Skip gap-finder prefix via binary search

[jserv]

find_free_gap_inner walked guest_t.regions[0..nregions) linearly and skipped any region with end <= gap_start using a cheap continue. With the cached post-allocation gap hint usually pointing high in the array, every mmap call still re-scanned the same skippable prefix on the way to the first interesting region, and the addr-hint direct caller atsys_mmap bypasses the hint cache entirely so it always paid the full prefix cost.

Hoist the prefix walk into a static lower_bound helper first_region_end_above and start the existing loop at its result. Correctness rests on invariants already maintained elsewhere: guest_region_add keeps regions[] sorted ascending by start, and sys_mmap MAP_FIXED removes overlapping ranges before insertion (with adjacent anonymous regions coalesced via regions_mergeable), so ends are monotonically non-decreasing and binary-searchable. The defensive end <= gap_start continue stays in the body because ALIGN_UP(regions[i].end, hps) past one region can still skip over a smaller adjacent region whose end is below the new gap_start.

No new data structures, no memory growth, no behavior change for callers; helps both the cached-hint path and the addr-hint direct call site.

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Summary by cubic

Speed up find_free_gap_inner by skipping the prefix of irrelevant regions using a binary search. This removes repeated O(n) scans on mmap calls and addr-hint probes with no behavior change.

• Performance
  • Added first_region_end_above() to binary-search the first region with end > gap_start, then start the loop from that index.
  • Kept the end <= gap_start guard for alignment edge cases; no new data structures or semantics.

^{Written for commit 6473537. Summary will update on new commits.}

[cubic-dev-ai]

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