Re: optimization/7120: Run once loop should *always* be unrolled

[Alan Modra <amodra@bigpond.net.au>]

The following reply was made to PR optimization/7120; it has been noted by GNATS.

From: Alan Modra <amodra@bigpond.net.au>
To: davem@gcc.gnu.org, gcc-bugs@gcc.gnu.org, nobody@gcc.gnu.org,
  rth@twiddle.net, rusty@rustcorp.com.au, gcc-gnats@gcc.gnu.org,
  gcc-patches@gcc.gnu.org
Cc:  
Subject: Re: optimization/7120: Run once loop should *always* be unrolled
Date: Wed, 26 Jun 2002 18:02:10 +0930

 On Wed, Jun 26, 2002 at 01:17:00PM +0930, Alan Modra wrote:
 > Bah, that's too simplistic.  Patch withdrawn.
 
 This one is better.  Handles horrible loops like:
 
   extern int *arr;
   int i;
   i = 0;
   do
     {
       arr[i] = 0;
       i++;
     }
   while (i == 1);
 
 I'm also setting loop_info->final_value and
 loop_info->final_equiv_value as they were reset to zero earlier.
 
 bootstrapped and regression tested i686-linux.
 
 	* unroll.c (loop_iterations): Handle EQ loops.
 
 Index: gcc/unroll.c
 ===================================================================
 RCS file: /cvs/gcc/gcc/gcc/unroll.c,v
 retrieving revision 1.168
 diff -u -p -r1.168 unroll.c
 --- gcc/unroll.c	15 Jun 2002 01:10:49 -0000	1.168
 +++ gcc/unroll.c	26 Jun 2002 04:58:40 -0000
 @@ -3941,12 +3941,6 @@ loop_iterations (loop)
  	}
        return 0;
      }
 -  else if (comparison_code == EQ)
 -    {
 -      if (loop_dump_stream)
 -	fprintf (loop_dump_stream, "Loop iterations: EQ comparison loop.\n");
 -      return 0;
 -    }
    else if (GET_CODE (final_value) != CONST_INT)
      {
        if (loop_dump_stream)
 @@ -3957,6 +3951,34 @@ loop_iterations (loop)
  	  fprintf (loop_dump_stream, ".\n");
  	}
        return 0;
 +    }
 +  else if (comparison_code == EQ)
 +    {
 +      if (loop_dump_stream)
 +	fprintf (loop_dump_stream, "Loop iterations: EQ comparison loop.\n");
 +
 +      if (((INTVAL (initial_value) + INTVAL (increment))
 +	   & (((unsigned HOST_WIDE_INT) 1
 +	       << (GET_MODE_BITSIZE (GET_MODE (iteration_var)) - 1)
 +	       << 1) - 1)) == INTVAL (final_value))
 +	{
 +	  /* The iterator value once through the loop is equal to the
 +	     comparision value.  Either we have an infinite loop, or
 +	     we'll loop twice.  */
 +	  if (INTVAL (increment) == 0)
 +	    return 0;
 +	  loop_info->n_iterations = 2;
 +	}
 +      else
 +	loop_info->n_iterations = 1;
 +
 +      loop_info->final_value
 +	= plus_constant (loop_info->initial_value,
 +			 loop_info->n_iterations * INTVAL (increment));
 +      loop_info->final_equiv_value
 +	= GEN_INT (INTVAL (initial_value)
 +		   + loop_info->n_iterations * INTVAL (increment));
 +      return loop_info->n_iterations;
      }
  
    /* Final_larger is 1 if final larger, 0 if they are equal, otherwise -1.  */
 
 
 -- 
 Alan Modra
 IBM OzLabs - Linux Technology Centre