[PATCH] Avoid optimizing memory references with side effects in compare-elim.c

[Paul Koning]

This fixes an ICE if post-reload compare elimination is done and the target 
supports post_inc and similar modes, as pdp11 does.  The ICE is caused by a 
generated PARALLEL that has the side effect twice, which is not legal.

(Ideally it would generate a similar RTL suitable for a matching constraint 
with the side effect omitted; I may try for that later on if that is still 
supported by the constraint machinery.)

Tested against my in-progress CCmode pdp11 target.  Ok to commit?

        paul

gcc/ChangeLog:

2018-06-05  Paul Koning  <n...@arrl.net>

        * compare-elim.c (addr_side_effect_check): New function.
        (addr_side_effect_p): Ditto.
        (try_merge_compare): Don't merge compare if address contains a
        side effect.

Index: compare-elim.c
===================================================================
--- compare-elim.c      (revision 261207)
+++ compare-elim.c      (working copy)
@@ -127,6 +127,23 @@
   
 static vec<comparison *> all_compares;
 
+/* Callback function used by addr_side_effect_p.  */
+static int
+addr_side_effect_check (rtx mem ATTRIBUTE_UNUSED, rtx op ATTRIBUTE_UNUSED,
+                       rtx dest ATTRIBUTE_UNUSED, rtx src ATTRIBUTE_UNUSED,
+                       rtx srcoff ATTRIBUTE_UNUSED, void *arg ATTRIBUTE_UNUSED)
+{
+  return 1;
+}
+
+/* Check if addr has side effects (contains autoinc or autodec
+   operations).  */
+static int
+addr_side_effect_p (rtx addr)
+{
+  return for_each_inc_dec (addr, addr_side_effect_check, NULL);
+} 
+  
 /* Look for a "conforming" comparison, as defined above.  If valid, return
    the rtx for the COMPARE itself.  */
 
@@ -690,6 +707,13 @@
     return false;
 
   rtx src = SET_SRC (set);
+
+  /* If the source uses addressing modes with side effects, we can't
+     do the merge because we'd end up with a PARALLEL that has two
+     instances of that side effect in it.  */
+  if (addr_side_effect_p (src))
+    return false;
+
   rtx flags = maybe_select_cc_mode (cmp, src, CONST0_RTX (GET_MODE (src)));
   if (!flags)
     {
@@ -809,6 +833,12 @@
   else
     return false;
 
+  /* If the source uses addressing modes with side effects, we can't
+     do the merge because we'd end up with a PARALLEL that has two
+     instances of that side effect in it.  */
+  if (addr_side_effect_p (cmp_src))
+    return false;
+
   /* Determine if we ought to use a different CC_MODE here.  */
   flags = maybe_select_cc_mode (cmp, cmp_src, in_b);
   if (flags == NULL)