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c3364a23d2
We don't query this information out of the promoted (it's basically a
single "unit" regardless of the complexity within it) and this saves on
re-initializing the SparseIntervalMatrix's backing IndexVec with mostly
empty rows for all of the leading regions in the function. Typical
promoteds will only contain a few regions that need up be uplifted,
while the parent function can have thousands.
For a simple function repeating println!("Hello world"); 50,000 times
this reduces compile times from 90 to 15 seconds in debug mode. The
previous implementations re-initialization led to an overall roughly n^2
runtime as each promoted initialized slots for ~n regions, now we scale
closer to linearly (5000 hello worlds takes 1.1 seconds).