[gtk/glyphcache-fiddling: 27/28] gl: Do less work on glyph caching

[Matthias Clasen <matthiasc src gnome org>]

commit 977ac2b31fb0d739c5386e4ba6799a53700fe4f5
Author: Matthias Clasen <mclasen redhat com>
Date:   Sat Oct 12 11:03:04 2019 -0400

    gl: Do less work on glyph caching
    
    There is no need for us to be very precise about
    aging the glyph entries. It is enough to check
    occasionally and mark old entries. This reduces
    the overhead of work we do every frame on the
    caches, at the cost of letting glyphs linger
    a bit longer in the cache.

 gsk/gl/gskglglyphcache.c        | 33 +++++++++++++++++----------------
 gsk/gl/gskglglyphcacheprivate.h |  6 +++---
 2 files changed, 20 insertions(+), 19 deletions(-)
---
diff --git a/gsk/gl/gskglglyphcache.c b/gsk/gl/gskglglyphcache.c
index 21121b1ce5..f68837444c 100644
--- a/gsk/gl/gskglglyphcache.c
+++ b/gsk/gl/gskglglyphcache.c
@@ -15,9 +15,9 @@
 
 /* Cache eviction strategy
  *
- * Each cached glyph has an age that gets reset every time a cached
- * glyph gets used. Glyphs that have not been used for the
- * MAX_FRAME_AGE frames are considered old.
+ * We mark glyphs as accessed every time we use them. Every
+ * few frames, we mark glyphs that haven't been accessed since
+ * the last check as old.
  *
  * We keep count of the pixels of each atlas that are taken up by old
  * data. When the fraction of old pixels gets too high, we drop the
@@ -264,12 +264,12 @@ gsk_gl_glyph_cache_lookup_or_add (GskGLGlyphCache         *cache,
 
   if (value)
     {
-      value->timestamp = cache->timestamp;
       if (value->atlas && !value->used)
         {
           gsk_gl_texture_atlas_mark_used (value->atlas, value->draw_width, value->draw_height);
           value->used = TRUE;
         }
+      value->accessed = TRUE;
 
       *cached_glyph_out = value;
       return;
@@ -292,7 +292,7 @@ gsk_gl_glyph_cache_lookup_or_add (GskGLGlyphCache         *cache,
     value->draw_y = ink_rect.y;
     value->draw_width = ink_rect.width;
     value->draw_height = ink_rect.height;
-    value->timestamp = cache->timestamp;
+    value->accessed = TRUE;
     value->atlas = NULL; /* For now */
 
     key = g_new0 (GlyphCacheKey, 1);
@@ -341,20 +341,21 @@ gsk_gl_glyph_cache_begin_frame (GskGLGlyphCache *self,
       GSK_NOTE(GLYPH_CACHE, if (dropped > 0) g_message ("Dropped %d glyphs", dropped));
     }
 
-  g_hash_table_iter_init (&iter, self->hash_table);
-  while (g_hash_table_iter_next (&iter, (gpointer *)&key, (gpointer *)&value))
+  if (self->timestamp % MAX_FRAME_AGE == 0)
     {
-      const guint age = self->timestamp - value->timestamp;
-
-      if (age > MAX_FRAME_AGE)
+      g_hash_table_iter_init (&iter, self->hash_table);
+      while (g_hash_table_iter_next (&iter, (gpointer *)&key, (gpointer *)&value))
         {
-          GskGLTextureAtlas *atlas = value->atlas;
-
-          if (atlas && value->used)
+          if (!value->accessed)
             {
-              gsk_gl_texture_atlas_mark_unused (atlas, value->draw_width, value->draw_height);
-              value->used = FALSE;
+              if (value->atlas && value->used)
+                {
+                  gsk_gl_texture_atlas_mark_unused (value->atlas, value->draw_width, value->draw_height);
+                  value->used = FALSE;
+                }
             }
-        }
+
+          value->accessed = FALSE;
+       }
     }
 }
diff --git a/gsk/gl/gskglglyphcacheprivate.h b/gsk/gl/gskglglyphcacheprivate.h
index 259688fbb7..75f7a8de19 100644
--- a/gsk/gl/gskglglyphcacheprivate.h
+++ b/gsk/gl/gskglglyphcacheprivate.h
@@ -15,7 +15,7 @@ typedef struct
   GHashTable *hash_table;
   GskGLTextureAtlases *atlases;
 
-  guint64 timestamp;
+  int timestamp;
 } GskGLGlyphCache;
 
 typedef struct
@@ -63,8 +63,8 @@ struct _GskGLCachedGlyph
   int draw_width;
   int draw_height;
 
-  guint used: 1;
-  guint64 timestamp;
+  guint accessed : 1; /* accessed since last check */
+  guint used     : 1; /* accounted as used in the atlas */
 };