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lzw: Always read three bytes on fast path to avoid swtich.

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Michael Drake 2021-04-18 17:31:36 +01:00 committed by Michael Drake
parent 98e8673dbc
commit a373fd766e
1 changed files with 13 additions and 15 deletions
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28
libvips/foreign/libnsgif/lzw.c
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@ -41,7 +41,7 @@ struct lzw_read_ctx {
uint32_t data_sb_next; /**< Offset to sub-block size */
const uint8_t *sb_data; /**< Pointer to current sub-block in data */
uint32_t sb_bit; /**< Current bit offset in sub-block */
size_t sb_bit; /**< Current bit offset in sub-block */
uint32_t sb_bit_count; /**< Bit count in sub-block */
};
@ -165,29 +165,25 @@ static lzw_result lzw__block_advance(struct lzw_read_ctx *restrict ctx)
*/
static inline lzw_result lzw__read_code(
struct lzw_read_ctx *restrict ctx,
uint8_t code_size,
uint32_t code_size,
uint32_t *restrict code_out)
{
uint32_t code = 0;
uint8_t current_bit = ctx->sb_bit & 0x7;
uint8_t byte_advance = (current_bit + code_size) >> 3;
uint32_t current_bit = ctx->sb_bit & 0x7;
assert(byte_advance <= 2);
if (ctx->sb_bit + code_size <= ctx->sb_bit_count) {
/* Fast path: code fully inside this sub-block */
if (ctx->sb_bit + 24 <= ctx->sb_bit_count) {
/* Fast path: read three bytes from this sub-block */
const uint8_t *data = ctx->sb_data + (ctx->sb_bit >> 3);
switch (byte_advance) {
case 2: code |= data[2] << 16; /* Fall through */
case 1: code |= data[1] << 8; /* Fall through */
case 0: code |= data[0] << 0;
}
code |= *data++ << 0;
code |= *data++ << 8;
code |= *data << 16;
ctx->sb_bit += code_size;
} else {
/* Slow path: code spans sub-blocks */
uint8_t byte_advance = (current_bit + code_size) >> 3;
uint8_t byte = 0;
uint8_t bits_remaining_0 = (code_size < (8 - current_bit)) ?
code_size : (8 - current_bit);
uint8_t bits_remaining_0 = (code_size < (8u - current_bit)) ?
code_size : (8u - current_bit);
uint8_t bits_remaining_1 = code_size - bits_remaining_0;
uint8_t bits_used[3] = {
[0] = bits_remaining_0,
@ -195,6 +191,8 @@ static inline lzw_result lzw__read_code(
[2] = bits_remaining_1 - 8,
};
assert(byte_advance <= 2);
while (true) {
const uint8_t *data = ctx->sb_data;
lzw_result res;