[PATCH] Don't add unnecessary var self-conflicts (PR tree-optimization/86214)

[Jakub Jelinek <jakub@redhat.com>]

Hi!

While looking at this PR (I've just started) I've noticed that
add_stack_var_conflict is quite often called with x == y.
We don't really need to record that a variable conflicts with itself,
the only reader of the conflicts bitmaps, stack_var_conflict_p,
starts with
  if (x == y)
    return false;
conflicts bitmap are set either by this function, or by the
              EXECUTE_IF_SET_IN_BITMAP (work, 0, i, bi)
                {
                  struct stack_var *a = &stack_vars[i];
                  if (!a->conflicts)
                    a->conflicts = BITMAP_ALLOC (&stack_var_bitmap_obstack);
                  bitmap_ior_into (a->conflicts, work);
                }
code (where work isn't derived from any conflicts bitmap though, so
doesn't care if we've added those self-conflicts or not).  The above
bitmap_ior_into stuff actually always sets self-conflicts (if you think
bitmap_clear_bit is worth it, I can add it afterwards though).

But I think the following patch is helpful, don't create the conflicts
bitmaps at all if all we'd record is just self-conflict which we'll ignore.

Bootstrapped/regtested on x86_64-linux and i686-linux, ok for trunk?

2019-01-16  Jakub Jelinek  <jakub@redhat.com>

	PR tree-optimization/86214
	* cfgexpand.c (add_stack_var_conflict): Don't add any conflicts
	if x == y.

--- gcc/cfgexpand.c.jj	2019-01-16 09:35:09.131247513 +0100
+++ gcc/cfgexpand.c	2019-01-16 20:14:11.445467399 +0100
@@ -470,6 +470,8 @@ add_stack_var_conflict (size_t x, size_t
 {
   struct stack_var *a = &stack_vars[x];
   struct stack_var *b = &stack_vars[y];
+  if (x == y)
+    return;
   if (!a->conflicts)
     a->conflicts = BITMAP_ALLOC (&stack_var_bitmap_obstack);
   if (!b->conflicts)

	Jakub