[clang] [llvm] [llvm][support] Implement tracing virtual file system (PR #88326)

[Jan Svoboda via cfe-commits]

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@@ -1125,6 +1125,54 @@ class YAMLVFSWriter {
   void write(llvm::raw_ostream &OS);
 };
 
+/// File system that tracks the number of calls to the underlying file system.
+/// This is particularly useful when wrapped around \c RealFileSystem to add
+/// lightweight tracking of expensive syscalls.
+class TracingFileSystem
+    : public llvm::RTTIExtends<TracingFileSystem, ProxyFileSystem> {
+public:
+  static const char ID;
+
+  std::size_t NumStatusCalls = 0;
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jansvoboda11 wrote:

I'm contemplating whether to make these atomic. The root `FileSystem` class inherits from `ThreadSafeRefCountedBase`, suggesting that VFSs may be thread-safe.

For example in `clang-scan-deps`, all workers _could_ use single `RealFileSystem`. If we wrapped that in a single thread-safe `TracingFileSystem`, we'd very easily get the complete VFS stats. The current non-thread-safe approach would require having N of these VFS stacks and then manually aggregating the individual VFS stats.

Thoughts?

https://github.com/llvm/llvm-project/pull/88326