shithub: dav1d

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Avoid CDF overreads in gather_top_partition_prob()

Explicitly take advantage of the fact that certain probabilities are zero
instead of loading zeros from the CDF padding.

The current code works just fine, but only because those values happen to
be zero due to what is essentially an implementation detail.

--- a/src/env.h

+++ b/src/env.h

@@ -90,21 +90,15 @@

           (((l->partition[yb8] >> (4 - bl)) & 1) << 1);

 }

-static inline unsigned cdf_element_prob(const uint16_t *const cdf, const int e) {

-    assert(e > 0);

-    return cdf[e - 1] - cdf[e];

-}

-

 static inline unsigned gather_left_partition_prob(const uint16_t *const in,

                                                   const enum BlockLevel bl)

 {

-    unsigned out = 0;

-    out += cdf_element_prob(in, PARTITION_H);

-    if (bl != BL_128X128)

-        out += cdf_element_prob(in, PARTITION_H4);

+    unsigned out = in[PARTITION_H - 1] - in[PARTITION_H];

     // Exploit the fact that cdfs for PARTITION_SPLIT, PARTITION_T_TOP_SPLIT,

-    //  PARTITION_T_BOTTOM_SPLIT and PARTITION_T_LEFT_SPLIT are neighbors.

+    // PARTITION_T_BOTTOM_SPLIT and PARTITION_T_LEFT_SPLIT are neighbors.

     out += in[PARTITION_SPLIT - 1] - in[PARTITION_T_LEFT_SPLIT];

+    if (bl != BL_128X128)

+        out += in[PARTITION_H4 - 1] - in[PARTITION_H4];

     return out;

 }

@@ -111,13 +105,16 @@

 static inline unsigned gather_top_partition_prob(const uint16_t *const in,

                                                  const enum BlockLevel bl)

 {

-    unsigned out = 0;

+    // Exploit the fact that cdfs for PARTITION_V, PARTITION_SPLIT and

+    // PARTITION_T_TOP_SPLIT are neighbors.

+    unsigned out = in[PARTITION_V - 1] - in[PARTITION_T_TOP_SPLIT];

+    // Exploit the facts that cdfs for PARTITION_T_LEFT_SPLIT and

+    // PARTITION_T_RIGHT_SPLIT are neighbors, the probability for

+    // PARTITION_V4 is always zero, and the probability for

+    // PARTITION_T_RIGHT_SPLIT is zero in 128x128 blocks.

+    out += in[PARTITION_T_LEFT_SPLIT - 1];

     if (bl != BL_128X128)

-        out += cdf_element_prob(in, PARTITION_V4);

-    // Exploit the fact that cdfs for PARTITION_T_LEFT_SPLIT and PARTITION_T_RIGHT_SPLIT,

-    //  and PARTITION_V, PARTITION_SPLIT and PARTITION_T_TOP_SPLIT are neighbors.

-    out += in[PARTITION_T_LEFT_SPLIT - 1] - in[PARTITION_T_RIGHT_SPLIT];

-    out += in[PARTITION_V - 1] - in[PARTITION_T_TOP_SPLIT];

+        out += in[PARTITION_V4 - 1] - in[PARTITION_T_RIGHT_SPLIT];

     return out;

 }