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

[Corey Minyard <minyard@acm.org>]

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Richard Henderson wrote:

>On Tue, Nov 27, 2001 at 03:26:43PM -0600, Corey Minyard wrote:
>
>>Are you moving back to the original loop test?  I think the original 
>>test you had didn't work because it wasn't the doloop case, but if you 
>>put it in the doloop code it should work properly.
>>
>
>I'm taking out the CONT test, yes.  But I'm keeping the LUID
>test against the BIV initializer rather than scanning with
>reg_set_between_p.
>
You will need to set the bl variable a little differently, in case its a 
GIV.  I've attached a patch.  That was the cause of my segv earlier. 
 Bootstrapping again....

-Corey



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Index: doloop.c
===================================================================
RCS file: /cvs/gcc/gcc/gcc/doloop.c,v
retrieving revision 1.12
diff -u -p -r1.12 doloop.c
--- doloop.c	2001/11/16 02:26:38	1.12
+++ doloop.c	2001/11/27 23:01:46
@@ -596,6 +596,59 @@ doloop_modify_runtime (loop, iterations_
 			      copy_rtx (neg_inc ? final_value : initial_value),
 			      NULL_RTX, unsigned_p, OPTAB_LIB_WIDEN);
 
+  /* Some code transformations can result in code akin to
+
+	  tmp = i + 1;
+	  ...
+	  goto scan_start;
+	top:
+	  tmp = tmp + 1;
+	scan_start:
+	  i = tmp;
+	  if (i < n) goto top;
+
+     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.  Adjust the difference to compensate.  */
+  /* ??? Logically, it would seem this belongs in loop_iterations.
+     However, this causes regressions e.g. on x86 execute/20011008-3.c,
+     so I do not believe we've properly characterized the exact nature
+     of the problem.  In the meantime, this fixes execute/20011126-2.c
+     on ia64 and some Ada front end miscompilation on ppc.  */
+
+  if (loop->scan_start)
+    {
+      rtx iteration_var;
+      struct loop_ivs *ivs = LOOP_IVS (loop);
+      struct iv_class *bl;
+
+      iteration_var = loop_info->iteration_var;
+
+      if (REG_IV_TYPE (ivs, REGNO (iteration_var)) == BASIC_INDUCT)
+	bl = REG_IV_CLASS (ivs, REGNO (iteration_var));
+      else if (REG_IV_TYPE (ivs, REGNO (iteration_var)) == GENERAL_INDUCT)
+	{
+	  struct induction *v = REG_IV_INFO (ivs, REGNO (iteration_var));
+	  bl = REG_IV_CLASS (ivs, REGNO (v->src_reg));
+	}
+      else
+	abort(); /* iteration var must be an induction variable to get
+		    to here. */
+
+      if (INSN_LUID (bl->biv->insn) < INSN_LUID (loop->scan_start))
+	{
+	  if (loop_dump_stream)
+	    fprintf (loop_dump_stream,
+	         "Doloop: Basic induction var skips initial incr.\n");
+
+	  diff = expand_simple_binop (mode, PLUS, diff, increment, diff,
+				      unsigned_p, OPTAB_LIB_WIDEN);
+	}
+    }
+
   if (abs_inc * loop_info->unroll_number != 1)
     {
       int shift_count;