mp_image_pool: add pool to avoid frequent image reallocations

Refcounting will conceptually allocate and free images all the time
when using the filter chain. Add a pool that makes these reallocations
cheap.

This only affects the image data, not mp_image structs and similar small
allocations. Small allocations are always fast with reasonable memory
managers, while large image data will trigger mmap/munmap calls each
time.

Makefile

@@ -227,6 +227,7 @@ SOURCES = talloc.c \
           video/image_writer.c \
           video/img_format.c \
           video/mp_image.c \
+          video/mp_image_pool.c \
           video/sws_utils.c \
           video/decode/dec_video.c \
           video/decode/lavc_dr1.c \

video/mp_image_pool.c

@@ -0,0 +1,133 @@
+/*
+ * This file is part of mpv.
+ *
+ * mpv is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 2 of the License, or
+ * (at your option) any later version.
+ *
+ * mpv is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License along
+ * with mpv; if not, write to the Free Software Foundation, Inc.,
+ * 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
+ */
+
+#include <assert.h>
+
+#include "talloc.h"
+
+#include "core/mp_common.h"
+#include "video/mp_image.h"
+
+#include "mp_image_pool.h"
+
+struct pool_trampoline {
+    struct mp_image_pool *pool;
+    int refcount;
+};
+
+static int image_pool_destructor(void *ptr)
+{
+    struct mp_image_pool *pool = ptr;
+    mp_image_pool_clear(pool);
+    pool->trampoline->pool = NULL;
+    if (!pool->trampoline->refcount)
+        talloc_free(pool->trampoline);
+    return 0;
+}
+
+struct mp_image_pool *mp_image_pool_new(int max_count)
+{
+    struct mp_image_pool *pool = talloc_ptrtype(NULL, pool);
+    talloc_set_destructor(pool, image_pool_destructor);
+    *pool = (struct mp_image_pool) {
+        .max_count = max_count,
+        .trampoline = talloc_struct(NULL, struct pool_trampoline, {
+            .pool = pool,
+        }),
+    };
+    return pool;
+}
+
+void mp_image_pool_clear(struct mp_image_pool *pool)
+{
+    for (int n = 0; n < pool->num_images; n++) {
+        struct mp_image *img = pool->images[n];
+        if (img->priv) {
+            // Image data is being used - detach from the pool, and make it
+            // free itself when pool_free_image() is called
+            img->priv = talloc_struct(NULL, struct pool_trampoline, {
+                .refcount = 1,
+            });
+        } else {
+            talloc_free(img);
+        }
+    }
+    pool->num_images = 0;
+    pool->trampoline->refcount = 0;
+}
+
+static void pool_free_image(void *ptr)
+{
+    struct mp_image *img = ptr;
+    struct pool_trampoline *trampoline = img->priv;
+    trampoline->refcount--;
+    img->priv = NULL;
+    if (!trampoline->pool) {
+        // pool was free'd while image reference was still held
+        if (trampoline->refcount == 0)
+            talloc_free(trampoline);
+        talloc_free(img);
+    }
+}
+
+// Return a new image of given format/size. The only difference to
+// mp_image_alloc() is that there is a transparent mechanism to recycle image
+// data allocations through this pool.
+// The image can be free'd with talloc_free().
+struct mp_image *mp_image_pool_get(struct mp_image_pool *pool, unsigned int fmt,
+                                   int w, int h)
+{
+    struct mp_image *new = NULL;
+    for (int n = 0; n < pool->num_images; n++) {
+        struct mp_image *img = pool->images[n];
+        if (!img->priv && img->imgfmt == fmt && img->w == w && img->h == h) {
+            new = img;
+            break;
+        }
+    }
+    if (!new) {
+        if (pool->num_images >= pool->max_count)
+            mp_image_pool_clear(pool);
+        new = mp_image_alloc(fmt, w, h);
+        MP_TARRAY_APPEND(pool, pool->images, pool->num_images, new);
+    }
+    pool->trampoline->refcount++;
+    new->priv = pool->trampoline;
+    return mp_image_new_custom_ref(new, new, pool_free_image);
+}
+
+// Like mp_image_new_copy(), but allocate the image out of the pool.
+struct mp_image *mp_image_pool_new_copy(struct mp_image_pool *pool,
+                                        struct mp_image *img)
+{
+    struct mp_image *new = mp_image_pool_get(pool, img->imgfmt, img->w, img->h);
+    mp_image_copy(new, img);
+    mp_image_copy_attributes(new, img);
+    return new;
+}
+
+// Like mp_image_make_writeable(), but if a copy has to be made, allocate it
+// out of the pool.
+void mp_image_pool_make_writeable(struct mp_image_pool *pool,
+                                  struct mp_image *img)
+{
+    if (mp_image_is_writeable(img))
+        return;
+    mp_image_steal_data(img, mp_image_pool_new_copy(pool, img));
+    assert(mp_image_is_writeable(img));
+}

video/mp_image_pool.h

@@ -0,0 +1,22 @@
+#ifndef MPV_MP_IMAGE_POOL_H
+#define MPV_MP_IMAGE_POOL_H
+
+struct mp_image_pool {
+    int max_count;
+
+    struct mp_image **images;
+    int num_images;
+    struct pool_trampoline *trampoline;
+};
+
+struct mp_image_pool *mp_image_pool_new(int max_count);
+struct mp_image *mp_image_pool_get(struct mp_image_pool *pool, unsigned int fmt,
+                                   int w, int h);
+void mp_image_pool_clear(struct mp_image_pool *pool);
+
+struct mp_image *mp_image_pool_new_copy(struct mp_image_pool *pool,
+                                        struct mp_image *img);
+void mp_image_pool_make_writeable(struct mp_image_pool *pool,
+                                  struct mp_image *img);
+
+#endif