[systemd-devel] [PATCH 04/17] journal: loop less in MATCH_AND_TERM conditionals

[Zbigniew Jędrzejewski-Szmek]

AND term usually don't have many subterms (4 seems to be the maximum
sensible number, e.g. _BOOT_ID && _SYSTEMD_UNIT && _PID && MESSAGE_ID).
Nevertheless, the cost of checking each subterm can be relatively
high, especially when the nested terms are compound, and it
makes sense to minimize the number of checks.

Instead of looping to the end and then again over the whole list once
again after at least one term changed the offset, start the loop at
the term which caused the change. This way ½ terms in the AND match
are not checked unnecessarily again.
---
 src/journal/sd-journal.c | 41 +++++++++++++++++------------------------
 src/shared/list.h        |  7 +++++++
 2 files changed, 24 insertions(+), 24 deletions(-)

diff --git a/src/journal/sd-journal.c b/src/journal/sd-journal.c
index e5b6968..0f7ea63 100644
--- a/src/journal/sd-journal.c
+++ b/src/journal/sd-journal.c
@@ -593,42 +593,35 @@ static int next_for_match(
                 }
 
         } else if (m->type == MATCH_AND_TERM) {
-                Match *i;
-                bool continue_looking;
+                Match *i, *last_moved;
 
                 /* Always jump to the next matching entry and repeat
-                 * this until we fine and offset that matches for all
+                 * this until we find an offset that matches for all
                  * matches. */
 
                 if (!m->matches)
                         return 0;
 
-                np = 0;
-                do {
-                        continue_looking = false;
+                r = next_for_match(j, m->matches, f, after_offset, direction, NULL, &np);
+                if (r <= 0)
+                        return r;
 
-                        LIST_FOREACH(matches, i, m->matches) {
-                                uint64_t cp, limit;
+                assert(direction == DIRECTION_DOWN ? np >= after_offset : np <= after_offset);
+                last_moved = m->matches;
 
-                                if (np == 0)
-                                        limit = after_offset;
-                                else if (direction == DIRECTION_DOWN)
-                                        limit = MAX(np, after_offset);
-                                else
-                                        limit = MIN(np, after_offset);
+                LIST_LOOP_BUT_ONE(matches, i, m->matches, last_moved) {
+                        uint64_t cp;
 
-                                r = next_for_match(j, i, f, limit, direction, NULL, &cp);
-                                if (r <= 0)
-                                        return r;
+                        r = next_for_match(j, i, f, np, direction, NULL, &cp);
+                        if (r <= 0)
+                                return r;
 
-                                if ((direction == DIRECTION_DOWN ? cp >= after_offset : cp <= after_offset) &&
-                                    (np == 0 || (direction == DIRECTION_DOWN ? cp > np : cp < np))) {
-                                        np = cp;
-                                        continue_looking = true;
-                                }
+                        assert(direction == DIRECTION_DOWN ? cp >= np : cp <= np);
+                        if (direction == DIRECTION_DOWN ? cp > np : cp < np) {
+                                np = cp;
+                                last_moved = i;
                         }
-
-                } while (continue_looking);
+                }
         }
 
         if (np == 0)
diff --git a/src/shared/list.h b/src/shared/list.h
index 47f275a..96d6237 100644
--- a/src/shared/list.h
+++ b/src/shared/list.h
@@ -123,3 +123,10 @@
 
 #define LIST_FOREACH_AFTER(name,i,p)                                    \
         for ((i) = (p)->name##_next; (i); (i) = (i)->name##_next)
+
+/* Loop starting from p->next until p->prev.
+   p can be adjusted meanwhile. */
+#define LIST_LOOP_BUT_ONE(name,i,head,p)                                \
+        for ((i) = (p)->name##_next ? (p)->name##_next : (head);        \
+             (i) != (p);                                                \
+             (i) = (i)->name##_next ? (i)->name##_next : (head))
-- 
1.8.2.562.g931e949