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[heap] Clean-up some weak map entries in scavenger
This change enables clean-up of weak map entries in the scavenger of the weak map is in the young generation. With this change, the scavenger treats keys in ephemerons as weak instead of strong, but does not implement full ephemeron semantics: Values are treated always as strong, independently of whether the key is live or not. This approach ensures that no value is cleaned up accidentally. After scavenging, all entries with dead keys are removed from weak maps. After that, some values that are not referenced anywhere anymore might still be in the heap, and those can be cleaned up in the next scavenge. What the scavenger does, amounts to one iteration of the fixed-point algorithm required to implement ephemeron semantics. We hope that this is a reasonable trade-off between time spent tracing and cleaned-up entries. This change does not affect weak maps that reside in old space. Bug: v8:8557 Change-Id: Ic5618b3b863ad8c314c87449571150e756fecbf0 Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1467182 Commit-Queue: Sigurd Schneider <sigurds@chromium.org> Reviewed-by: Ulan Degenbaev <ulan@chromium.org> Cr-Commit-Position: refs/heads/master@{#60101}
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