[clang][deps] Share FileManager between modules

[jansvoboda11]

The FileManager sharing between module-building CompilerInstances was disabled a while ago due to FileEntry::getName() being unreliable. Now that we use FileEntryRef::getNameAsRequested() in places where it matters, re-enabling FileManager is sound and improves performance of clang-scan-deps by ~6.2%.

[llvmbot]

@llvm/pr-subscribers-clang

Author: Jan Svoboda (jansvoboda11)

Changes

The FileManager sharing between module-building CompilerInstances was disabled a while ago due to FileEntry::getName() being unreliable. Now that we use FileEntryRef::getNameAsRequested() in places where it matters, re-enabling FileManager is sound and improves performance of clang-scan-deps by ~6.2%.

---

Full diff: https://github.com/llvm/llvm-project/pull/115065.diff

1 Files Affected:

• (modified) clang/lib/Tooling/DependencyScanning/DependencyScanningWorker.cpp (+3-1)

diff --git a/clang/lib/Tooling/DependencyScanning/DependencyScanningWorker.cpp b/clang/lib/Tooling/DependencyScanning/DependencyScanningWorker.cpp
index d77187bfb1f2b8..1deffe68003804 100644
--- a/clang/lib/Tooling/DependencyScanning/DependencyScanningWorker.cpp
+++ b/clang/lib/Tooling/DependencyScanning/DependencyScanningWorker.cpp
@@ -335,7 +335,9 @@ class DependencyScanningAction : public tooling::ToolAction {
 
     ScanInstance.getFrontendOpts().GenerateGlobalModuleIndex = false;
     ScanInstance.getFrontendOpts().UseGlobalModuleIndex = false;
-    ScanInstance.getFrontendOpts().ModulesShareFileManager = false;
+    // This will prevent us compiling individual modules asynchronously since
+    // FileManager is not thread-safe, but it does improve performance for now.
+    ScanInstance.getFrontendOpts().ModulesShareFileManager = true;
     ScanInstance.getHeaderSearchOpts().ModuleFormat = "raw";
     ScanInstance.getHeaderSearchOpts().ModulesIncludeVFSUsage =
         any(OptimizeArgs & ScanningOptimizations::VFS);