[un-revert] Replace per-save-point array snapshots with an undo trail - #881
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Creating a save point previously copied the four mutable register arrays (ints, positions, values, and storedCaptures) to enable restoration during backtracking. This led to a large amount of reference counting traffic and copy-on-write of the original register arrays whenever they were updated. This change moves the restoration data from using full copies of each array in every save point, to tracking a log of the changes to each array in the processor itself and only storing the current state of the logs in each save point. This allows `SavePoint` to be a trivial type (almost) and removes all the copy-on-write traffic. Backtracking now uses the tracking logs to unwind the registers to the specific previous state. This change also includes a minor refactor, moving the `storedCaptures` array to the `Registers` struct, alongside the other mutable registers.
signalFailure only calls `undo(to:)`, which rewinds a register's log, but doesn't touch its values or add to the log.
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@swift-ci Please test Linux platform |
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Checked that the macOS build worked with the |
AnthonyLatsis
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Revert "[un-revert] Replace per-save-point array snapshots with an undo trail (#881)"
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Second attempt at landing #870, which simplifies the
SavePointtype and logs register mutations in anUndoableArray, rather than relying on copy-on-write for the registers during backtracking. Updated from the original to include a proper import that was missed without theMemberImportVisibilityflag, and some additional performance tuning.