Perf: Only scan the affected suffix of the provisional cache - #161130
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Perf: Only scan the affected suffix of the provisional cache#161130Dandandan wants to merge 1 commit into
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`insert_provisional` walks the whole provisional cache on every insert to
lower `reached_depth`. Everything it can affect was inserted while
`from_dfn` was on the stack, so it is a suffix of this insertion-ordered
map and the walk can stop at the first older entry.
The full walk is quadratic in the size of the cache, which only fills up
for mutually recursive types. Proving `Send` for a graph of N such types
did about N^2 iterations of this loop -- 641M of them for N=480.
`evaluate_obligation` on that graph:
N=60 104ms -> 61ms
N=120 315ms -> 118ms
N=240 886ms -> 243ms
N=480 3.06s -> 0.55s
That is the linear growth the same graph already has when it is acyclic.
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insert_provisionalwalks the whole provisional cache on every insert, to lowerreached_depthon entries at or abovefrom_dfn. Everything it can affect wasinserted while
from_dfnwas on the stack, so those entries are a suffix of thisinsertion-ordered map, and the walk can stop at the first older entry.
The full walk is quadratic in the size of the cache. That cache only fills up for
mutually recursive types, so this shows up when proving
SendorSyncfor agraph of such types: for a graph of N of them the loop ran about N² times.
evaluate_obligationself time on that graph:Growth goes from superlinear to linear, which is what the same graph already does
when it has no cycles.
I ran into this looking at DataFusion, whose
ExprandLogicalPlantypes aremutually recursive across a few hundred types.
AI Disclosure
I used LLM (Claude) to run some experiments and change the code