gpu: Add a front cache

This copies the approach taken in the gl renderer to avoid much
of the hash table lookup overhead by means of a front cache.

gsk/gpu/gskgpudevice.c

@@ -32,6 +32,15 @@ typedef struct _GskGpuCachedGlyph GskGpuCachedGlyph;
 typedef struct _GskGpuCachedTexture GskGpuCachedTexture;
 typedef struct _GskGpuDevicePrivate GskGpuDevicePrivate;
 
+typedef struct _GlyphKey GlyphKey;
+struct _GlyphKey
+{
+  PangoFont *font;
+  PangoGlyph glyph;
+  GskGpuGlyphLookupFlags flags;
+  float scale;
+};
+
 struct _GskGpuDevicePrivate
 {
   GdkDisplay *display;
@@ -45,6 +54,11 @@ struct _GskGpuDevicePrivate
   GHashTable *glyph_cache;
 
   GskGpuCachedAtlas *current_atlas;
+
+  struct {
+    GlyphKey key;
+    GskGpuCachedGlyph *value;
+  } front[256];
 };
 
 G_DEFINE_TYPE_WITH_PRIVATE (GskGpuDevice, gsk_gpu_device, G_TYPE_OBJECT)
@@ -392,10 +406,7 @@ struct _GskGpuCachedGlyph
 {
   GskGpuCached parent;
 
-  PangoFont *font;
-  PangoGlyph glyph;
-  GskGpuGlyphLookupFlags flags;
-  float scale;
+  GlyphKey key;
 
   GskGpuImage *image;
   graphene_rect_t bounds;
@@ -411,7 +422,7 @@ gsk_gpu_cached_glyph_free (GskGpuDevice *device,
 
   g_hash_table_remove (priv->glyph_cache, self);
 
-  g_object_unref (self->font);
+  g_object_unref (self->key.font);
   g_object_unref (self->image);
 
   g_free (self);
@@ -434,10 +445,10 @@ gsk_gpu_cached_glyph_hash (gconstpointer data)
 {
   const GskGpuCachedGlyph *glyph = data;
 
-  return GPOINTER_TO_UINT (glyph->font) ^
-         glyph->glyph ^
-         (glyph->flags << 24) ^
-         ((guint) glyph->scale * PANGO_SCALE);
+  return GPOINTER_TO_UINT (glyph->key.font) ^
+         glyph->key.glyph ^
+         (glyph->key.flags << 24) ^
+         ((guint) glyph->key.scale * PANGO_SCALE);
 }
 
 static gboolean
@@ -447,10 +458,10 @@ gsk_gpu_cached_glyph_equal (gconstpointer v1,
   const GskGpuCachedGlyph *glyph1 = v1;
   const GskGpuCachedGlyph *glyph2 = v2;
 
-  return glyph1->font == glyph2->font
-      && glyph1->glyph == glyph2->glyph
-      && glyph1->flags == glyph2->flags
-      && glyph1->scale == glyph2->scale;
+  return glyph1->key.font == glyph2->key.font
+      && glyph1->key.glyph == glyph2->key.glyph
+      && glyph1->key.flags == glyph2->key.flags
+      && glyph1->key.scale == glyph2->key.scale;
 }
 
 static const GskGpuCachedClass GSK_GPU_CACHED_GLYPH_CLASS =
@@ -762,10 +773,10 @@ gsk_gpu_device_lookup_glyph_image (GskGpuDevice           *self,
 {
   GskGpuDevicePrivate *priv = gsk_gpu_device_get_instance_private (self);
   GskGpuCachedGlyph lookup = {
-    .font = font,
-    .glyph = glyph,
-    .flags = flags,
-    .scale = scale
+    .key.font = font,
+    .key.glyph = glyph,
+    .key.flags = flags,
+    .key.scale = scale
   };
   GskGpuCachedGlyph *cache;
   PangoRectangle ink_rect;
@@ -773,14 +784,32 @@ gsk_gpu_device_lookup_glyph_image (GskGpuDevice           *self,
   graphene_point_t origin;
   GskGpuImage *image;
   gsize atlas_x, atlas_y, padding;
+  guint64 timestamp = gsk_gpu_frame_get_timestamp (frame);
+  guint front_index = glyph & 0xFF;
+
+  if (memcmp (&lookup.key, &priv->front[front_index], sizeof (GlyphKey)) == 0)
+    {
+      cache = priv->front[front_index].value;
+
+      gsk_gpu_cached_use (self, (GskGpuCached *) cache, timestamp);
+
+      *out_bounds = cache->bounds;
+      *out_origin = cache->origin;
+
+      return cache->image;
+    }
 
   cache = g_hash_table_lookup (priv->glyph_cache, &lookup);
   if (cache)
     {
-      gsk_gpu_cached_use (self, (GskGpuCached *) cache, gsk_gpu_frame_get_timestamp (frame));
+      memcpy (&priv->front[front_index].key, &lookup.key, sizeof (GlyphKey));
+      priv->front[front_index].value = cache;
+
+      gsk_gpu_cached_use (self, (GskGpuCached *) cache, timestamp);
 
       *out_bounds = cache->bounds;
       *out_origin = cache->origin;
+
       return cache->image;
     }
 
@@ -792,7 +821,7 @@ gsk_gpu_device_lookup_glyph_image (GskGpuDevice           *self,
   padding = 1;
 
   image = gsk_gpu_device_add_atlas_image (self,
-                                          gsk_gpu_frame_get_timestamp (frame),
+                                          timestamp,
                                           rect.size.width + 2 * padding, rect.size.height + 2 * padding,
                                           &atlas_x, &atlas_y);
   if (image)
@@ -811,10 +840,10 @@ gsk_gpu_device_lookup_glyph_image (GskGpuDevice           *self,
       cache = gsk_gpu_cached_new (self, &GSK_GPU_CACHED_GLYPH_CLASS, NULL);
     }
 
-  cache->font = g_object_ref (font),
-  cache->glyph = glyph,
-  cache->flags = flags,
-  cache->scale = scale,
+  cache->key.font = g_object_ref (font),
+  cache->key.glyph = glyph,
+  cache->key.flags = flags,
+  cache->key.scale = scale,
   cache->bounds = rect,
   cache->image = image,
   ((GskGpuCached *) cache)->pixels = (rect.size.width + 2 * padding) * (rect.size.height + 2 * padding);
@@ -836,10 +865,14 @@ gsk_gpu_device_lookup_glyph_image (GskGpuDevice           *self,
                                                  cache->origin.y + padding));
 
   g_hash_table_insert (priv->glyph_cache, cache, cache);
-  gsk_gpu_cached_use (self, (GskGpuCached *) cache, gsk_gpu_frame_get_timestamp (frame));
+  gsk_gpu_cached_use (self, (GskGpuCached *) cache, timestamp);
+
+  memcpy (&priv->front[front_index].key, &lookup.key, sizeof (GlyphKey));
+  priv->front[front_index].value = cache;
 
   *out_bounds = cache->bounds;
   *out_origin = cache->origin;
+
   return cache->image;
 }