Re: optimization/8492: [3.3 regression] GCC spins forever compiling loop

[Eric Botcazou <ebotcazou@libertysurf.fr>]

The following reply was made to PR optimization/8492; it has been noted by GNATS.

From: Eric Botcazou <ebotcazou@libertysurf.fr>
To: Jan Hubicka <jh@suse.cz>
Cc: janis187@us.ibm.com,
 gcc-gnats@gcc.gnu.org,
 gcc-prs@gcc.gnu.org,
 gcc-bugs@gcc.gnu.org,
 nobody@gcc.gnu.org,
 thorpej@shagadelic.org,
 jh@suse.cz
Subject: Re: optimization/8492: [3.3 regression] GCC spins forever compiling loop
Date: Sat, 21 Dec 2002 13:37:21 +0100

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 > I think deleting unreachable blocks is cheap enought to do in the case
 > conditional jump was eliminated.
 
 Ok. But then the GCSE code needs to be (at least partially) re-initialize=
 d=20
 because the number of basic blocks may change. I've attached a naive patc=
 h I=20
 wrote some time ago: while fixing the PR (and doing some housekeeping wor=
 k),=20
 it introduces many regressions because of this problem.
 
 > Alternatively we may prevent first local cprop pass from modifying CFG.
 
 I don't know enough of the global organization of optimization passes to=20
 comment. Will the optimizations missed at that point be caught elsewhere =
 ?
 
 --=20
 Eric Botcazou
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 --- gcse.c.orig=09Sun Dec  1 09:28:09 2002
 +++ gcse.c=09Sun Dec  1 11:00:05 2002
 @@ -606,13 +606,13 @@
  static void find_used_regs=09PARAMS ((rtx *, void *));
  static int try_replace_reg=09PARAMS ((rtx, rtx, rtx));
  static struct expr *find_avail_set PARAMS ((int, rtx));
 -static int cprop_jump=09=09PARAMS ((basic_block, rtx, rtx, rtx, rtx));
  static void mems_conflict_for_gcse_p PARAMS ((rtx, rtx, void *));
  static int load_killed_in_block_p    PARAMS ((basic_block, int, rtx, int=
 ));
  static void canon_list_insert        PARAMS ((rtx, rtx, void *));
  static int cprop_insn=09=09PARAMS ((rtx, int));
  static int cprop=09=09PARAMS ((int));
  static int one_cprop_pass=09PARAMS ((int, int));
 +static bool constprop_jump=09PARAMS ((basic_block, rtx, rtx, rtx, rtx));
  static bool constprop_register=09PARAMS ((rtx, rtx, rtx, int));
  static struct expr *find_bypass_set PARAMS ((int, int));
  static int bypass_block=09=09    PARAMS ((basic_block, rtx, rtx));
 @@ -701,7 +701,7 @@
  static rtx gcse_emit_move_after=09=09PARAMS ((rtx, rtx, rtx));
  static bool do_local_cprop=09=09PARAMS ((rtx, rtx, int, rtx*));
  static bool adjust_libcall_notes=09PARAMS ((rtx, rtx, rtx, rtx*));
 -static void local_cprop_pass=09=09PARAMS ((int));
 +static bool local_cprop_pass=09=09PARAMS ((int));
  =0C
  /* Entry point for global common subexpression elimination.
     F is the first instruction in the function.  */
 @@ -3674,6 +3674,11 @@
  static sbitmap *cprop_avin;
  static sbitmap *cprop_avout;
 =20
 +/* Note whether or not the CFG was modified between two points.  This ma=
 y
 +   happen during the local cprop pass if we changed any jumps.  */
 +
 +static bool cfg_was_modified;
 +
  /* Allocate vars used for copy/const propagation.  N_BLOCKS is the numbe=
 r of
     basic blocks.  N_SETS is the number of sets.  */
 =20
 @@ -3964,7 +3969,7 @@
    return success;
  }
 =20
 -/* Find a set of REGNOs that are available on entry to INSN's block.  Re=
 turns
 +/* Find a SET of REGNO that is available on entry to INSN's block.  Retu=
 rns
     NULL no such set is found.  */
 =20
  static struct expr *
 @@ -4034,15 +4039,15 @@
    return set1;
  }
 =20
 -/* Subroutine of cprop_insn that tries to propagate constants into
 +/* Subroutine of constprop_register that tries to propagate constants in=
 to
     JUMP_INSNS.  JUMP must be a conditional jump.  If SETCC is non-NULL
     it is the instruction that immediately preceeds JUMP, and must be a
     single SET of a register.  FROM is what we will try to replace,
 -   SRC is the constant we will try to substitute for it.  Returns nonzer=
 o
 +   SRC is the constant we will try to substitute for it.  Returns TRUE
     if a change was made.  */
 =20
 -static int
 -cprop_jump (bb, setcc, jump, from, src)
 +static bool
 +constprop_jump (bb, setcc, jump, from, src)
       basic_block bb;
       rtx setcc;
       rtx jump;
 @@ -4071,7 +4076,7 @@
    /* If no simplification can be made, then try the next
       register.  */
    if (rtx_equal_p (new, new_set) || rtx_equal_p (new, SET_SRC (set)))
 -    return 0;
 +    return false;
 =20
    /* If this is now a no-op delete it, otherwise this must be a valid in=
 sn.  */
    if (new =3D=3D pc_rtx)
 @@ -4082,9 +4087,9 @@
           to one computed by setcc.  */
        if (setcc=20
  =09  && modified_in_p (new, setcc))
 -=09return 0;
 +=09return false;
        if (! validate_change (jump, &SET_SRC (set), new, 0))
 -=09return 0;
 +=09return false;
 =20
        /* If this has turned into an unconditional jump,
  =09 then put a barrier after it so that the unreachable
 @@ -4110,11 +4115,17 @@
        print_rtl (gcse_file, src);
        fprintf (gcse_file, "\n");
      }
 -  purge_dead_edges (bb);
 =20
 -  return 1;
 +  cfg_was_modified |=3D purge_dead_edges (bb);
 +
 +  return true;
  }
 =20
 +/* Perform constant propagation on INSN.  FROM is what we will try to
 +   replace, TO is the constant we will try to substitute for it.  If
 +   ALTER_JUMPS is non-zero, propagate constants into JUMP_INSNS too.
 +   Returns TRUE if a change was made.  */
 +
  static bool
  constprop_register (insn, from, to, alter_jumps)
       rtx insn;
 @@ -4132,14 +4143,14 @@
      {
        rtx dest =3D SET_DEST (sset);
        if ((REG_P (dest) || CC0_P (dest))
 -=09  && cprop_jump (BLOCK_FOR_INSN (insn), insn, NEXT_INSN (insn), from,=
  to))
 -=09return 1;
 +=09  && constprop_jump (BLOCK_FOR_INSN (insn), insn, NEXT_INSN (insn), f=
 rom, to))
 +=09return true;
      }
 =20
    /* Handle normal insns next.  */
    if (GET_CODE (insn) =3D=3D INSN
        && try_replace_reg (from, to, insn))
 -    return 1;
 +    return true;
 =20
    /* Try to propagate a CONST_INT into a conditional jump.
       We're pretty specific about what we will handle in this
 @@ -4148,8 +4159,9 @@
       Right now the insn in question must look like
       (set (pc) (if_then_else ...))  */
    else if (alter_jumps && any_condjump_p (insn) && onlyjump_p (insn))
 -    return cprop_jump (BLOCK_FOR_INSN (insn), NULL, insn, from, to);
 -  return 0;
 +    return constprop_jump (BLOCK_FOR_INSN (insn), NULL, insn, from, to);
 +
 +  return false;
  }
 =20
  /* Perform constant and copy propagation on INSN.
 @@ -4332,6 +4344,7 @@
     their REG_EQUAL notes need updating to reflect that OLDREG has been
     replaced with NEWVAL in INSN.  Return true if all substitutions could
     be made.  */
 +
  static bool
  adjust_libcall_notes (oldreg, newval, insn, libcall_sp)
       rtx oldreg, newval, insn, *libcall_sp;
 @@ -4369,7 +4382,11 @@
 =20
  #define MAX_NESTED_LIBCALLS 9
 =20
 -static void
 +/* Perform one local copy/constant propagation pass.  ALTER_JUMPS is
 +   non-zero if we are allowed to modify JUMP_INSNS.  Return TRUE if
 +   the CFG was modified.  */
 +
 +static bool
  local_cprop_pass (alter_jumps)
       int alter_jumps;
  {
 @@ -4377,6 +4394,8 @@
    struct reg_use *reg_used;
    rtx libcall_stack[MAX_NESTED_LIBCALLS + 1], *libcall_sp;
 =20
 +  cfg_was_modified =3D false;
 +
    cselib_init ();
    libcall_sp =3D &libcall_stack[MAX_NESTED_LIBCALLS];
    *libcall_sp =3D 0;
 @@ -4414,6 +4433,8 @@
        cselib_process_insn (insn);
      }
    cselib_finish ();
 +=20
 +  return cfg_was_modified;
  }
 =20
  /* Forward propagate copies.  This includes copies and constants.  Retur=
 n
 @@ -4463,22 +4484,25 @@
    return changed;
  }
 =20
 -/* Perform one copy/constant propagation pass.
 -   F is the first insn in the function.
 -   PASS is the pass count.  */
 +/* Perform one copy/constant propagation pass.  PASS is the
 +   pass count, ALTER_JUMPS is non-zero if we are allowed to
 +   modify JUMP_INSNS.  Return nonzero if a change was made.  */
 =20
  static int
  one_cprop_pass (pass, alter_jumps)
       int pass;
       int alter_jumps;
  {
 +  bool do_cleanup_cfg;
    int changed =3D 0;
 =20
    const_prop_count =3D 0;
    copy_prop_count =3D 0;
 =20
 -  local_cprop_pass (alter_jumps);
 -
 +  do_cleanup_cfg =3D local_cprop_pass (alter_jumps);
 +  if (do_cleanup_cfg)
 +    cleanup_cfg (0);
 + =20
    alloc_hash_table (max_cuid, &set_hash_table, 1);
    compute_hash_table (&set_hash_table);
    if (gcse_file)
 
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