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Slightly over-allocate picture buffers

Allows simplified SIMD function implementations that don't exactly
respect picture boundaries when reading picture data. Fixes #251 and
#250.

--- a/include/dav1d/picture.h

+++ b/include/dav1d/picture.h

@@ -34,6 +34,11 @@

 #include "common.h"

 #include "headers.h"

+/* Number of bytes to align AND pad picture memory buffers by, so that SIMD

+ * implementations can over-read by a few bytes, and use aligned read/write

+ * instructions. */

+#define DAV1D_PICTURE_ALIGNMENT 32

+

 typedef struct Dav1dPictureParameters {

     int w; ///< width (in pixels)

     int h; ///< height (in pixels)

@@ -85,8 +90,10 @@

     /**

      * Allocate the picture buffer based on the Dav1dPictureParameters.

      *

-     * The data[0], data[1] and data[2] must be 32 byte aligned and with a

-     * pixel width/height multiple of 128 pixels.

+     * The data[0], data[1] and data[2] must be DAV1D_PICTURE_ALIGNMENT byte

+     * aligned and with a pixel width/height multiple of 128 pixels. Any

+     * allocated memory area should also be padded by DAV1D_PICTURE_ALIGNMENT

+     * bytes.

      * data[1] and data[2] must share the same stride[1].

      *

      * This function will be called on the main thread (the thread which calls

--- a/src/picture.c

+++ b/src/picture.c

@@ -59,7 +59,8 @@

     const size_t uv_sz = p->stride[1] * (aligned_h >> ss_ver);

     const size_t pic_size = y_sz + 2 * uv_sz;

-    uint8_t *data = dav1d_alloc_aligned(pic_size, 32);

+    uint8_t *data = dav1d_alloc_aligned(pic_size + DAV1D_PICTURE_ALIGNMENT,

+                                        DAV1D_PICTURE_ALIGNMENT);

     if (data == NULL) {

         return -1;

     }