[Dependency Scanning] Use a standalone DependencyScanningFilesystemon the scanner, sharing a common status cache from Clang's dependency service cache
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cachemeifyoucan
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| new clang::tooling::dependencies::DependencyScanningWorkerFilesystem( | ||
| globalScanningService.ClangScanningService->getSharedCache(), | ||
| ScanASTContext.SourceMgr.getFileSystem()); | ||
| ScanASTContext.SourceMgr.setFileSystem(std::move(DepFS)); |
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What's the lifetime of ScanASTContext?
I'm asking because each instance of DependencyScanningWorkerFilesystem has a copy of a subset of the shared cache to reduce lock contention in the shared cache. We should create as few of these VFSs as possible in order to avoid re-populating the local cache from the shared cache over and over again.
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Its lifetime is the entire scan. And until we enable parallel-scanning by-default, there is only one of these workers.
… on the scanner, sharing a common `status` cache from Clang's dependency service cache
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Built-in Clang dependency scanner uses a
DependencyScanningFilesystemwhich relies on thestatcache owned by its parentDependencyScanningService. This way repeated attempts to query a filesystem entry hit the cache during scanning. Until now, the Swift scanner did not share thisstatcache so filesystem entry lookups by the Swift scanner went directly to the "real" underlying filesystem, resulting in often duplicated calls tostat. This change makes the Swift scanner use/share Clang Scanner'sstatcache, unifying the filesystem abstraction for the two.