[PATCH 03/12] mode-switching: Avoid quadractic list operation

[Richard Sandiford]

add_seginfo chained insn information to the end of a list
by starting at the head of the list.  This patch avoids the
quadraticness by keeping track of the tail pointer.

gcc/
        * mode-switching.cc (add_seginfo): Replace head pointer with
        a pointer to the tail pointer.
        (optimize_mode_switching): Update calls accordingly.
---
 gcc/mode-switching.cc | 24 ++++++++----------------
 1 file changed, 8 insertions(+), 16 deletions(-)

diff --git a/gcc/mode-switching.cc b/gcc/mode-switching.cc
index 8577069bde1..bebe89d5fd2 100644
--- a/gcc/mode-switching.cc
+++ b/gcc/mode-switching.cc
@@ -162,23 +162,14 @@ new_seginfo (int mode, rtx_insn *insn, const HARD_REG_SET 
&regs_live)
 }
 
 /* Add a seginfo element to the end of a list.
-   HEAD is a pointer to the list beginning.
+   TAIL is a pointer to the list's null terminator.
    INFO is the structure to be linked in.  */
 
 static void
-add_seginfo (struct bb_info *head, struct seginfo *info)
+add_seginfo (struct seginfo ***tail_ptr, struct seginfo *info)
 {
-  struct seginfo *ptr;
-
-  if (head->seginfo == NULL)
-    head->seginfo = info;
-  else
-    {
-      ptr = head->seginfo;
-      while (ptr->next != NULL)
-       ptr = ptr->next;
-      ptr->next = info;
-    }
+  **tail_ptr = info;
+  *tail_ptr = &info->next;
 }
 
 /* Record in LIVE that register REG died.  */
@@ -574,6 +565,7 @@ optimize_mode_switching (void)
         Also compute the initial transparency settings.  */
       FOR_EACH_BB_FN (bb, cfun)
        {
+         struct seginfo **tail_ptr = &info[bb->index].seginfo;
          struct seginfo *ptr;
          int last_mode = no_mode;
          bool any_set_required = false;
@@ -599,7 +591,7 @@ optimize_mode_switching (void)
                if (ins_pos != BB_END (bb))
                  ins_pos = NEXT_INSN (ins_pos);
                ptr = new_seginfo (no_mode, ins_pos, live_now);
-               add_seginfo (info + bb->index, ptr);
+               add_seginfo (&tail_ptr, ptr);
                for (i = 0; i < no_mode; i++)
                  clear_mode_bit (transp[bb->index], j, i);
              }
@@ -617,7 +609,7 @@ optimize_mode_switching (void)
                      any_set_required = true;
                      last_mode = mode;
                      ptr = new_seginfo (mode, insn, live_now);
-                     add_seginfo (info + bb->index, ptr);
+                     add_seginfo (&tail_ptr, ptr);
                      for (i = 0; i < no_mode; i++)
                        clear_mode_bit (transp[bb->index], j, i);
                    }
@@ -646,7 +638,7 @@ optimize_mode_switching (void)
          if (!any_set_required)
            {
              ptr = new_seginfo (no_mode, BB_END (bb), live_now);
-             add_seginfo (info + bb->index, ptr);
+             add_seginfo (&tail_ptr, ptr);
              if (last_mode != no_mode)
                for (i = 0; i < no_mode; i++)
                  clear_mode_bit (transp[bb->index], j, i);
-- 
2.25.1