shithub: dav1d

Info  •  Files  •  Log  •  Branches

Avoid calling get_uniform(max=1)

Calling get_uniform(max=1) results in a read_bits(n=0),
In get_uniform, the n param is renamed to max to clarify the
semantics. Asserts are added to detect calls to get_uniform()
and get_bits() that don't actually read anything.

Closes #76

--- a/src/getbits.c

+++ b/src/getbits.c

@@ -62,6 +62,7 @@

 unsigned dav1d_get_bits(GetBits *const c, const unsigned n) {

     assert(n <= 32 /* can go up to 57 if we change return type */);

+    assert(n /* can't shift state by 64 */);

     if (n > c->bits_left) refill(c, n);

@@ -78,11 +79,13 @@

     return res >> shift;

 }

-unsigned dav1d_get_uniform(GetBits *const c, const unsigned n) {

-    assert(n > 0);

-    const int l = ulog2(n) + 1;

-    assert(l > 0);

-    const unsigned m = (1U << l) - n;

+unsigned dav1d_get_uniform(GetBits *const c, const unsigned max) {

+    // Output in range [0..max-1]

+    // max must be > 1, or else nothing is read from the bitstream

+    assert(max > 1);

+    const int l = ulog2(max) + 1;

+    assert(l > 1);

+    const unsigned m = (1U << l) - max;

     const unsigned v = dav1d_get_bits(c, l - 1);

     return v < m ? v : (v << 1) - m + dav1d_get_bits(c, 1);

 }

--- a/src/getbits.h

+++ b/src/getbits.h

@@ -41,7 +41,9 @@

 void dav1d_init_get_bits(GetBits *c, const uint8_t *data, size_t sz);

 unsigned dav1d_get_bits(GetBits *c, unsigned n);

 int dav1d_get_sbits(GetBits *c, unsigned n);

-unsigned dav1d_get_uniform(GetBits *c, unsigned range);

+

+// Output in range 0..max-1

+unsigned dav1d_get_uniform(GetBits *c, unsigned max);

 unsigned dav1d_get_vlc(GetBits *c);

 int dav1d_get_bits_subexp(GetBits *c, int ref, unsigned n);

 const uint8_t *dav1d_flush_get_bits(GetBits *c);

--- a/src/obu.c

+++ b/src/obu.c

@@ -464,8 +464,10 @@

         hdr->tiling.cols = 0;

         int widest_tile = 0, max_tile_area_sb = sbw * sbh;

         for (int sbx = 0; sbx < sbw; hdr->tiling.cols++) {

-            const int tile_w = 1 + dav1d_get_uniform(gb, imin(sbw - sbx,

-                                                              max_tile_width_sb));

+            const int tile_width_sb = imin(sbw - sbx, max_tile_width_sb);

+            const int tile_w = (tile_width_sb > 1) ?

+                                   1 + dav1d_get_uniform(gb, tile_width_sb) :

+                                   1;

             hdr->tiling.col_start_sb[hdr->tiling.cols] = sbx;

             sbx += tile_w;

             widest_tile = imax(widest_tile, tile_w);

@@ -476,8 +478,10 @@

         hdr->tiling.rows = 0;

         for (int sby = 0; sby < sbh; hdr->tiling.rows++) {

-            const int tile_h = 1 + dav1d_get_uniform(gb, imin(sbh - sby,

-                                                              max_tile_height_sb));

+            const int tile_height_sb = imin(sbh - sby, max_tile_height_sb);

+            const int tile_h = (tile_height_sb > 1) ?

+                                   1 + dav1d_get_uniform(gb, tile_height_sb) :

+                                   1;

             hdr->tiling.row_start_sb[hdr->tiling.rows] = sby;

             sby += tile_h;

         }