && SET_SRC (this_rtx) == SET_DEST (this_rtx))
 151         ;
 152       else if (GET_CODE (this_rtx) != CLOBBER
 153                && GET_CODE (this_rtx) != USE)
 154         return NULL_RTX;
 155     }
 156
 157   return XVECEXP (tmp, 0, 0);
 158 }
 159
 160 /* Free the list of csa_reflist nodes.  */
 161
 162 static void
 163 free_csa_reflist (struct csa_reflist *reflist)
 164 {
 165   struct csa_reflist *next;
 166   for (; reflist ; reflist = next)
 167     {
 168       next = reflist->next;
 169       free (reflist);
 170     }
 171 }
 172
 173 /* Create a new csa_reflist node from the given stack reference.
 174    It is already known that the reference is either a MEM satisfying the
 175    predicate stack_memref_p or a REG representing the stack pointer.  */
 176
 177 static struct csa_reflist *
 178 record_one_stack_ref (rtx insn, rtx *ref, struct csa_reflist *next_reflist)
 179 {
 180   struct csa_reflist *ml;
 181
 182   ml = XNEW (struct csa_reflist);
 183
 184   if (REG_P (*ref) || XEXP (*ref, 0) == stack_pointer_rtx)
 185     ml->sp_offset = 0;
 186   else
 187     ml->sp_offset = INTVAL (XEXP (XEXP (*ref, 0), 1));
 188
 189   ml->insn = insn;
 190   ml->ref = ref;
 191   ml->next = next_reflist;
 192
 193   return ml;
 194 }
 195
 196 /* Attempt to apply ADJUST to the stack adjusting insn INSN, as well
 197    as each of the memories and stack references in REFLIST.  Return true
 198    on success.  */
 199
 200 static int
 201 try_apply_stack_adjustment (rtx insn, struct csa_reflist *reflist,
 202                             HOST_WIDE_INT new_adjust, HOST_WIDE_INT delta)
 203 {
 204   struct csa_reflist *ml;
 205   rtx set;
 206
 207   set = single_set_for_csa (insn);
 208   if (MEM_P (SET_DEST (set)))
 209     validate_change (insn, &SET_DEST (set),
 210                      replace_equiv_address (SET_DEST (set), stack_pointer_rtx),
 211                      1);
 212   else
 213     validate_change (insn, &XEXP (SET_SRC (set), 1), GEN_INT (new_adjust), 1);
 214
 215   for (ml = reflist; ml ; ml = ml->next)
 216     {
 217       rtx new_addr = plus_constant (stack_pointer_rtx, ml->sp_offset - delta);
 218       rtx new_val;
 219
 220       if (MEM_P (*ml->ref))
 221         new_val = replace_equiv_address_nv (*ml->ref, new_addr);
 222       else if (GET_MODE (*ml->ref) == GET_MODE (stack_pointer_rtx))
 223         new_val = new_addr;
 224       else
 225         new_val = lowpart_subreg (GET_MODE (*ml->ref), new_addr,
 226                                   GET_MODE (new_addr));
 227       validate_change (ml->insn, ml->ref, new_val, 1);
 228     }
 229
 230   if (apply_change_group ())
 231     {
 232       /* Succeeded.  Update our knowledge of the stack references.  */
 233       for (ml = reflist; ml ; ml = ml->next)
 234         ml->sp_offset -= delta;
 235
 236       return 1;
 237     }
 238   else
 239     return 0;
 240 }
 241
 242 /* Called via for_each_rtx and used to record all stack memory and other
 243    references in the insn and discard all other stack pointer references.  */
 244 struct record_stack_refs_data
 245 {
 246   rtx insn;
 247   struct csa_reflist *reflist;
 248 };
 249
 250 static int
 251 record_stack_refs (rtx *xp, void *data)
 252 {
 253   rtx x = *xp;
 254   struct record_stack_refs_data *d =
 255     (struct record_stack_refs_data *) data;
 256   if (!x)
 257     return 0;
 258   switch (GET_CODE (x))
 259     {
 260     case MEM:
 261       if (!reg_mentioned_p (stack_pointer_rtx, x))
 262         return -1;
 263       /* We are not able to handle correctly all possible memrefs containing
 264          stack pointer, so this check is necessary.  */
 265       if (stack_memref_p (x))
 266         {
 267           d->reflist = record_one_stack_ref (d->insn, xp, d->reflist);
 268           return -1;
 269         }
 270       /* Try harder for DEBUG_INSNs, handle e.g. (mem (mem (sp + 16) + 4).  */
 271       return !DEBUG_INSN_P (d->insn);
 272     case REG:
 273       /* ??? We want be able to handle non-memory stack pointer
 274          references later.  For now just discard all insns referring to
 275          stack pointer outside mem expressions.  We would probably
 276          want to teach validate_replace to simplify expressions first.
 277
 278          We can't just compare with STACK_POINTER_RTX because the
 279          reference to the stack pointer might be in some other mode.
 280          In particular, an explicit clobber in an asm statement will
 281          result in a QImode clobber.
 282
 283          In DEBUG_INSNs, we want to replace all occurrences, otherwise
 284          they will cause -fcompare-debug failures.  */
 285       if (REGNO (x) == STACK_POINTER_REGNUM)
 286         {
 287           if (!DEBUG_INSN_P (d->insn))
 288             return 1;
 289           d->reflist = record_one_stack_ref (d->insn, xp, d->reflist);
 290           return -1;
 291         }
 292       break;
 293     default:
 294       break;
 295     }
 296   return 0;
 297 }
 298
 299 /* If INSN has a REG_ARGS_SIZE note, move it to LAST.
 300    AFTER is true iff LAST follows INSN in the instruction stream.  */
 301
 302 static void
 303 maybe_move_args_size_note (rtx last, rtx insn, bool after)
 304 {
 305   rtx note, last_note;
 306
 307   note = find_reg_note (insn, REG_ARGS_SIZE, NULL_RTX);
 308   if (note == NULL)
 309     return;
 310
 311   last_note = find_reg_note (last, REG_ARGS_SIZE, NULL_RTX);
 312   if (last_note)
 313     {
 314       /* The ARGS_SIZE notes are *not* cumulative.  They represent an
 315          absolute value, and the "most recent" note wins.  */
 316       if (!after)
 317         XEXP (last_note, 0) = XEXP (note, 0);
 318     }
 319   else
 320     add_reg_note (last, REG_ARGS_SIZE, XEXP (note, 0));
 321 }
 322
 323 /* Subroutine of combine_stack_adjustments, called for each basic block.  */
 324
 325 static void
 326 combine_stack_adjustments_for_block (basic_block bb)
 327 {
 328   HOST_WIDE_INT last_sp_adjust = 0;
 329   rtx last_sp_set = NULL_RTX;
 330   struct csa_reflist *reflist = NULL;
 331   rtx insn, next, set;
 332   struct record_stack_refs_data data;
 333   bool end_of_block = false;
 334
 335   for (insn = BB_HEAD (bb); !end_of_block ; insn = next)
 336     {
 337       end_of_block = insn == BB_END (bb);
 338       next = NEXT_INSN (insn);
 339
 340       if (! INSN_P (insn))
 341         continue;
 342
 343       set = single_set_for_csa (insn);
 344       if (set)
 345         {
 346           rtx dest = SET_DEST (set);
 347           rtx src = SET_SRC (set);
 348
 349           /* Find constant additions to the stack pointer.  */
 350           if (dest == stack_pointer_rtx
 351               && GET_CODE (src) == PLUS
 352               && XEXP (src, 0) == stack_pointer_rtx
 353               && CONST_INT_P (XEXP (src, 1)))
 354             {
 355               HOST_WIDE_INT this_adjust = INTVAL (XEXP (src, 1));
 356
 357               /* If we've not seen an adjustment previously, record
 358                  it now and continue.  */
 359               if (! last_sp_set)
 360                 {
 361                   last_sp_set = insn;
 362                   last_sp_adjust = this_adjust;
 363                   continue;
 364                 }
 365
 366               /* If not all recorded refs can be adjusted, or the
 367                  adjustment is now too large for a constant addition,
 368                  we cannot merge the two stack adjustments.
 369
 370                  Also we need to be careful to not move stack pointer
 371                  such that we create stack accesses outside the allocated
 372                  area.  We can combine an allocation into the first insn,
 373                  or a deallocation into the second insn.  We can not
 374                  combine an allocation followed by a deallocation.
 375
 376                  The only somewhat frequent occurrence of the later is when
 377                  a function allocates a stack frame but does not use it.
 378                  For this case, we would need to analyze rtl stream to be
 379                  sure that allocated area is really unused.  This means not
 380                  only checking the memory references, but also all registers
 381                  or global memory references possibly containing a stack
 382                  frame address.
 383
 384                  Perhaps the best way to address this problem is to teach
 385                  gcc not to allocate stack for objects never used.  */
 386
 387               /* Combine an allocation into the first instruction.  */
 388               if (STACK_GROWS_DOWNWARD ? this_adjust <= 0 : this_adjust >= 0)
 389                 {
 390                   if (try_apply_stack_adjustment (last_sp_set, reflist,
 391                                                   last_sp_adjust + this_adjust,
 392                                                   this_adjust))
 393                     {
 394                       maybe_move_args_size_note (last_sp_set, insn, false);
 395
 396                       /* It worked!  */
 397                       delete_insn (insn);
 398                       last_sp_adjust += this_adjust;
 399                       continue;
 400                     }
 401                 }
 402
 403               /* Otherwise we have a deallocation.  Do not combine with
 404                  a previous allocation.  Combine into the second insn.  */
 405               else if (STACK_GROWS_DOWNWARD
 406                        ? last_sp_adjust >= 0 : last_sp_adjust <= 0)
 407                 {
 408                   if (try_apply_stack_adjustment (insn, reflist,
 409                                                   last_sp_adjust + this_adjust,
 410                                                   -last_sp_adjust))
 411                     {
 412                       maybe_move_args_size_note (insn, last_sp_set, true);
 413
 414                       /* It worked!  */
 415                       delete_insn (last_sp_set);
 416                       last_sp_set = insn;
 417                       last_sp_adjust += this_adjust;
 418                       free_csa_reflist (reflist);
 419                       reflist = NULL;
 420                       continue;
 421                     }
 422                 }
 423
 424               /* Combination failed.  Restart processing from here.  If
 425                  deallocation+allocation conspired to cancel, we can
 426                  delete the old deallocation insn.  */
 427               if (last_sp_set && last_sp_adjust == 0)
 428                 delete_insn (last_sp_set);
 429               free_csa_reflist (reflist);
 430               reflist = NULL;
 431               last_sp_set = insn;
 432               last_sp_adjust = this_adjust;
 433               continue;
 434             }
 435
 436           /* Find a store with pre-(dec|inc)rement or pre-modify of exactly
 437              the previous adjustment and turn it into a simple store.  This
 438              is equivalent to anticipating the stack adjustment so this must
 439              be an allocation.  */
 440           if (MEM_P (dest)
 441               && ((STACK_GROWS_DOWNWARD
 442                    ? (GET_CODE (XEXP (dest, 0)) == PRE_DEC
 443                       && last_sp_adjust
 444                          == (HOST_WIDE_INT) GET_MODE_SIZE (GET_MODE (dest)))
 445                    : (GET_CODE (XEXP (dest, 0)) == PRE_INC
 446                       && last_sp_adjust
 447                          == -(HOST_WIDE_INT) GET_MODE_SIZE (GET_MODE (dest))))
 448                   || ((STACK_GROWS_DOWNWARD
 449                        ? last_sp_adjust >= 0 : last_sp_adjust <= 0)
 450                       && GET_CODE (XEXP (dest, 0)) == PRE_MODIFY
 451                       && GET_CODE (XEXP (XEXP (dest, 0), 1)) == PLUS
 452                       && XEXP (XEXP (XEXP (dest, 0), 1), 0)
 453                          == stack_pointer_rtx
 454                       && GET_CODE (XEXP (XEXP (XEXP (dest, 0), 1), 1))
 455                          == CONST_INT
 456                       && INTVAL (XEXP (XEXP (XEXP (dest, 0), 1), 1))
 457                          == -last_sp_adjust))
 458               && XEXP (XEXP (dest, 0), 0) == stack_pointer_rtx
 459               && !reg_mentioned_p (stack_pointer_rtx, src)
 460               && memory_address_p (GET_MODE (dest), stack_pointer_rtx)
 461               && try_apply_stack_adjustment (insn, reflist, 0,
 462                                              -last_sp_adjust))
 463             {
 464               delete_insn (last_sp_set);
 465               free_csa_reflist (reflist);
 466               reflist = NULL;
 467               last_sp_set = NULL_RTX;
 468               last_sp_adjust = 0;
 469               continue;
 470             }
 471         }
 472
 473       data.insn = insn;
 474       data.reflist = reflist;
 475       if (!CALL_P (insn) && last_sp_set
 476           && !for_each_rtx (&PATTERN (insn), record_stack_refs, &data))
 477         {
 478            reflist = data.reflist;
 479            continue;
 480         }
 481       reflist = data.reflist;
 482
 483       /* Otherwise, we were not able to process the instruction.
 484          Do not continue collecting data across such a one.  */
 485       if (last_sp_set
 486           && (CALL_P (insn)
 487               || reg_mentioned_p (stack_pointer_rtx, PATTERN (insn))))
 488         {
 489           if (last_sp_set && last_sp_adjust == 0)
 490             delete_insn (last_sp_set);
 491           free_csa_reflist (reflist);
 492           reflist = NULL;
 493           last_sp_set = NULL_RTX;
 494           last_sp_adjust = 0;
 495         }
 496     }
 497
 498   if (last_sp_set && last_sp_adjust == 0)
 499     delete_insn (last_sp_set);
 500
 501   if (reflist)
 502     free_csa_reflist (reflist);
 503 }
 504 \f
 505
 506 static bool
 507 gate_handle_stack_adjustments (void)
 508 {
 509   return flag_combine_stack_adjustments;
 510 }
 511
 512 static unsigned int
 513 rest_of_handle_stack_adjustments (void)
 514 {
 515   cleanup_cfg (flag_crossjumping ? CLEANUP_CROSSJUMP : 0);
 516
 517   /* This is kind of a heuristic.  We need to run combine_stack_adjustments
 518      even for machines with possibly nonzero TARGET_RETURN_POPS_ARGS
 519      and ACCUMULATE_OUTGOING_ARGS.  We expect that only ports having
 520      push instructions will have popping returns.  */
 521 #ifndef PUSH_ROUNDING
 522   if (!ACCUMULATE_OUTGOING_ARGS)
 523 #endif
 524     {
 525       df_note_add_problem ();
 526       df_analyze ();
 527       combine_stack_adjustments ();
 528     }
 529   return 0;
 530 }
 531
 532 struct rtl_opt_pass pass_stack_adjustments =
 533 {
 534  {
 535   RTL_PASS,
 536   "csa",                                /* name */
 537   gate_handle_stack_adjustments,        /* gate */
 538   rest_of_handle_stack_adjustments,     /* execute */
 539   NULL,                                 /* sub */
 540   NULL,                                 /* next */
 541   0,                                    /* static_pass_number */
 542   TV_COMBINE_STACK_ADJUST,              /* tv_id */
 543   0,                                    /* properties_required */
 544   0,                                    /* properties_provided */
 545   0,                                    /* properties_destroyed */
 546   0,                                    /* todo_flags_start */
 547   TODO_df_finish | TODO_verify_rtl_sharing |
 548   TODO_ggc_collect,                     /* todo_flags_finish */
 549  }
 550 };