[PATCH] fix prefetch optimization in loop.c

[Janis Johnson <janis187 at us dot ibm dot com>]

This patch allows the mainline to bootstrap on i686-pc-linux-gnu with
BOOT_CFLAGS="-O2 -fprefetch-loop-arrays -mcpu=pentium3".  Without this
patch, reload.c and reload1.c were being miscompiled when loops were
considered and then rejected for prefetch optimization.  This happened
because of an assumption that rtx_copy was returning a copy of the
address rtx when it was actually returning a pointer to the original
rtx.

Bootstrapped and tested on i686-pc-linux-gnu and ia64-unknown-linux,
along with the following patches:
  http://gcc.gnu.org/ml/gcc-patches/2002-01/msg01071.html
  http://gcc.gnu.org/ml/gcc-patches/2002-01/msg01393.html
  http://gcc.gnu.org/ml/gcc-patches/2002-01/msg01395.html

OK?

2002-01-21  Janis Johnson  <janis187@us.ibm.com>

        * loop.c (emit_prefetch_instructions): Don't change addresses if
        they aren't new copies.

testsuite:

        * gcc.dg/20020121-2.c: New test.

Index: loop.c
===================================================================
RCS file: /cvs/gcc/gcc/gcc/loop.c,v
retrieving revision 1.378
diff -u -p -r1.378 loop.c
--- loop.c	2002/01/03 16:05:54	1.378
+++ loop.c	2002/01/20 20:59:46
@@ -54,6 +54,7 @@ Software Foundation, 59 Temple Place - S
 #include "toplev.h"
 #include "predict.h"
 #include "insn-flags.h"
+#include "optabs.h"
 
 /* Not really meaningful values, but at least something.  */
 #ifndef SIMULTANEOUS_PREFETCHES
@@ -3856,10 +3857,18 @@ emit_prefetch_instructions (loop)
 	  address = copy_rtx (iv->add_val);
 	  temp = copy_rtx (bl->initial_value);
 
-	  address = simplify_gen_binary (PLUS, Pmode, temp, address);
-	  index = remove_constant_addition (&address);
+	  /* COPY_RTX sometimes returns a pointer to the original rather
+	     than making a copy.  We can't change the existing rtx in case
+	     we end up not doing a prefetch.
+	     ??? If we can always get a real copy, do that instead of
+	     skipping the following code.  */
+	  if (temp != bl->initial_value)
+	    {
+	      address = simplify_gen_binary (PLUS, Pmode, temp, address);
+	      index = remove_constant_addition (&address);
+	      index += size;
+	    }
 
-	  index += size;
 	  d.mem_write = 0;
 	  d.mem_address = *iv->location;
 
--- /dev/null	Tue May 23 09:27:54 2000
+++ testsuite/gcc.dg/20020121-2.c	Sat Jan 19 20:12:03 2002
@@ -0,0 +1,43 @@
+/* This code is from the beginning of combine_reloads in reload.c in
+   GCC 3.1-20020117, with simplifications.  It compiled incorrectly
+   for -O2 -fprefetch-loop-arrays for ix86 targets.  */
+
+/* { dg-options "-O2 -fprefetch-loop-arrays -mcpu=pentium3" } */
+/* { dg-do run { target-i?86-*-* } } */
+
+struct reload
+{
+  int first_member;
+  int out;
+  int final_member;
+};
+
+int n_reloads;
+struct reload rld[10];
+
+static int
+combine_reloads ()
+{
+  int i;
+  int output_reload = -1;
+  int secondary_out = -1;
+
+  for (i = 0; i < n_reloads; i++)
+    if (rld[i].out != 0)
+      {
+	if (output_reload >= 0)
+	  return output_reload;
+	output_reload = i;
+      }
+  return output_reload;
+}
+
+int
+main ()
+{
+  n_reloads = 4;
+  rld[2].out = 2;
+  if (combine_reloads () != 2)
+    abort ();
+  exit (0);
+}