gcc/init-regs.c

   1 /* Initialization of uninitialized regs.
   2    Copyright (C) 2007-2021 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 it under
   7 the terms of the GNU General Public License as published by the Free
   8 Software Foundation; either version 3, or (at your option) any later
   9 version.
  10
  11 GCC is distributed in the hope that it will be useful, but WITHOUT ANY
  12 WARRANTY; without even the implied warranty of MERCHANTABILITY or
  13 FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
  14 for more details.
  15
  16 You should have received a copy of the GNU General Public License
  17 along with GCC; see the file COPYING3.  If not see
  18 <http://www.gnu.org/licenses/>.  */
  19
  20 #include "config.h"
  21 #include "system.h"
  22 #include "coretypes.h"
  23 #include "backend.h"
  24 #include "rtl.h"
  25 #include "tree.h"
  26 #include "df.h"
  27 #include "memmodel.h"
  28 #include "emit-rtl.h"
  29 #include "expr.h"
  30 #include "tree-pass.h"
  31
  32 /* Check all of the uses of pseudo variables.  If any use that is MUST
  33    uninitialized, add a store of 0 immediately before it.  For
  34    subregs, this makes combine happy.  For full word regs, this makes
  35    other optimizations, like the register allocator and the reg-stack
  36    happy as well as papers over some problems on the arm and other
  37    processors where certain isa constraints cannot be handled by gcc.
  38    These are of the form where two operands to an insn my not be the
  39    same.  The ra will only make them the same if they do not
  40    interfere, and this can only happen if one is not initialized.
  41
  42    There is also the unfortunate consequence that this may mask some
  43    buggy programs where people forget to initialize stack variable.
  44    Any programmer with half a brain would look at the uninitialized
  45    variable warnings.  */
  46
  47 static void
  48 initialize_uninitialized_regs (void)
  49 {
  50   basic_block bb;
  51   auto_bitmap already_genned;
  52
  53   if (optimize == 1)
  54     {
  55       df_live_add_problem ();
  56       df_live_set_all_dirty ();
  57     }
  58
  59   df_analyze ();
  60
  61   FOR_EACH_BB_FN (bb, cfun)
  62     {
  63       rtx_insn *insn;
  64       bitmap lr = DF_LR_IN (bb);
  65       bitmap ur = DF_LIVE_IN (bb);
  66       bitmap_clear (already_genned);
  67
  68       FOR_BB_INSNS (bb, insn)
  69         {
  70           df_ref use;
  71           if (!NONDEBUG_INSN_P (insn))
  72             continue;
  73
  74           FOR_EACH_INSN_USE (use, insn)
  75             {
  76               unsigned int regno = DF_REF_REGNO (use);
  77
  78               /* Only do this for the pseudos.  */
  79               if (regno < FIRST_PSEUDO_REGISTER)
  80                 continue;
  81
  82               /* Ignore pseudo PIC register.  */
  83               if (pic_offset_table_rtx
  84                   && regno == REGNO (pic_offset_table_rtx))
  85                 continue;
  86
  87               /* Do not generate multiple moves for the same regno.
  88                  This is common for sequences of subreg operations.
  89                  They would be deleted during combine but there is no
  90                  reason to churn the system.  */
  91               if (bitmap_bit_p (already_genned, regno))
  92                 continue;
  93
  94               /* A use is MUST uninitialized if it reaches the top of
  95                  the block from the inside of the block (the lr test)
  96                  and no def for it reaches the top of the block from
  97                  outside of the block (the ur test).  */
  98               if (bitmap_bit_p (lr, regno)
  99                   && (!bitmap_bit_p (ur, regno)))
 100                 {
 101                   rtx_insn *move_insn;
 102                   rtx reg = DF_REF_REAL_REG (use);
 103
 104                   bitmap_set_bit (already_genned, regno);
 105
 106                   start_sequence ();
 107                   emit_clobber (reg);
 108                   /* PR98872: Only emit an initialization if MODE has a
 109                      CONST0_RTX defined.  */
 110                   if (CONST0_RTX (GET_MODE (reg)))
 111                     emit_move_insn (reg, CONST0_RTX (GET_MODE (reg)));
 112                   move_insn = get_insns ();
 113                   end_sequence ();
 114                   emit_insn_before (move_insn, insn);
 115                   if (dump_file)
 116                     fprintf (dump_file,
 117                              "adding initialization in %s of reg %d at in block %d for insn %d.\n",
 118                              current_function_name (), regno, bb->index,
 119                              INSN_UID (insn));
 120                 }
 121             }
 122         }
 123     }
 124
 125   if (optimize == 1)
 126     {
 127       if (dump_file)
 128         df_dump (dump_file);
 129       df_remove_problem (df_live);
 130     }
 131 }
 132
 133 namespace {
 134
 135 const pass_data pass_data_initialize_regs =
 136 {
 137   RTL_PASS, /* type */
 138   "init-regs", /* name */
 139   OPTGROUP_NONE, /* optinfo_flags */
 140   TV_NONE, /* tv_id */
 141   0, /* properties_required */
 142   0, /* properties_provided */
 143   0, /* properties_destroyed */
 144   0, /* todo_flags_start */
 145   TODO_df_finish, /* todo_flags_finish */
 146 };
 147
 148 class pass_initialize_regs : public rtl_opt_pass
 149 {
 150 public:
 151   pass_initialize_regs (gcc::context *ctxt)
 152     : rtl_opt_pass (pass_data_initialize_regs, ctxt)
 153   {}
 154
 155   /* opt_pass methods: */
 156   virtual bool gate (function *) { return optimize > 0; }
 157   virtual unsigned int execute (function *)
 158     {
 159       initialize_uninitialized_regs ();
 160       return 0;
 161     }
 162
 163 }; // class pass_initialize_regs
 164
 165 } // anon namespace
 166
 167 rtl_opt_pass *
 168 make_pass_initialize_regs (gcc::context *ctxt)
 169 {
 170   return new pass_initialize_regs (ctxt);
 171 }