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reduce the number of malloc/free cycles in huffman.c

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pre-allocating a sorted[] array for most common cases of small
alphabet size cuts a lot of traffic.

Change-Id: I73ff2f6e507f81b0b0bb7d9801a344aa4bcb038a
This commit is contained in:
skal 2016-07-12 12:06:31 -07:00
parent 7b4b05e0dc
commit fc8cad9f29
1 changed files with 31 additions and 13 deletions
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44
src/utils/huffman.c
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@ -75,11 +75,13 @@ static WEBP_INLINE int NextTableBitSize(const int* const count,
return len - root_bits;
}
int VP8LBuildHuffmanTable(HuffmanCode* const root_table, int root_bits,
const int code_lengths[], int code_lengths_size) {
// sorted[code_lengths_size] is a pre-allocated array for sorting symbols
// by code length.
static int BuildHuffmanTable(HuffmanCode* const root_table, int root_bits,
const int code_lengths[], int code_lengths_size,
uint16_t sorted[]) {
HuffmanCode* table = root_table; // next available space in table
int total_size = 1 << root_bits; // total size root table + 2nd level table
int* sorted = NULL; // symbols sorted by code length
int len; // current code length
int symbol; // symbol index in original or sorted table
// number of codes of each length:
@ -114,11 +116,6 @@ int VP8LBuildHuffmanTable(HuffmanCode* const root_table, int root_bits,
offset[len + 1] = offset[len] + count[len];
}
sorted = (int*)WebPSafeMalloc(code_lengths_size, sizeof(*sorted));
if (sorted == NULL) {
return 0;
}
// Sort symbols by length, by symbol order within each length.
for (symbol = 0; symbol < code_lengths_size; ++symbol) {
const int symbol_code_length = code_lengths[symbol];
@ -133,7 +130,6 @@ int VP8LBuildHuffmanTable(HuffmanCode* const root_table, int root_bits,
code.bits = 0;
code.value = (uint16_t)sorted[0];
ReplicateValue(table, 1, total_size, code);
WebPSafeFree(sorted);
return total_size;
}
@ -153,7 +149,6 @@ int VP8LBuildHuffmanTable(HuffmanCode* const root_table, int root_bits,
num_nodes += num_open;
num_open -= count[len];
if (num_open < 0) {
WebPSafeFree(sorted);
return 0;
}
for (; count[len] > 0; --count[len]) {
@ -172,7 +167,6 @@ int VP8LBuildHuffmanTable(HuffmanCode* const root_table, int root_bits,
num_nodes += num_open;
num_open -= count[len];
if (num_open < 0) {
WebPSafeFree(sorted);
return 0;
}
for (; count[len] > 0; --count[len]) {
@ -195,11 +189,35 @@ int VP8LBuildHuffmanTable(HuffmanCode* const root_table, int root_bits,
// Check if tree is full.
if (num_nodes != 2 * offset[MAX_ALLOWED_CODE_LENGTH] - 1) {
WebPSafeFree(sorted);
return 0;
}
}
WebPSafeFree(sorted);
return total_size;
}
// Maximum code_lengths_size is 2328 (reached for 11-bit color_cache_bits).
// More commonly, the value is around ~280.
#define MAX_CODE_LENGTHS_SIZE \
((1 << MAX_CACHE_BITS) + NUM_LITERAL_CODES + NUM_LENGTH_CODES)
// Cut-off value for switching between heap and stack allocation.
#define SORTED_SIZE_CUTOFF 512
int VP8LBuildHuffmanTable(HuffmanCode* const root_table, int root_bits,
const int code_lengths[], int code_lengths_size) {
int total_size;
assert(code_lengths_size <= MAX_CODE_LENGTHS_SIZE);
if (code_lengths_size <= SORTED_SIZE_CUTOFF) {
// use local stack-allocated array.
uint16_t sorted[SORTED_SIZE_CUTOFF];
total_size = BuildHuffmanTable(root_table, root_bits,
code_lengths, code_lengths_size, sorted);
} else { // rare case. Use heap allocation.
uint16_t* const sorted =
(uint16_t*)WebPSafeMalloc(code_lengths_size, sizeof(*sorted));
if (sorted == NULL) return 0;
total_size = BuildHuffmanTable(root_table, root_bits,
code_lengths, code_lengths_size, sorted);
WebPSafeFree(sorted);
}
return total_size;
}