[clang] [clang][deps] Share `FileManager` between modules (PR #115065)

[Jan Svoboda via cfe-commits]

https://github.com/jansvoboda11 created https://github.com/llvm/llvm-project/pull/115065

The `FileManager` sharing between module-building `CompilerInstance`s 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%.

>From 0045351e322bde4a16e566db05df7e1ac8ed0e92 Mon Sep 17 00:00:00 2001
From: Jan Svoboda <jan_svoboda at apple.com>
Date: Tue, 5 Nov 2024 12:46:55 -0800
Subject: [PATCH] [clang][deps] Share `FileManager` between modules

The `FileManager` sharing between module-building `CompilerInstance`s 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%.
---
 .../Tooling/DependencyScanning/DependencyScanningWorker.cpp   | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

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);