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The previous logic here created a separate pool of arcs for each state, so that the out-arcs of each state were physically stored within it. Perhaps this choice was driven by trying to not include a "from" pointer within each arc; but Spencer gave up on that idea long ago, and it's hard to see what the value is now. The approach turns out to be fairly disastrous in terms of memory consumption, though. In the first place, NFAs built by this engine seem to have about 4 arcs per state on average, with a majority having only one or two out-arcs. So pre-allocating 10 out-arcs for each state is already cause for a factor of two or more bloat. Worse, the NFA optimization phase moves arcs around with abandon. In a large NFA, some of the states will have hundreds of out-arcs, so towards the end of the optimization phase we have a significant number of states whose arc pools have room for hundreds of arcs each, even though only a few of those arcs are in use. We have seen real-world regexes in which this effect bloats the memory requirement by 25X or even more. Hence, get rid of the per-state arc pools in favor of a single arc pool for the whole NFA, with variable-sized allocation batches instead of always asking for 10 at a time. While we're at it, let's batch the allocations of state structs too, to further reduce the malloc traffic. This incidentally allows moveouts() to be optimized in a similar way to moveins(): when moving an arc to another state, it's now valid to just re-link the same arc struct into a different outchain, where before the code invariants required us to make a physically new arc and then free the old one. These changes reduce the regex compiler's typical space consumption for average-size regexes by about a factor of two, and much more for large or complicated regexes. In a large test set of real-world regexes, we formerly had half a dozen cases that failed with "regular expression too complex" due to exceeding the REG_MAX_COMPILE_SPACE limit (about 150MB); we would have had to raise that limit to something close to 400MB to make them work with the old code. Now, none of those cases need more than 13MB to compile. Furthermore, the test set is about 10% faster overall due to less malloc traffic. Discussion: https://postgr.es/m/168861.1614298592@sss.pgh.pa.us
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