Re: optimization/7130: miscompiled code for gcc-3.1 in powerpc linux with -funroll-all-loops

[Alan Modra <amodra at bigpond dot net dot au>]

On Thu, Jul 11, 2002 at 05:21:57PM +0930, Alan Modra wrote:
> doloop.c:doloop_modify_runtime says:
> 
>      If the loop has been unrolled, then the loop body has been
>      preconditioned to iterate a multiple of unroll_number times.  If
>      abs_inc is != 1, the full calculation is
> 
>        t1 = abs_inc * unroll_number;
>        n = abs (final - initial) / t1;
>        n += (abs (final - initial) % t1) > t1 - abs_inc;
> 
> This is wrong.  Taking the example in the PR, we have
> 
> abs_inc = 1
> unroll_number = 4
> abs (final - initial) = 10
> 
> => t1 == 4
>    abs (final - initial) % t1 == 2
> => n == 2
> 
> We want n == 3, to go around the loop fully twice, and once partially.
> 
> A little thought shows the correct calculation is
> 
> 	The amount we increment per (partially) unrolled loop
>         t1 = abs_inc * unroll_number;
> 
> 	The number of time we'll go fully round the loop.
>         n = abs (final - initial) / t1;
> 
> 	Plus any partial loops.
>         n += (abs (final - initial) % t1) >= abs_inc;

And a little more thought, prodded by bootstrap failing :), says

	Plus any partial loops.
	  n += (abs (final - initial) % t1) != 0;

Why?  Well, GE loops don't care about final - initial being a multiple
of the loop increment.  Besides, for unroll_number == 1, the above
calculations ought to agree with this extract from unroll.c

     The number of iterations (prior to any loop unrolling) is given by:

       n = (abs (final - initial) + abs_inc - 1) / abs_inc.

     However, it is possible for the summation to overflow, and a
     safer method is:

       n = abs (final - initial) / abs_inc;
       n += (abs (final - initial) % abs_inc) != 0;


gcc/ChangeLog
 	PR optimization/7130
 	* doloop.c (doloop_modify_runtime): Correct count for unrolled loops.

gcc/testsuite/ChangeLog
	* gcc.dg/unroll.c: New.

(And this time, I waited for bootstrap to get through building the stage2
 compiler before firing off emails.)

-- 
Alan Modra
IBM OzLabs - Linux Technology Centre

Index: gcc/doloop.c
===================================================================
RCS file: /cvs/gcc/gcc/gcc/doloop.c,v
retrieving revision 1.20
diff -u -p -r1.20 doloop.c
--- gcc/doloop.c	24 Jun 2002 02:16:42 -0000	1.20
+++ gcc/doloop.c	11 Jul 2002 09:07:06 -0000
@@ -552,6 +552,7 @@ doloop_modify_runtime (loop, iterations_
 {
   const struct loop_info *loop_info = LOOP_INFO (loop);
   HOST_WIDE_INT abs_inc;
+  HOST_WIDE_INT abs_loop_inc;
   int neg_inc;
   rtx diff;
   rtx sequence;
@@ -601,7 +602,7 @@ doloop_modify_runtime (loop, iterations_
 
        t1 = abs_inc * unroll_number;
        n = abs (final - initial) / t1;
-       n += (abs (final - initial) % t1) > t1 - abs_inc;
+       n += (abs (final - initial) % t1) != 0;
 
      The division and modulo operations can be avoided by requiring
      that the increment is a power of 2 (precondition_loop_p enforces
@@ -667,20 +668,21 @@ doloop_modify_runtime (loop, iterations_
 	}
     }
 
-  if (abs_inc * loop_info->unroll_number != 1)
+  abs_loop_inc = abs_inc * loop_info->unroll_number;
+  if (abs_loop_inc != 1)
     {
       int shift_count;
 
-      shift_count = exact_log2 (abs_inc * loop_info->unroll_number);
+      shift_count = exact_log2 (abs_loop_inc);
       if (shift_count < 0)
 	abort ();
 
-      if (abs_inc != 1)
-	diff = expand_simple_binop (GET_MODE (diff), PLUS,
-				    diff, GEN_INT (abs_inc - 1),
-				    diff, 1, OPTAB_LIB_WIDEN);
+      diff = expand_simple_binop (GET_MODE (diff), PLUS,
+				  diff, GEN_INT (abs_loop_inc - 1),
+				  diff, 1, OPTAB_LIB_WIDEN);
 
-      /* (abs (final - initial) + abs_inc - 1) / (abs_inc * unroll_number)  */
+      /* (abs (final - initial) + abs_inc * unroll_number - 1)
+	 / (abs_inc * unroll_number)  */
       diff = expand_simple_binop (GET_MODE (diff), LSHIFTRT,
 				  diff, GEN_INT (shift_count),
 				  diff, 1, OPTAB_LIB_WIDEN);
Index: gcc/testsuite/gcc.dg/unroll.c
===================================================================
RCS file: gcc/testsuite/gcc.dg/unroll.c
diff -N gcc/testsuite/gcc.dg/unroll.c
--- /dev/null	1 Jan 1970 00:00:00 -0000
+++ gcc/testsuite/gcc.dg/unroll.c	11 Jul 2002 09:31:12 -0000
@@ -0,0 +1,39 @@
+/* PR opt/7130 */
+/* { dg-do run } */
+/* { dg-options "-O2 -funroll-all-loops" } */
+
+#define TYPE long
+
+void
+scale (TYPE *alpha, TYPE *x, int n)
+{
+  int i, ix;
+
+  if (*alpha != 1)
+    for (i = 0, ix = 0; i < n; i++, ix += 2)
+      {
+	TYPE tmpr, tmpi;
+	tmpr = *alpha * x[ix];
+	tmpi = *alpha * x[ix + 1];
+	x[ix] = tmpr;
+	x[ix + 1] = tmpi;
+      }
+}
+
+int
+main (void)
+{
+  int i;
+  TYPE x[10];
+  TYPE alpha = 2;
+
+  for (i = 0; i < 10; i++)
+    x[i] = i;
+
+  scale (&alpha, x, 5);
+
+  if (x[9] != 18)
+    abort ();
+
+  return 0;
+}