Reland "cppgc-js: Add snapshot for C++ objects"
This reverts commit fba14bde. Reland fixes: - const vector<const string> -> const vector<string> Original message: The following implements a snapshotting algorithm for C++ objects that also filters strongly-connected components (SCCs) of only "hidden" objects that are not (transitively) referencing any non-hidden objects. C++ objects come in two versions. a. Named objects that have been assigned a name through NameProvider. b. Unnamed objects, that are potentially hidden if the build configuration requires Oilpan to hide such names. Hidden objects have their name set to NameProvider::kHiddenName. The main challenge for the algorithm is to avoid blowing up the final object graph with hidden nodes that do not carry information. For that reason, the algorithm filters SCCs of only hidden objects, e.g.: ... -> (object) -> (object) -> (hidden) -> (hidden) In this case the (hidden) objects are filtered from the graph. The trickiest part is maintaining visibility state for objects referencing other objects that are currently being processed. Main algorithm idea (two passes): 1. First pass marks all non-hidden objects and those that transitively reach non-hidden objects as visible. Details: - Iterate over all objects. - If object is non-hidden mark it as visible and also mark parent as visible if needed. - If object is hidden, traverse children as DFS to find non-hidden objects. Post-order process the objects and mark those objects as visible that have child nodes that are visible themselves. - Maintain an epoch counter (StateStorage::state_count_) to allow deferring the visibility decision to other objects in the same SCC. This is similar to the "lowlink" value in Tarjan's algorithm for SCC. - After the first pass it is guaranteed that all deferred visibility decisions can be resolved. 2. Second pass adds nodes and edges for all visible objects. - Upon first checking the visibility state of an object, all deferred visibility states are resolved. For practical reasons, the recursion is transformed into an iteration. We do not use plain Tarjan's algorithm to avoid another pass over all nodes to create SCCs. Follow ups: 1. Adding wrapper nodes for cpp objects that are wrappables for V8 wrappers. 2. Adding detachedness information. Bug: chromium:1056170 Change-Id: Ib47df5c912c57d644d052f209276e9d926cece0f Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2480362 Commit-Queue: Michael Lippautz <mlippautz@chromium.org> Commit-Queue: Ulan Degenbaev <ulan@chromium.org> Auto-Submit: Michael Lippautz <mlippautz@chromium.org> Reviewed-by:Ulan Degenbaev <ulan@chromium.org> Cr-Commit-Position: refs/heads/master@{#70577}
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