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Keyhan Vakil
0736d0b3f5
Currently we use external strings for internalized builtin source code. However when a snapshot is taken, any external string whose resource is not registered is flattened into a SeqString (see ref). The result is that module source code stored in the snapshot does not use external strings after deserialization. This patch registers an external string resource for each internalized builtin's source. The savings are substantial: ~1.9 MB of heap memory per isolate, or ~44% of an otherwise empty isolate's heap usage: ```console $ node --expose-gc -p 'gc(),process.memoryUsage().heapUsed' 4190968 $ ./node --expose-gc -p 'gc(),process.memoryUsage().heapUsed' 2327536 ``` The savings can be even higher for user snapshots which may include more internal modules. The existing UnionBytes implementation was ill-suited, because it only created an external string resource when ToStringChecked was called, but we need to register the external string resources before the isolate even exists. We change UnionBytes to no longer own the data, and shift ownership of the data to a new external resource class called StaticExternalByteResource. StaticExternalByteResource are either statically allocated (for internalized builtin code) or owned by the static `externalized_builtin_sources` map, so they will only be destructed when static resources are destructed. We change JS2C to emit statements to register a string resource for each internalized builtin. Refs: https://github.com/v8/v8/blob/d2c8fbe9ccd1a6ce5591bb7dd319c3c00d6bf489/src/snapshot/serializer.cc#L633 PR-URL: https://github.com/nodejs/node/pull/47055 Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com> Reviewed-By: Chengzhong Wu <legendecas@gmail.com>