[PATCH] D124816: [LibTooling] use ToolFileManager to store file managers for each CWD

[Duncan P. N. Exon Smith via Phabricator via cfe-commits]

dexonsmith added a comment.

This looks mostly correct to me, but perhaps pessimistic enough it'll regress performance unnecessarily. A few comments inline.



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Comment at: clang/lib/Tooling/DependencyScanning/DependencyScanningWorker.cpp:169-170
     ScanInstance.getFrontendOpts().UseGlobalModuleIndex = false;
-
+    auto FileMgr = ToolFileMgr->getOrCreateFileManager().get();
     FileMgr->getFileSystemOpts().WorkingDir = std::string(WorkingDirectory);
     ScanInstance.setFileManager(FileMgr);
----------------
Should `WorkingDirectory` be an argument to `getOrCreateFileManager()` here?


================
Comment at: clang/lib/Tooling/DependencyScanning/DependencyScanningWorker.cpp:171-172
     FileMgr->getFileSystemOpts().WorkingDir = std::string(WorkingDirectory);
     ScanInstance.setFileManager(FileMgr);
     ScanInstance.createSourceManager(*FileMgr);
 
----------------
Do these need to be moved lower, after the FileMgr/VFS might change?


================
Comment at: clang/lib/Tooling/DependencyScanning/DependencyScanningWorker.cpp:202-203
       // filesystem.
-      FileMgr->setVirtualFileSystem(createVFSFromCompilerInvocation(
+      ToolFileMgr->setVirtualFileSystem(createVFSFromCompilerInvocation(
           ScanInstance.getInvocation(), ScanInstance.getDiagnostics(), DepFS));
 
----------------
I don't see how calling this on `ToolFileMgr` threads through to updating the `FileMgr` in use, which has already been sent to `ScanInstance`.

Note that `createVFSFromCompilerInvocation()` *always* returns a new VFS, even if it's equivalent to the one used before (same DepFS options and same `-ivfsoverlay`s from `ScanInstance.getInvocation()`). Threading it through would mean that `FileMgr` is never reused.

IMO, never-reuse-the-filemanager is the right/correct behaviour for clang-scan-deps, since DepFS provides caching for things that can be cached soundly, but probably better to explicitly stop reusing the FileMgr (in a separate patch ahead of time) rather than leaving code that looks like it might get reused.


================
Comment at: clang/lib/Tooling/Tooling.cpp:447-448
   appendArgumentsAdjuster(getClangStripDependencyFileAdjuster());
   if (Files)
     Files->setVirtualFileSystem(OverlayFileSystem);
 }
----------------
This will *never* reuse the file manager, since it's *always* a new VFS.
- Maybe that's correct, since we want to inject different things into InMemoryFS?
- Or, maybe that's overly pessimistic, and we should pass BaseFS into the filemanager if `appendArgumentsAdjuster()` never added anything to InMemoryFS?


Repository:
  rG LLVM Github Monorepo

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  https://reviews.llvm.org/D124816/new/

https://reviews.llvm.org/D124816