[gcc r12-7401] rtl-optimization/104686 - speed up conflict iteration

[Richard Biener]

https://gcc.gnu.org/g:a8250bbaeb2e8250c20db477fe67fd085214be7c

commit r12-7401-ga8250bbaeb2e8250c20db477fe67fd085214be7c
Author: Richard Biener <rguenther@suse.de>
Date:   Fri Feb 25 14:19:44 2022 +0100

    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.

Diff:
---
 gcc/ira-int.h | 10 ++++++----
 1 file changed, 6 insertions(+), 4 deletions(-)

diff --git a/gcc/ira-int.h b/gcc/ira-int.h
index 957604b22e9..f42a314fa7f 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;