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Optimize vp9_tree_probs_from_distribution

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The previous implementation visited each node in the tree multiple times
because it used each symbol's encoding to revisit the branches taken and
increment its count. Instead, we can traverse the tree depth first and
calculate the probabilities and branch counts as we walk back up. The
complexity goes from somewhere between O(nlogn) and O(n^2) (depending on
how balanced the tree is) to O(n).

Only tested one clip (256kbps, CIF), saw 13% decoding perf improvement.

Note that this optimization should port trivially to VP8 as well. In VP8,
the decoder doesn't use this function, but it does routinely show up
on the profile for realtime encoding.

Change-Id: I4f2848e4f41dc9a7694f73f3e75034bce08d1b12
This commit is contained in:
John Koleszar
2013-03-10 13:39:30 -07:00
parent 4209bba462
commit bd84685f78
6 changed files with 82 additions and 134 deletions
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81
vp9/common/vp9_treecoder.c
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@@ -48,66 +48,37 @@ void vp9_tokens_from_tree_offset(struct vp9_token_struct *p, vp9_tree t,
tree2tok(p - offset, t, 0, 0, 0);
}
static void branch_counts(
int n, /* n = size of alphabet */
vp9_token tok [ /* n */ ],
vp9_tree tree,
unsigned int branch_ct [ /* n-1 */ ] [2],
const unsigned int num_events[ /* n */ ]
) {
const int tree_len = n - 1;
int t = 0;
#if CONFIG_DEBUG
assert(tree_len);
#endif
do {
branch_ct[t][0] = branch_ct[t][1] = 0;
} while (++t < tree_len);
t = 0;
do {
int L = tok[t].Len;
const int enc = tok[t].value;
const unsigned int ct = num_events[t];
vp9_tree_index i = 0;
do {
const int b = (enc >> --L) & 1;
const int j = i >> 1;
#if CONFIG_DEBUG
assert(j < tree_len && 0 <= L);
#endif
branch_ct [j] [b] += ct;
i = tree[ i + b];
} while (i > 0);
#if CONFIG_DEBUG
assert(!L);
#endif
} while (++t < n);
static unsigned int convert_distribution(unsigned int i,
vp9_tree tree,
vp9_prob probs[],
unsigned int branch_ct[][2],
const unsigned int num_events[],
unsigned int tok0_offset) {
unsigned int left, right;
if (tree[i] <= 0) {
left = num_events[-tree[i] - tok0_offset];
} else {
left = convert_distribution(tree[i], tree, probs, branch_ct,
num_events, tok0_offset);
}
if (tree[i + 1] <= 0) {
right = num_events[-tree[i + 1] - tok0_offset];
} else {
right = convert_distribution(tree[i + 1], tree, probs, branch_ct,
num_events, tok0_offset);
}
probs[i>>1] = get_binary_prob(left, right);
branch_ct[i>>1][0] = left;
branch_ct[i>>1][1] = right;
return left + right;
}
void vp9_tree_probs_from_distribution(
int n, /* n = size of alphabet */
vp9_token tok [ /* n */ ],
vp9_tree tree,
vp9_prob probs [ /* n-1 */ ],
unsigned int branch_ct [ /* n-1 */ ] [2],
const unsigned int num_events[ /* n */ ]
) {
const int tree_len = n - 1;
int t = 0;
branch_counts(n, tok, tree, branch_ct, num_events);
do {
probs[t] = get_binary_prob(branch_ct[t][0], branch_ct[t][1]);
} while (++t < tree_len);
const unsigned int num_events[ /* n */ ],
unsigned int tok0_offset) {
convert_distribution(0, tree, probs, branch_ct, num_events, tok0_offset);
}