CDATA codegen updates, part 3 #33
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These fields are only used at the end of input, and moving them into a different struct will make the per-state data accessed during DFA execution more compact.
Each state has two 256-bitsets, stored as a uint64_t[4], but the individual words in those have a lot of duplication. Add a table with every unique word, sorted descending by frequency, and replace the per-state labels and label_group_starts arrays with an array of offsets into the label_word table. Typically these offsets will fit in a uint8_t (though the code generation will switch to a uint16_t when necessary), making the per-state data much smaller. The label_word table's most commonly used entries are all grouped together and should stay in cache.
The edge sets leak when halting with FSM_DETERMINISE_WITH_CONFIG_STATE_LIMIT_REACHED.
Previously, the CDATA codegen wrote eager_outputs directly into the caller's match bit buffer as they were encountered. Instead, set them in a stack-allocated buffer, and then copy them to the caller's if the DFA match succeeds overall. In order to avoid repeatedly checking for whether an eager_output has already been set (in the buffer), this collects the set of all distinct eager_output IDs and then remaps the array with eager output IDs to offsets into the unique set. This condenses the (sparse) set into a dense series 0..n that can be represented by flags in a stack-allocated bit vector (with a size known at compile time), and redundant eager_outputs harmlessly set flag bits that are already set. If the overall match succeeds, that bit vector is matched up with the unique ID array and the sparse values are written into the caller's buffer. Because the unique ID array is sorted, the relative ordering of the sparse and dense IDs is preserved (and 0 stays 0), so using non-ascending values as terminators still works.
deg4uss3r
approved these changes
Dec 11, 2024
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Some further updates, for a new Device Detection canary. This still isn't quite ready to go upstream, but it's getting closer.
Changes:
.endand.endid_offsetfields from the state struct table to a separate table. Those fields aren't used until the end of input, moving them out makes the state table more dense and improves locality.uint8_toruint16_toffsets into a shared word table the overall binary ends up considerably smaller. They are sorted by use (descending), so the most frequently used ones are likely to stay in cache.fsm_determinise_with_config's early exit katef/libfsm#504