In particular, on even moderately compressible data most blocks take the following form:
- 1 token byte + 2 distance bytes
- a somewhat unpredictable number of literal bytes
If these streams are interleaved, then it's harder for the out-of-order core to process the token bytes of several blocks in advance (as the position of the next token byte depends on the unpredictable number of literal bytes in the current block). However, if you separate them (all token bytes in a prefix, and literal bytes in a suffix), the CPU can speculatively parse token bytes of a lot of blocks in advance (as most of the time, it just has to step forward by three bytes to go to the next block's token byte).
I used the same "separate stream" design (one of them coded backwards from the end, to eliminate the need to send a separate offset) when working on video compression, to separate out arithmetic-coded bits from literal bits. It's a good idea. The only reason it isn't in AV1 is because hardware wanted to do DRM decryption in order.
Yes, that prevents streaming for now.
What is the advantage of this? Out-of-order execution (see my decoder's hot loop: https://github.com/welcome-to-the-sunny-side/misa77/blob/3a9...).
In particular, on even moderately compressible data most blocks take the following form:
If these streams are interleaved, then it's harder for the out-of-order core to process the token bytes of several blocks in advance (as the position of the next token byte depends on the unpredictable number of literal bytes in the current block). However, if you separate them (all token bytes in a prefix, and literal bytes in a suffix), the CPU can speculatively parse token bytes of a lot of blocks in advance (as most of the time, it just has to step forward by three bytes to go to the next block's token byte).I used the same "separate stream" design (one of them coded backwards from the end, to eliminate the need to send a separate offset) when working on video compression, to separate out arithmetic-coded bits from literal bits. It's a good idea. The only reason it isn't in AV1 is because hardware wanted to do DRM decryption in order.