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Avoid excessive L2 collisions with certain frame widths

Memory addresses with certain power-of-two offsets will map to the
same set of cache lines. Using such offsets as strides will cause
excessive cache evictions resulting in more cache misses.

Avoid this by adding a small padding when the stride is a multiple
of 1024 (somewhat arbitrarily chosen as the specific number depends
on the hardware implementation) when allocating picture buffers.

--- a/src/picture.c

+++ b/src/picture.c

@@ -52,17 +52,24 @@

     const int has_chroma = p->p.layout != DAV1D_PIXEL_LAYOUT_I400;

     const int ss_ver = p->p.layout == DAV1D_PIXEL_LAYOUT_I420;

     const int ss_hor = p->p.layout != DAV1D_PIXEL_LAYOUT_I444;

-    p->stride[0] = aligned_w << hbd;

-    p->stride[1] = has_chroma ? (aligned_w >> ss_hor) << hbd : 0;

-    const size_t y_sz = p->stride[0] * aligned_h;

-    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 + DAV1D_PICTURE_ALIGNMENT,

-                                        DAV1D_PICTURE_ALIGNMENT);

-    if (data == NULL) {

-        return DAV1D_ERR(ENOMEM);

-    }

+    ptrdiff_t y_stride = aligned_w << hbd;

+    ptrdiff_t uv_stride = has_chroma ? y_stride >> ss_hor : 0;

+    /* Due to how mapping of addresses to sets works in most L1 and L2 cache

+     * implementations, strides of multiples of certain power-of-two numbers

+     * may cause multiple rows of the same superblock to map to the same set,

+     * causing evictions of previous rows resulting in a reduction in cache

+     * hit rate. Avoid that by slightly padding the stride when necessary. */

+    if (!(y_stride & 1023))

+        y_stride += DAV1D_PICTURE_ALIGNMENT;

+    if (!(uv_stride & 1023) && has_chroma)

+        uv_stride += DAV1D_PICTURE_ALIGNMENT;

+    p->stride[0] = y_stride;

+    p->stride[1] = uv_stride;

+    const size_t y_sz = y_stride * aligned_h;

+    const size_t uv_sz = uv_stride * (aligned_h >> ss_ver);

+    const size_t pic_size = y_sz + 2 * uv_sz + DAV1D_PICTURE_ALIGNMENT;

+    uint8_t *data = dav1d_alloc_aligned(pic_size, DAV1D_PICTURE_ALIGNMENT);

+    if (!data) return DAV1D_ERR(ENOMEM);

     p->data[0] = data;

     p->data[1] = has_chroma ? data + y_sz : NULL;