rtl-optimization/104686 - speed up conflict iteration

The following replaces

       /* Skip bits that are zero.  */
       for (; (word & 1) == 0; word >>= 1)
         bit_num++;

idioms in ira-int.h in the attempt to speedup update_conflict_hard_regno_costs
which we're bound on in PR104686.  The trick is to use ctz_hwi here
which should pay off even with dense bitmaps on architectures that
have HW support for this.

For the PR in question this speeds up compile-time from 31s to 24s for
me.

2022-02-25  Richard Biener  <rguenther@suse.de>

	PR rtl-optimization/104686
	* ira-int.h (minmax_set_iter_cond): Use ctz_hwi to elide loop
	skipping bits that are zero.
	(ira_object_conflict_iter_cond): Likewise.

1 files changed, 6 insertions, 4 deletions

diff --git a/gcc/ira-int.h b/gcc/ira-int.h
index 957604b..f42a314 100644
--- a/gcc/ira-int.h
+++ b/gcc/ira-int.h
@@ -764,8 +764,9 @@ minmax_set_iter_cond (minmax_set_iterator *i, int *n)
     }
 
   /* Skip bits that are zero.  */
-  for (; (i->word & 1) == 0; i->word >>= 1)
-    i->bit_num++;
+  int off = ctz_hwi (i->word);
+  i->bit_num += off;
+  i->word >>= off;
 
   *n = (int) i->bit_num + i->start_val;
 
@@ -1379,8 +1380,9 @@ ira_object_conflict_iter_cond (ira_object_conflict_iterator *i,
 	}
 
       /* Skip bits that are zero.  */
-      for (; (word & 1) == 0; word >>= 1)
-	bit_num++;
+      int off = ctz_hwi (word);
+      bit_num += off;
+      word >>= off;
 
       obj = ira_object_id_map[bit_num + i->base_conflict_id];
       i->bit_num = bit_num + 1;