Re: Loop optimization bug with Ada front end on PPC (and probably Alpha)

[Richard Henderson <rth@redhat.com>]

On Mon, Nov 26, 2001 at 10:59:25AM -0600, Corey Minyard wrote:
> Not all loops have loop->top set in this situation, because that's only 
> set in bottom entry loops, it seems.

Oh, duh.  Look at scan_loop: loop->top is set in exactly the condition
that we're interested in and no other.  That simplifies the code in
loop_iterations greatly.

ia64 bootstrap underway.


r~



Index: unroll.c
===================================================================
RCS file: /cvs/gcc/gcc/gcc/unroll.c,v
retrieving revision 1.147
diff -c -p -d -r1.147 unroll.c
*** unroll.c	2001/11/21 00:50:56	1.147
--- unroll.c	2001/11/26 21:46:30
*************** loop_iterations (loop)
*** 3480,3485 ****
--- 3480,3486 ----
    int unsigned_p, compare_dir, final_larger;
    rtx last_loop_insn;
    rtx reg_term;
+   rtx insn;
    struct iv_class *bl;
  
    loop_info->n_iterations = 0;
*************** loop_iterations (loop)
*** 3705,3710 ****
--- 3706,3747 ----
  
    if (initial_value == 0)
      return 0;
+ 
+   /* Some code transformations can result in code akin to
+ 
+ 	LOOP_BEG
+ 	  goto start;
+ 	top:
+ 	  i++;
+ 	start:
+ 	  ...
+ 	LOOP_CONT
+ 	  if (i < n) goto top;
+ 	LOOP_END
+ 
+      We'll have already detected this form of loop in scan_loop,
+      and set loop->top and loop->scan_start appropriately.
+ 
+      In this situation, we skip the increment the first time through
+      the loop, which results in an incorrect estimate of the number
+      of iterations.  As we did for GIVs above, adjust the initial value
+      to compensate.  */
+ 
+   if (loop->top
+       && reg_set_between_p (bl->biv->src_reg, loop->top, loop->scan_start))
+     {
+       if (loop_dump_stream)
+ 	fprintf (loop_dump_stream,
+ 		 "Loop iterations: Basic induction var skips initial incr.\n");
+       if (GET_CODE (increment) != CONST_INT)
+ 	{
+ 	  if (loop_dump_stream)
+ 	    fprintf (loop_dump_stream,
+ 		     "Loop iterations: Can't adjust with non-constant incr.\n");
+ 	  return 0;
+ 	}
+       initial_value = plus_constant (initial_value, -INTVAL (increment));
+     }
  
    unsigned_p = 0;
    off_by_one = 0;