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timers: cross JS/C++ border less frequently
This removes the `process._needImmediateCallback` property and its semantics of having a 1/0 switch that tells C++ whether immediates are currently scheduled. Instead, a counter keeping track of all immediates is created, that can be increased on `setImmediate()` or decreased when an immediate is run or cleared. This is faster, because rather than reading/writing a C++ getter, this operation can be performed as a direct memory read/write via a typed array. The only C++ call that is left to make is activating the native handles upon creation of the first `Immediate` after the queue is empty. One other (good!) side-effect is that `immediate._destroyed` now reliably tells whether an `immediate` is still scheduled to run or not. Also, as a nice extra, this should make it easier to implement an internal variant of `setImmediate` for C++ that piggybacks off the same mechanism, which should be useful at least for async hooks and HTTP/2. Benchmark results: $ ./node benchmark/compare.js --new ./node --old ./node-master-1b093cb93df0 --runs 10 --filter immediate timers | Rscript benchmark/compare.R [00:08:53|% 100| 4/4 files | 20/20 runs | 1/1 configs]: Done improvement confidence p.value timers/immediate.js type="breadth" thousands=2000 25.61 % ** 1.432301e-03 timers/immediate.js type="breadth1" thousands=2000 7.66 % 1.320233e-01 timers/immediate.js type="breadth4" thousands=2000 4.61 % 5.669053e-01 timers/immediate.js type="clear" thousands=2000 311.40 % *** 3.896291e-07 timers/immediate.js type="depth" thousands=2000 17.54 % ** 9.755389e-03 timers/immediate.js type="depth1" thousands=2000 17.09 % *** 7.176229e-04 timers/set-immediate-breadth-args.js millions=5 10.63 % * 4.250034e-02 timers/set-immediate-breadth.js millions=10 20.62 % *** 9.150439e-07 timers/set-immediate-depth-args.js millions=10 17.97 % *** 6.819135e-10 PR-URL: #17064 Reviewed-By: Refael Ackermann <email@example.com> Reviewed-By: Minwoo Jung <firstname.lastname@example.org> Reviewed-By: James M Snell <email@example.com> Reviewed-By: Ben Noordhuis <firstname.lastname@example.org>
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